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• How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

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To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

Scribbr slides are free to use, customize, and distribute for educational purposes.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
• Poisson distribution

Research bias

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• Hawthorne effect
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A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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• Published: 21 March 2024

Expert review of the science underlying nature-based climate solutions

• B. Buma   ORCID: orcid.org/0000-0003-2402-7737 1 , 2   na1 ,
• D. R. Gordon   ORCID: orcid.org/0000-0001-6398-2345 1 , 3   na1 ,
• K. M. Kleisner 1 ,
• A. Bartuska 1 , 4 ,
• A. Bidlack 5 ,
• R. DeFries   ORCID: orcid.org/0000-0002-3332-4621 6 ,
• P. Ellis   ORCID: orcid.org/0000-0001-7933-8298 7 ,
• P. Friedlingstein   ORCID: orcid.org/0000-0003-3309-4739 8 , 9 ,
• S. Metzger 10   nAff15   nAff16 ,
• G. Morgan 11 ,
• K. Novick   ORCID: orcid.org/0000-0002-8431-0879 12 ,
• J. N. Sanchirico 13 ,
• J. R. Collins   ORCID: orcid.org/0000-0002-5705-9682 1 , 14 ,
• A. J. Eagle   ORCID: orcid.org/0000-0003-0841-2379 1 ,
• R. Fujita 1 ,
• E. Holst 1 ,
• J. M. Lavallee   ORCID: orcid.org/0000-0002-3028-7087 1 ,
• R. N. Lubowski 1   nAff17 ,
• C. Melikov 1   nAff18 ,
• L. A. Moore   ORCID: orcid.org/0000-0003-0239-6080 1   nAff19 ,
• E. E. Oldfield   ORCID: orcid.org/0000-0002-6181-1267 1 ,
• J. Paltseva 1   nAff20 ,
• A. M. Raffeld   ORCID: orcid.org/0000-0002-5036-6460 1 ,
• N. A. Randazzo 1   nAff21   nAff22 ,
• C. Schneider 1 ,
• N. Uludere Aragon 1   nAff23 &
• S. P. Hamburg 1  

Nature Climate Change ( 2024 ) Cite this article

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• Climate-change ecology
• Climate-change mitigation
• Environmental impact

Viable nature-based climate solutions (NbCS) are needed to achieve climate goals expressed in international agreements like the Paris Accord. Many NbCS pathways have strong scientific foundations and can deliver meaningful climate benefits but effective mitigation is undermined by pathways with less scientific certainty. Here we couple an extensive literature review with an expert elicitation on 43 pathways and find that at present the most used pathways, such as tropical forest conservation, have a solid scientific basis for mitigation. However, the experts suggested that some pathways, many with carbon credit eligibility and market activity, remain uncertain in terms of their climate mitigation efficacy. Sources of uncertainty include incomplete GHG measurement and accounting. We recommend focusing on resolving those uncertainties before broadly scaling implementation of those pathways in quantitative emission or sequestration mitigation plans. If appropriate, those pathways should be supported for their cobenefits, such as biodiversity and food security.

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Nature-based climate solutions (NbCS) are conservation, restoration and improved management strategies (pathways) in natural and working ecosystems with the primary motivation to mitigate GHG emissions and remove CO 2 from the atmosphere 1 (similar to ecosystem-based mitigation 2 ). GHG mitigation through ecosystem stewardship is integral to meeting global climate goals, with the greatest benefit coming from near-term maximization of emission reductions, followed by CO 2 removal 3 . Many countries (for example, Indonesia, China and Colombia) use NbCS to demonstrate progress toward national climate commitments.

The scope of NbCS is narrower than that of nature-based solutions (NbS) which include interventions that prioritize non-climate benefits alongside climate (for example, biodiversity, food provisioning and water quality improvement) 4 . In many cases, GHG mitigation is considered a cobenefit that results from NbS actions focused on these other challenges 2 . In contrast, NbCS are broader than natural climate solutions, which are primarily focused on climate mitigation through conservation, restoration and improved land management, generally not moving ecosystems beyond their unmodified structure, function or composition 5 . NbCS may involve moving systems beyond their original function, for example by cultivating macroalgae in water deeper than their natural habitat.

The promise of NbCS has generated a proliferation of interest in using them in GHG mitigation plans 6 , 7 ; 104 of the 168 signatories to the Paris Accord included nature-based actions as part of their mitigation plans 8 . Success in long-term GHG management requires an accurate accounting of inputs and outputs to the atmosphere at scale, so NbCS credits must have robust, comprehensive and transparent scientific underpinnings 9 . Given the urgency of the climate problem, our goal is to identify NbCS pathways with a sufficient scientific foundation to provide broad confidence in their potential GHG mitigation impact, provide resources for confident implementation and identify priority research areas in more uncertain pathways. Evaluating implementation of mitigation projects is beyond our scope; this effort focuses on understanding the underlying science. The purpose is not evaluating any specific carbon crediting protocol or implementation framework but rather the current state of scientific understanding necessary to provide confidence in any NbCS.

In service of this goal, we first investigated nine biomes (boreal forests, coastal marine (salt marsh, mangrove, seagrass and coral reef), freshwater wetlands, grasslands, open ocean (large marine animal and mesopelagic zone biomass, seabed), peatlands, shrublands, temperate forests and tropical forests) and three cultivation types (agroforestry, croplands and macroalgae aquaculture); these were chosen because of their identified potential scale of global impact. In this context, impact is assessed as net GHG mitigation: the CO 2 sequestered or emissions reduced, for example, discounted by understood simultaneous emissions of other GHG (as when N 2 O is released simultaneously with carbon sequestration in cropland soils). From there, we identified 43 NbCS pathways which have been formally implemented (with or without market action) or informally proposed. We estimated the scale of mitigation impact for each pathway on the basis of this literature and, as a proxy measure of NbCS implementation, determined eligibility and activity under existing carbon crediting protocols. Eligibility means that the pathway is addressed by an existing GHG mitigation protocol; market activity means that credits are actively being bought under those eligibility requirements. We considered pathways across a spectrum from protection to improved management to restoration to manipulated systems, but some boundaries were necessary. We excluded primarily abiotically driven pathways (for example, ocean alkalinity enhancement) or where major land use or land-use trade-offs exist (for example, afforestation) 10 , 11 , 12 . Of the 43 pathways, 79% are at present eligible for carbon crediting (sometimes under several methodologies) and at least 65% of those have been implemented (Supplementary Table 1 ). This review was then appraised by 30 independent scholars (at least three per pathway; a complete review synthesis is given in the Supplementary Data ).

Consolidation of a broad body of scientific knowledge, with inherent variance, requires expert judgement. We used an expert elicitation process 13 , 14 , 15 with ten experts to place each proposed NbCS pathway into one of three readiness categories following their own assessment of the scientific literature, categorized by general sources of potential uncertainty: category 1, sufficient scientific basis to support a high-quality carbon accounting system or to support the development of such a system today; category 2, a >25% chance that focused research and reasonable funding would support development of high-quality carbon accounting (that is, move to category 1) within 5 years; or category 3, a <25% chance of development of high-quality carbon accounting within 5 years (for example, due to measurement challenges, unconstrained leakage, external factors which constrain viability).

If an expert ranked a pathway as category 2, they were also asked to rank general research needs to resolve: leakage/displacement (spillover to other areas), measuring, reporting and verification (the ability to quantify all salient stocks and fluxes), basic mechanisms of action (fundamental science), durability (ability to predict or compensate for uncertainty in timescale of effectiveness due to disturbances, climate change, human activity or other factors), geographic uncertainty (place-to-place variation), scaling potential (ability to estimate impact) and setting of a baseline (ability to estimate additionality over non-action; a counterfactual). To avoid biasing towards a particular a priori framework for evaluation of the scientific literature, reviewers could use their own framework for evaluating the NbCS literature about potential climate impact and so could choose to ignore or add relevant categorizations as well. Any pathway in category 1 would not need fundamental research for implementation; research gaps were considered too extensive for useful guidance on reducing uncertainty in category 3 pathways. Estimates of the global scale of likely potential impact (PgCO 2 e yr −1 ) and cobenefits were also collected from expert elicitors. See Methods and Supplementary Information for the survey instrument.

Four pathways with the highest current carbon market activity and high mitigation potential (tropical and temperate forest conservation and reforestation; Table 1 and Supplementary Data ), were consistently rated as high-confidence pathways in the expert elicitation survey. Other NbCS pathways, especially in the forestry sector, were rated relatively strongly by the experts for both confidence in scientific basis and scale of potential impact, with some spread across the experts (upper right quadrant, Fig. 1 ). Conversely, 13 pathways were consistently marked by experts as currently highly uncertain/low confidence (median score across experts: 2.5–3.0) and placed in category 3 (for example, cropland microbial amendments and coral reef restoration; Supplementary Tables 1 and 2 ). For the full review, including crediting protocols currently used, literature estimates of scale and details of sub-pathways, see Supplementary Data .

Pathways in the upper right quadrant have both high confidence in the scientific foundations and the largest potential scale of global impact; pathways in the lower left have the lowest confidence in our present scientific body of knowledge and an estimated smaller potential scale of impact. Designations of carbon credit eligibility under existing protocols and market activity at the present time are noted. Grassland enhanced mineral weathering (EMW) is not shown (mean category rating 2.9) as no scale of impact was estimated. See Supplementary Table 1 for specific pathway data. Bars represent 20th to 80th percentiles of individual estimates, if there was variability in estimates. A small amount of random noise was added to avoid overlap.

The experts assessed 26 pathways as having average confidence scores between 1.5 and 2.4, suggesting the potential for near-term resolution of uncertainties. This categorization arose from either consensus amongst experts on the uncertain potential (for example, boreal forest reforestation consistently rated category 2, with primary concerns about durability) or because experts disagreed, with some ranking category 1 and others category 3 (for example, pasture management). We note that where expert disagreement exists (seen as the spread of responses in Fig. 1 and Supplementary Table 1 ; also see Data availability for link to original data), this suggests caution against overconfidence in statements about these pathways. These results also suggest that confidence may be increased by targeted research on the identified sources of uncertainty (Supplementary Table 3 ).

Sources of uncertainty

Durability and baseline-setting were rated as high sources of uncertainty across all pathways ranked as category 2 by the experts (mean ratings of 3.6 and 3.4 out of 5, respectively; Supplementary Table 3 ). Understanding of mechanisms and geographic spread had the lowest uncertainty ratings (2.1 and 2.3, respectively), showing confidence in the basic science. Different subsets of pathways had different prioritizations, however, suggesting different research needs: forest-centric pathways were most uncertain in their durability and additionality (3.8 and 3.4, respectively), suggesting concerns about long-term climate and disturbance trajectories. Agricultural and grassland systems, however, had higher uncertainty in measurement methods and additionality (3.9 and 3.5 respectively). Although there were concerns about durability from some experts (for example, due to sea-level rise), some coastal blue carbon pathways such as mangrove restoration (mean category ranking: 1.7 (20th to 80th percentile 1.0–2.0)) have higher confidence than others (for example, seagrass restoration: mean category ranking 2.8, 20th to 80th percentile 2.6–3.0)), which are relatively poorly constrained in terms of net radiative forcing potential despite a potentially large carbon impact (seagrass median: 1.60 PgCO 2 e yr −1 ; see Supplementary Data for more scientific literature estimates).

Scale of impact

For those pathways with lower categorization by the expert elicitation (category 2 or 3) at the present time, scale of global impact is a potential heuristic for prioritizing further research. High variability, often two orders of magnitude, was evident in the mean estimated potential PgCO 2 e yr −1 impacts for the different pathways (Fig. 1 and Supplementary Table 2 ) and the review of the literature found even larger ranges produced by individual studies (Supplementary Data ). A probable cause of this wide range was different constraints on the estimated potential, with some studies focusing on potential maximum impact and others on more constrained realizable impacts. Only avoided loss of tropical forest and cropland biochar amendment were consistently estimated as having the likely potential to mitigate >2 PgCO 2 e yr −1 , although biochar was considered more uncertain by experts due to other factors germane to its overall viability as a climate solution, averaging a categorization of 2.2. The next four highest potential impact pathways, ranging from 1.6 to 1.7 PgCO 2 e yr −1 , spanned the spectrum from high readiness (temperate forest restoration) to moderate (cropland conversion from annual to perennial vegetation and grassland restoration) to low (seagrass restoration, with main uncertainties around scale of potential impact and durability).

There was high variability in the elicitors’ estimated potential scale of impact, even in pathways with strong support, such as tropical forest avoided loss (20th to 80th percentile confidence interval: 1–8 PgCO 2 e yr −1 ), again emphasizing the importance of consistent definitions and constraints on how NbCS are measured, evaluated and then used in broad-scale climate change mitigation planning and budgeting. Generally, as pathway readiness decreased (moving from category 1 to 3), the elicitor-estimated estimates of GHG mitigation potential decreased (Supplementary Fig. 1 ). Note that individual studies from the scientific literature may have higher or lower estimates (Supplementary Data ).

Expert elicitation meta-analyses suggest that 6–12 responses are sufficient for a robust and stable quantification of responses 15 . We tested that assumption via a Monte Carlo-based sensitivity assessment. Readiness categorizations by the ten experts were robust to a Monte Carlo simulation test, where further samples were randomly drawn from the observed distribution of responses: mean difference between the original and the boot-strapped data was 0.02 (s.d. = 0.05) with an absolute difference average of 0.06 (s.d. = 0.06). The maximum difference in readiness categorization means across all pathways was 0.20 (s.d. = 0.20) (Supplementary Table 2 ). The full dataset of responses is available online (see ʻData availabilityʼ).

These results highlight opportunities to accelerate implementation of NbCS in well-supported pathways and identify critical research needs in others (Fig. 1 ). We suggest focusing future efforts on resolving identified uncertainties for pathways at the intersection between moderate average readiness (for example, mean categorizations between ~1.5 and 2.0) and high potential impact (for example, median >0.5 PgCO 2 e yr −1 ; Supplementary Table 1 ): agroforestry, improved tropical and temperate forest management, tropical and boreal peatlands avoided loss and peatland restoration. Many, although not all, experts identified durability and baseline/additionality as key concerns to resolve in those systems; research explicitly targeted at those specific uncertainties (Supplementary Table 3 ) could rapidly improve confidence in those pathways.

We recommend a secondary research focus on the lower ranked (mean category 2.0 to 3.0) pathways with estimated potential impacts >1 PgCO 2 e yr −1 (Supplementary Fig. 2 ). For these pathways, explicit, quantitative incorporation into broad-scale GHG management plans will require further focus on systems-level carbon/GHG understandings to inspire confidence at all stages of action and/or identifying locations likely to support durable GHG mitigation, for example ref. 16 . Examples of this group include avoided loss and degradation of boreal forests (for example, fire, pests and pathogens and albedo 16 ) and effective mesopelagic fishery management, which some individual studies estimate would avoid future reductions of the currently sequestered 1.5–2.0 PgC yr −1 (refs. 17 , 18 ). These pathways may turn out to have higher or lower potential than the expert review suggests, on the basis of individual studies (Supplementary Data ) but strong support will require further, independent verification of that potential.

We note that category 3 rankings by expert elicitation do not necessarily imply non-viability but simply that much more research is needed to confidently incorporate actions into quantitative GHG mitigation plans. We found an unsurprising trend of lower readiness categorization with lower pathway familiarity (Supplementary Fig. 3 ). This correlation may result from two, non-exclusive potential causes: (1) lower elicitor expertise in some pathways (inevitable, although the panel was explicitly chosen for global perspectives, connections and diverse specialties) and (2) an actual lack of scientific evidence in the literature, which leads to that self-reported lack of familiarity, a common finding in the literature review (Supplementary Data ). Both explanations suggest a need to better consolidate, develop and disseminate the science in each pathway for global utility and recognition.

Our focus on GHG-related benefits in no way diminishes the substantial conservation, environmental and social cobenefits of these pathways (Supplementary Table 4 ), which often exceed their perceived climate benefits 1 , 19 , 20 , 21 . Where experts found climate impacts to remain highly uncertain but other NbS benefits are clear (for example, biodiversity and water quality; Supplementary Table 4 ), other incentives or financing mechanisms independent of carbon crediting should be pursued. While the goals here directly relate to using NbCS as a reliably quantifiable part of global climate action planning and thus strong GHG-related scientific foundations, non-climate NbS projects may provide climate benefits that are less well constrained (and thus less useful from a GHG budgeting standpoint) but also valuable. Potential trade-offs, if any, between ecosystem services and management actions, such as biodiversity and positive GHG outcomes, should be explored to ensure the best realization of desired goals 2 .

Finally, our focus in this study was on broad-scale NbCS potential in quantitative mitigation planning because of the principal and necessary role of NbCS in overall global warming targets. We recognize the range of project conditions that may increase, or decrease, the rigour of any pathway outside the global-scale focus here. We did not specifically evaluate the large and increasing number of crediting concepts (by pathway: Supplementary Data ), focusing rather on the underlying scientific body of knowledge within those pathways. Some broad pathways may have better defined sub-pathways within them, with a smaller potential scale of impact but potentially lower uncertainty (for example, macroalgae harvest cycling). Poorly enacted NbCS actions and/or crediting methodologies at project scales may result in loss of benefits even from high-ranking pathways 22 , 23 , 24 and attention to implementation should be paramount. Conversely, strong, careful project-scale methodologies may make lower readiness pathways beneficial for a given site.

Viable NbCS are vital to global climate change mitigation but NbCS pathways that lack strong scientific underpinnings threaten global accounting by potentially overestimating future climate benefits and eroding public trust in rigorous natural solutions. Both the review of the scientific literature and the expert elicitation survey identified high potential ready-to-implement pathways (for example, tropical reforestation), reinforcing present use of NbCS in planning.

However, uncertainty remains about the quantifiable GHG mitigation of some active and nascent NbCS pathways. On the basis of the expert elicitation survey and review of the scientific literature, we are concerned that large-scale implementation of less scientifically well-founded NbCS pathways in mitigation plans may undermine net GHG budget planning; those pathways require more study before they can be confidently promoted at broad scales and life-cycle analyses to integrate system-level emissions when calculating totals. The expert elicitation judgements suggest a precautionary approach to scaling lower confidence pathways until the scientific foundations are strengthened, especially for NbCS pathways with insufficient measurement and monitoring 10 , 24 , 25 or poorly understood or measured net GHG mitigation potentials 16 , 26 , 27 , 28 . While the need to implement more NbCS pathways for reducing GHG emissions and removing carbon from the atmosphere is urgent, advancing the implementation of poorly quantified pathways (in relation to their GHG mitigation efficacy) could give the false impression that they can balance ongoing, fossil emissions, thereby undermining overall support for more viable NbCS pathways. Explicitly targeting research to resolve these uncertainties in the baseline science could greatly bolster confidence in the less-established NbCS pathways, benefiting efforts to reduce GHG concentrations 29 .

The results of this study should inform both market-based mechanisms and non-market approaches to NbCS pathway management. Research and action that elucidates and advances pathways to ensure a solid scientific basis will provide confidence in the foundation for successfully implementing NbCS as a core component of global GHG management.

NbCS pathway selection

We synthesized scientific publications for nine biomes (boreal forests, coastal blue carbon, freshwater wetlands, grasslands, open ocean blue carbon, peatlands, shrublands, temperate forests and tropical forests) and three cultivation types (agroforestry, croplands and macroalgae aquaculture) (hereafter, systems) and the different pathways through which they may be able to remove carbon or reduce GHG emissions. Shrublands and grasslands were considered as independent ecosystems; nonetheless, we acknowledge that there is overlap in the numbers presented here because shrublands are often included with grasslands 5 , 30 , 31 , 32 , 33 .

The 12 systems were chosen because they have each been identified as having potential for emissions reductions or carbon removal at globally relevant scales. Within these systems, we identified 43 pathways which either have carbon credit protocols formally established or informally proposed for review (non-carbon associated credits were not evaluated). We obtained data on carbon crediting protocols from international, national and regional organizations and registries, such as Verra, American Carbon Registry, Climate Action Reserve, Gold Standard, Clean Development Mechanism, FAO and Nori. We also obtained data from the Voluntary Registry Offsets Database developed by the Berkeley Carbon Trading Project and Carbon Direct company 34 . While we found evidence of more Chinese carbon crediting protocols, we were not able to review these because of limited publicly available information. To maintain clarity and avoid misrepresentation, we used the language as written in each protocol. A full list of the organizations and registries for each system can be found in the Supplementary Data .

Literature searches and synthesis

We reviewed scientific literature and reviews (for example, IPCC special reports) to identify studies reporting data on carbon stocks, GHG dynamics and sequestration potential of each system. Peer-reviewed studies and meta-analyses were identified on Scopus, Web of Science and Google Scholar using simple queries combining the specific practice or pathway names or synonyms (for example, no-tillage, soil amendments, reduced stocking rates, improved forest management, avoided forest conversion and degradation, avoided mangrove conversion and degradation) and the following search terms: ‘carbon storage’, ‘carbon stocks’, ‘carbon sequestration’, ‘carbon sequestration potential’, ‘additional carbon storage’, ‘carbon dynamics’, ‘areal extent’ or ‘global’.

The full literature review was conducted between January and October 2021. We solicited an independent, external review of the syntheses (obtaining from at least three external reviewers per natural or working system; see p. 2 of the Supplementary Data ) as a second check against missing key papers or misinterpretation of data. The review was generally completed in March 2022. Data from additional relevant citations were added through October 2022 as they were discovered. For a complete list of all literature cited, see pp. 217–249 of the Supplementary Data .

From candidate papers, the papers were considered if their results/data could be applied to the following central questions:

How much carbon is stored (globally) at present in the system (total and on average per hectare) and what is the confidence?

At the global level, is the system a carbon source or sink at this time? What is the business-as-usual projection for its carbon dynamics?

Is it possible, through active management, to either increase net carbon sequestration in the system or prevent carbon emissions from that system? (Note that other GHG emissions and forcings were included here as well.)

What is the range of estimates for how much extra carbon could be sequestered globally?

How much confidence do we have in the present methods to detect any net increases in carbon sequestration in a system or net changes in areal extent of that?

From each paper, quantitative estimates for the above questions were extracted for each pathway, including any descriptive information/metadata necessary to understand the estimate. In addition, information on sample size, sampling scheme, geographic coverage, timeline of study, timeline of projections (if applicable) and specific study contexts (for example, wind-break agroforestry) were recorded.

We also tracked where the literature identified trade-offs between carbon sequestered or CO 2 emissions reduced and emissions of other GHG (for example, N 2 O or methane) for questions three and five above. For example, wetland restoration can result in increased CO 2 uptake from the atmosphere. However, it can also increase methane and N 2 O emissions to the atmosphere. Experts were asked to consider the uncertainty in assessing net GHG mitigation as they categorized the NbCS pathways.

Inclusion of each pathway in mitigation protocols and the specific carbon registries involved were also identified. These results are reported (grouped or individually as appropriate) in the Supplementary Data , organized by the central questions and including textual information for interpretation. The data and protocol summaries for each of the 12 systems were reviewed by at least three scientists each and accordingly revised.

These summaries were provided to the expert elicitation group as optional background information.

Unit conversions

Since this synthesis draws on literature from several sources that use different methods and units, all carbon measurements were standardized to the International System of Units (SI units). When referring to total stocks for each system, numbers are reported in SI units of elemental carbon (that is, PgC). When referring to mitigation potential, elemental carbon was converted to CO 2 by multiplying by 3.67. Differences in methodology, such as soil sampling depth, make it difficult to standardize across studies. Where applicable, the specific measurement used to develop each stock estimate is reported.

Expert elicitation process

To assess conclusions brought about by the initial review process described above, we conducted an expert elicitation survey to consolidate and add further, independent assessments to the original literature review. The expert elicitation survey design followed best practice recommendations 14 , with a focus on participant selection, explicitly defining uncertainty, minimizing cognitive and overconfidence biases and clarity of focus. Research on expert elicitation suggests that 6–12 responses are sufficient for a stable quantification of responses 15 . We identified >40 potential experts via a broad survey of leading academics, science-oriented NGO and government agency publications and products. These individuals have published on several NbCS pathways or could represent larger research efforts that spanned the NbCS under consideration. Careful attention was paid to the gender and sectoral breakdown of respondents to ensure equitable representation. Of the invitees, ten completed the full elicitation effort. Experts were offered compensation for their time.

Implementation of the expert elicitation process followed the IDEA protocol 15 . Briefly, after a short introductory interview, the survey was sent to the participants. Results were anonymized and standardized (methods below) and a meeting held with the entire group to discuss the initial results and calibrate understanding of questions. The purpose of this meeting was not to develop consensus on a singular answer but to discuss and ensure that all questions are being considered in the same way (for example, clarifying any potentially confusing language, discussing any questions that emerged as part of the process). The experts then revisited their initial rankings to provide final, anonymous rankings which were compiled in the same way. These final rankings are the results presented here and may be the same or different from the initial rankings, which were discarded.

Survey questions

The expert elicitation survey comprised five questions for each pathway. The data were collected via Google Forms and collated anonymously at the level of pathways, with each respondent contributing one datapoint for each pathway. The experts reported their familiarity (or the familiarity of the organization whose work they were representing) with the pathway and other cobenefits for the pathways.

The initial question ranked the NbCS pathway by category, from one to three.

Category 1 was defined as a pathway with sufficient scientific knowledge to support a high-quality carbon accounting system today (for example, meets the scientific criteria identified in the WWF-EDF-Oeko Institut and ICAO TAB) or to support the development of such a system today. The intended interpretation is that sufficient science is available for quantifying and verifying net GHG mitigation. Note that experts were not required to reference any given ‘high-quality’ crediting framework, which were provided only as examples. In other words, the evaluation was not intended to rank a given framework (for example, ref. 35 ) but rather expert confidence in the fundamental scientific understandings that underpin potential for carbon accounting overall. To this end, no categorization of uncertainty was required (reviewers could skip categorizations they felt were not necessary) and space was available to fill in new categories by individual reviewers (if they felt a category was missing or needed). Uncertainties at this category 1 level are deemed ‘acceptable’, for example, not precluding accounting now, although more research may further substantiate high-quality credits.

Category 2 pathways have a good chance (>25%) that with more research and within the next 5 years, the pathway could be developed into a high-quality pathway for carbon accounting and as a nature-based climate solution pathway. For these pathways, further understanding is needed for factors such as baseline processes, long-term stability, unconstrained fluxes, possible leakage or other before labelling as category 1 but the expert is confident that information can be developed, in 5 years or less, with more work. The >25% chance threshold and 5-year timeframe were determined a priori to reflect and identify pathways that experts identified as having the potential to meet the Paris Accord 2030 goal. Other thresholds (for example, longer timeframes) could have been chosen, which would impact the relative distribution of pathways in categories 2 and 3 (for example, a longer timeframe allowed could move some pathways from category 3 into category 2, for some reviewers). We emphasize that category 3 pathways do not necessarily mean non-valuable approaches but longer timeframes required for research than the one set here.

Category 3 responses denoted pathways that the expert thought had little chance (<25%) that with more research and within the next 5 years, this pathway could be developed into a suitable pathway for managing as a natural solutions pathway, either because present evidence already suggests GHG reduction is not likely to be viable, co-emissions or other biophysical feedbacks may offset those gains or because understanding of key factors is lacking and unlikely to be developed within the next 5 years. Notably, the last does not mean that the NbCS pathway is not valid or viable in the long-term, simply that physical and biological understandings are probably not established enough to enable scientific rigorous and valid NbCS activity in the near term.

The second question asked the experts to identify research gaps associated with those that they ranked as category 2 pathways to determine focal areas for further research. The experts were asked to rank concerns about durability (ability to predict or compensate for uncertainty in timescale of effectiveness due to disturbances, climate change, human activity or other factors), geographic uncertainty (place-to-place variation), leakage or displacement (spillover of activities to other areas), measuring, reporting and verification (MRV, referring to the ability to quantify all salient stocks and fluxes to fully assess climate impacts), basic mechanisms of action (fundamental science), scaling potential (ability to estimate potential growth) and setting of a baseline (ability to reasonably quantify additionality over non-action, a counterfactual). Respondents could also enter a different category if desired. For complete definitions of these categories, see the survey instrument ( Supplementary Information ). This question was not asked if the expert ranked the pathway as category 1, as those were deemed acceptable, or for category 3, respecting the substantial uncertainty in that rating. Note that responses were individual and so the same NbCS pathway could receive (for example) several individual category 1 rankings, which would indicate reasonable confidence from those experts, and several category 2 rankings from others, which would indicate that those reviewers have lingering concerns about the scientific basis, along with their rankings of the remaining key uncertainties in those pathways. These are important considerations, as they reflect the diversity of opinions and research priorities; individual responses are publicly available (anonymized: https://doi.org/10.5281/zenodo.7859146 ).

The third question involved quantification of the potential for moving from category 2 to 1 explicitly. Following ref. 14 , the respondents first reported the lowest plausible value for the potential likelihood of movement (representing the lower end of a 95% confidence interval), then the upper likelihood and then their best guess for the median/most likely probability. They were also asked for the odds that their chosen interval contained the true value, which was used to scale responses to standard 80% credible intervals and limit overconfidence bias 13 , 15 . This question was not asked if the expert ranked the pathway as category 3, respecting the substantial uncertainty in that rating.

The fourth question involved the scale of potential impact from the NbCS, given the range of uncertainties associated with effectiveness, area of applicability and other factors. The question followed the same pattern as the third, first asking about lowest, then highest, then best estimate for potential scale of impact (in PgCO 2 e yr −1 ). Experts were again asked to express their confidence in their own range, which was used to scale to a standard 80% credible interval. This estimate represents a consolidation of the best-available science by the reviewers. For a complete review including individual studies and their respective findings, see the Supplementary Data . This question was not asked if the expert ranked the pathway as category 3, respecting the substantial uncertainty in that rating.

Final results

After collection of the final survey responses, results were anonymized and compiled by pathway. For overall visualization and discussion purposes, responses were combined into a mean and 20th to 80th percentile range. The strength of the expert elicitation process lies in the collection of several independent assessments. Those different responses represent real differences in data interpretation and synthesis ascribed by experts. This can have meaningful impacts on decision-making by different individuals and organizations (for example, those that are more optimistic or pessimistic about any given pathway). Therefore, individual anonymous responses were retained by pathway to show the diversity of responses for any given pathway. The experts surveyed, despite their broad range of expertise, ranked themselves as less familiar with category 3 pathways than category 1 or 2 (linear regression, P  < 0.001, F  = 59.6 2, 394 ); this could be because of a lack of appropriate experts—although they represented all principal fields—or simply because the data are limited in those areas.

Sensitivity

To check for robustness against sample size variation, we conducted a Monte Carlo sensitivity analysis of the data on each pathway to generate responses of a further ten hypothetical experts. Briefly, the extra samples were randomly drawn from the observed category ranking mean and standard deviations for each individual pathway and appended to the original list; values <1 or >3 were truncated to those values. This analysis resulted in only minor differences in the mean categorization across all pathways: the mean difference between the original and the boot-strapped data was 0.02 (s.d. = 0.05) with an absolute difference average of 0.06 (s.d. = 0.06). The maximum difference in means across all pathways was 0.20 (s.d. = 0.20) (Supplementary Table 2 ). The results suggest that the response values are stable to additional responses.

All processing was done in R 36 , with packages including fmsb 37 and forcats 38 .

Data availability

Anonymized expert elicitation responses are available on Zenodo 39 : https://doi.org/10.5281/zenodo.7859146 .

Code availability

R code for analysis available on Zenodo 39 : https://doi.org/10.5281/zenodo.7859146 .

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Acknowledgements

This research was supported through gifts to the Environmental Defense Fund from the Bezos Earth Fund, King Philanthropies and Arcadia, a charitable fund of L. Rausing and P. Baldwin. We thank J. Rudek for help assembling the review and 30 experts who reviewed some or all of those data and protocol summaries (Supplementary Data ). S.M. was supported by a cooperative agreement between the National Science Foundation and Battelle that sponsors the National Ecological Observatory Network programme.

Author information

Present address: Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, WI, USA

Present address: AtmoFacts, Longmont, CO, USA

R. N. Lubowski

Present address: Lombard Odier Investment Managers, New York, NY, USA

Present address: Ecological Carbon Offset Partners LLC, dba EP Carbon, Minneapolis, MN, USA

L. A. Moore

Present address: , San Francisco, CA, USA

J. Paltseva

Present address: ART, Arlington, VA, USA

N. A. Randazzo

Present address: NASA/GSFC, Greenbelt, MD, USA

Present address: University of Maryland, College Park, MD, USA

N. Uludere Aragon

Present address: Numerical Terradynamic Simulation Group, University of Montana, Missoula, MT, USA

These authors contributed equally: B. Buma, D. R. Gordon.

Authors and Affiliations

Environmental Defense Fund, New York, NY, USA

B. Buma, D. R. Gordon, K. M. Kleisner, A. Bartuska, J. R. Collins, A. J. Eagle, R. Fujita, E. Holst, J. M. Lavallee, R. N. Lubowski, C. Melikov, L. A. Moore, E. E. Oldfield, J. Paltseva, A. M. Raffeld, N. A. Randazzo, C. Schneider, N. Uludere Aragon & S. P. Hamburg

Department of Integrative Biology, University of Colorado, Denver, CO, USA

Department of Biology, University of Florida, Gainesville, FL, USA

D. R. Gordon

Resources for the Future, Washington, DC, USA

A. Bartuska

International Arctic Research Center, University of Alaska, Fairbanks, AK, USA

Department of Ecology Evolution and Environmental Biology and the Climate School, Columbia University, New York, NY, USA

The Nature Conservancy, Arlington, VA, USA

Faculty of Environment, Science and Economy, University of Exeter, Exeter, UK

P. Friedlingstein

Laboratoire de Météorologie Dynamique/Institut Pierre-Simon Laplace, CNRS, Ecole Normale Supérieure/Université PSL, Sorbonne Université, Ecole Polytechnique, Palaiseau, France

National Ecological Observatory Network, Battelle, Boulder, CO, USA

Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA, USA

O’Neill School of Public and Environmental Affairs, Indiana University, Bloomington, IN, USA

Department of Environmental Science and Policy, University of California, Davis, CA, USA

J. N. Sanchirico

Department of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

J. R. Collins

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D.R.G. and B.B. conceived of and executed the study design. D.R.G., K.M.K., J.R.C., A.J.E., R.F., E.H., J.M.L., R.N.L., C.M., L.A.M., E.E.O., J.P., A.M.R., N.A.R., C.S. and N.U.A. coordinated and conducted the literature review. G.M. and B.B. primarily designed the survey. A. Bartuska, A. Bidlack, B.B., J.N.S., K.N., P.E., P.F., R.D. and S.M. contributed to the elicitation. B.B. conducted the analysis and coding. S.P.H. coordinated funding. B.B. and D.R.G. were primary writers; all authors were invited to contribute to the initial drafting.

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Buma, B., Gordon, D.R., Kleisner, K.M. et al. Expert review of the science underlying nature-based climate solutions. Nat. Clim. Chang. (2024). https://doi.org/10.1038/s41558-024-01960-0

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Associations between Nature Exposure and Health: A Review of the Evidence

Marcia p. jimenez.

1 Department of Epidemiology, Harvard T. H. Chan School of Public Health, Boston, MA 02215, USA

2 Department of Population Medicine, Harvard Pilgrim Health Care Institute and Harvard Medical School, Boston, MA 02215, USA; ude.dravrah.hpsh@semajp

Nicole V. DeVille

3 Channing Division of Network Medicine, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA 02215, USA; ude.dravrah.hpsh@ttoillee (E.G.E.); ude.dravrah.gninnahc@hcjer (J.E.H.)

Elise G. Elliott

4 Department of Environmental Health, Harvard T. H. Chan School of Public Health, Boston, MA 02215, USA; ude.dravrah.hpsh@ffihcsj (J.E.S.); ude.dravrah.g@tliwg (G.E.W.)

Jessica E. Schiff

Grete e. wilt, jaime e. hart, peter james.

There is extensive empirical literature on the association between exposure to nature and health. In this narrative review, we discuss the strength of evidence from recent (i.e., the last decade) experimental and observational studies on nature exposure and health, highlighting research on children and youth where possible. We found evidence for associations between nature exposure and improved cognitive function, brain activity, blood pressure, mental health, physical activity, and sleep. Results from experimental studies provide evidence of protective effects of exposure to natural environments on mental health outcomes and cognitive function. Cross-sectional observational studies provide evidence of positive associations between nature exposure and increased levels of physical activity and decreased risk of cardiovascular disease, and longitudinal observational studies are beginning to assess long-term effects of nature exposure on depression, anxiety, cognitive function, and chronic disease. Limitations of current knowledge include inconsistent measures of exposure to nature, the impacts of the type and quality of green space, and health effects of duration and frequency of exposure. Future directions include incorporation of more rigorous study designs, investigation of the underlying mechanisms of the association between green space and health, advancement of exposure assessment, and evaluation of sensitive periods in the early life-course.

1. Introduction

The “biophilia hypothesis” posits that humans have evolved with nature to have an affinity for nature [ 1 ]. Building on this concept, two major theories—Attention Restoration Theory and Stress Reduction Theory—have provided insight into the mechanisms through which spending time in nature might affect human health. Attention Restoration Theory (ART) posits that the mental fatigue associated with modern life is associated with a depleted capacity to direct attention [ 2 ]. According to this theory, spending time in natural environments enables people to overcome this mental fatigue and to restore the capacity to direct attention [ 3 ]. The Stress Reduction Theory (SRT) describes how spending time in nature might influence feelings or emotions by activating the parasympathetic nervous system to reduce stress and autonomic arousal because of people’s innate connection to the natural world [ 4 , 5 ]. Further, proponents of the biophilia hypothesis postulate that green spaces provide children with opportunities such as discovery, creativity, risk taking, mastery, and control, which positively influence different aspects of brain development [ 6 ]. Beyond the biophilia hypothesis, there are a number of other pathways through which nature may affect health, including but not limited to increasing opportunities for social engagement and space for physical activity, while mitigating harmful environmental exposures (e.g., air pollution, noise, heat) [ 7 , 8 , 9 , 10 ]. Though evidence is inconsistent, physical activity may serve as an important mechanistic pathway to beneficial health outcomes by providing increased opportunities for outdoor exercise (e.g., walking) and play [ 7 , 8 , 9 ]. Facilitation of social contact is a promising mechanism emerging from recent literature, where natural environments and green space provide an avenue for increased contact with others and a greater sense of community [ 9 , 10 ]. The mechanism’s underlying associations between nature exposure and health outcomes are many, not completely understood, and could act in isolation or synergistically [ 11 ].

While the study of exposure to nature and health outcomes has expanded substantially over recent years, there remain many understudied relationships, mechanisms, and populations. For instance, there is a much more expansive evidence base for associations between nature and health, particularly with experimental studies, in adults than in children. This narrative review synthesizes recent scientific literature on associations between nature and health, highlighting studies conducted among children and youth where possible, published throughout August 2020 and based on: (1) randomized experimental studies of short-term exposure to nature and acute responses; and (2) observational studies of exposure to nature.

A narrative review synthesizes the results of quantitative studies that employ diverse methodologies and/or theoretical frameworks without a focus on the statistical significance of the studies’ results [ 12 , 13 ]. We conducted a keyword search-based review using PubMed Advanced Search on 31 August 2020 for studies published in the last ten years with titles or abstracts containing “greenness”, “green space” or “NDVI” (i.e., normalized difference vegetation index) as the exposure, and “health, “children’s health” or “youth health” as the outcome (National Library of Medicine, Bethesda, MD, USA). Using World Health Organization definitions, we categorized a child as a person younger than 10 years and youth from 10 to 24 years inclusive [ 14 ]. We limited this narrative review to research on human subjects only and included English-language-based, international peer-reviewed articles (e.g., primary research, reviews), online reports, electronic books, and press releases. We included both experimental and observational studies and applied snowballing search methodology using the references cited in the articles identified in the literature search. Each identified item was assessed for relevance by a member of the study team. This review is not comprehensive but is intended to summarize recent literature on nature exposure and health.

In retrieving literature on associations of nature and health, we reviewed a range of research from multiple health-related disciplines, geographic regions, and study populations. Evidence from the experimental and observational studies presented below represents more recent literature (e.g., the last decade) on nature exposure and health, primarily from Western countries.

3.1. Experimental Studies

We found a substantial body of research on natural environment interventions to evaluate the effects of nature on health from an experimental approach. The interventions consisted of active engagement in the natural environment (e.g., walking, running, or other activities), passive engagement (e.g., resting outside or living with a view), or virtual exposure (e.g., watching videos or viewing images of nature) [ 15 , 16 ]. The majority of experimental studies assessed mental health and neurologic outcomes. Results from experimental studies suggested a protective effect of exposure to natural environments on mental health outcomes and cognitive function.

3.1.1. Stress

Several experimental studies have examined perceived stress and other subjective measures of stress, such as sleep quality. A recent systematic review of more than 40 experimental studies indicates that measures of heart rate, blood pressure, and perceived stress provide the most convincing evidence that exposure to nature or outdoor environments may reduce the negative effects of stress [ 17 ]. The results from perceived or reported stress after exposure to natural environments were more consistent than findings from studies using physiological stress measurements (e.g., cortisol levels) among adults. A recent meta-analysis found evidence suggesting that exposure to natural environments may reduce cortisol levels, one of the most frequently studied biological markers of stress. Song et al. [ 18 ] reviewed 52 articles from Japan that examined the physiological effects of nature therapy. There was overwhelming evidence that cortisol levels decreased when participants were exposed to a natural environment. In numerous studies, salivary cortisol levels decreased after mild to moderate exercise in a natural environment compared with an urban environment [ 18 ].

Although many studies have observed significant decreases in measured salivary cortisol levels after exposure to natural environments, others have not observed any significant differences in salivary cortisol levels before and after exposure to natural environments [ 17 , 19 ]. However, a key limitation of using cortisol as a biomarker of stress in experimental studies is the fluctuation of cortisol over a 24-h period. Diurnal cortisol levels need to be taken into account in order to make a fair comparison, and most of the literature on exposure to nature and stress have only studied cortisol levels before and after exposure [ 17 ].

Experimental studies focusing on children or youth are sparse [ 20 , 21 ]. One quasi-experimental study conducted in 10–12 year-olds in a school setting examined the influence of natural environments on stress response [ 22 ]. The researchers observed higher tonic vagal tone, a measure of heart rate variability, in natural environments but found no associations with event or phasic vagal tone.

3.1.2. Affective State

Exposure to natural environments has also been studied in relation to the self-reported affective state, or the underlying experience of feeling, emotion or mood. Although study measures vary, studies among adults have generally observed relationships between exposure to natural environments and affective state, with positive associations with positive emotions and negative associations with negative emotions [ 16 , 22 , 23 ]. A study randomly assigned sixty adults to a 50-min walk in either a natural or an urban environment in Palo Alto, California, and found that compared to urban experience, nature experience led to affective benefits (decreased anxiety, rumination, and negative affect, and preservation of positive affect) as well as cognitive benefits (increased working memory performance) [ 23 ]. In a study investigating forest bathing, or shinrin-yoku, researchers found that time spent in forests was associated with a reduction in reported feelings of hostility, depression, and anxiety among adults with acute and chronic stress [ 24 ]. Another study examining walking in different environments observed the largest and most consistent improvements in psychological states associated with forest walks [ 25 ]. Forest bathing may play an important role in health promotion and disease prevention. However, the lack of studies focused on children or youth limits the generalizability of these findings across a wide age range [ 26 ].

3.1.3. Anxiety and Depressive Mood

Exposure to natural environments has been linked with decreases in anxiety and rumination, which are associated with negative mental health outcomes, such as depression and anxiety [ 23 , 27 ]. Nature-based health interventions (NBI) are interventions that aim to engage people in nature-based experiences with the goal of improving health and wellness outcomes [ 28 ]. One study evaluated a wetland NBI in Gloucestershire, UK, that was designed to facilitate engagement with nature as a treatment for individuals diagnosed with anxiety and/or depression. The study found that the wetland site provided a sense of escape from participants’ everyday environments, facilitating relaxation and reductions in stress [ 27 ]. A recent systematic review and meta-analysis found a reduction in depressive mood following short-term exposure to natural environments [ 21 ]. However, the authors noted that the reviewed studies were generally of low quality due to a lack of blinding of study participants and a lack of information on randomization quality among randomized trials.

3.1.4. Cognitive Function

Experimental studies have examined the impact of brief nature experiences and cognition among adults, investigating cognitive function related to exposure to natural environments, and are consistent with the results from studies among school-aged children. A growing number of studies have found that exposure to natural environments compared with urban environments is associated with improved attention, executive function, and perceived restorativeness [ 16 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ]. These studies have found statistically significant associations with positive cognitive outcomes, even after short periods of time spent in natural environments. Additionally, an emerging area of research is virtual reality (VR), using eye-tracking and wearable biomonitoring sensors to measure short-term physiological and cognitive responses to different biophilic indoor environments. These studies have found consistent physiological and cognitive benefits in indoor environments with diverse biophilic design features [ 38 , 39 ].

3.1.5. Brain Activity

Exposure to nature has been associated with alterations in brain activity in the prefrontal cortex, an area of the brain that plays an important role in emotional regulation [ 18 , 19 ]. One experimental study among female university students in Japan investigated physiological and psychological responses to looking at real plants compared with images of the same plants [ 40 ]. Although participants reported feelings of comfort and relaxation after seeing either real plants or images of the same plants, a physiological response was observed only after seeing real plants. Seeing real plants was associated with increased oxy-hemoglobin concentrations in the prefrontal cortex, suggesting that real plants may have physiological benefits for brain activity not replicated by images of plants.

3.1.6. Blood Pressure

Two meta-analyses [ 18 , 41 ] found evidence suggesting that exposure to a natural environment reduced blood pressure. Song et al. [ 18 ] reviewed the research in Japan from 52 studies on the physiological effects of nature therapy and found overwhelming evidence that blood pressure levels decreased when participants were exposed to a natural environment. Decreases in both systolic and diastolic blood pressure levels were observed across young healthy populations, as well as populations with hypertension. This suggests that forest walking may lead to a state of physiological relaxation [ 18 ]. Ideno et al. [ 41 ] conducted another systematic review and meta-analysis to synthesize the effects of forest bathing on blood pressure, including 20 trials involving 732 participants including high-school and college-aged youth. The authors found that both systolic and diastolic blood pressure taken in the forest environment were significantly lower than in non-forest environments [ 41 ].

3.1.7. Immune Function

In Japan, forest bathing has been positively associated with human immune function [ 42 ]. A study was conducted in which subjects experienced a 3-day/2-night bathing trip to forest areas, and blood and urine were sampled on days 2 and 3 of the trip. On days 7 and 30 after the trip, it was found that the mean values of natural killer (NK) cells (which play a major role in the immune system) and NK activity were higher on forest bathing days compared with control days [ 43 ]. This effect persisted for 30 days after the trip. A potential pathway for improved immune function is exposure to phytoncides (a substance emitted by plants and trees to protect themselves from harmful insects and germs), which could decrease stress hormones in the human body and increase NK cell activity. Additionally, the findings indicated that a day trip to a forest park also increased the levels of intracellular anti-cancer proteins [ 43 ].

3.1.8. Postoperative Recovery

While there is limited research on the effect of nature on postoperative recovery, a seminal study by Ulrich [ 4 ] investigated recovery after a cholecystectomy on patients with and without a room with a window view of a natural setting. Patients with a view of a natural setting had shorter hospital stays, received fewer negative evaluative comments in the nurse’s notes section of their charts, and took fewer potent analgesics (e.g., opiates) than those patients whose windows faced a brick building wall [ 4 ]. More recent research has successfully replicated the concept that plants and foliage in the hospital environment may have beneficial impacts on surgical recovery in randomized trials [ 44 , 45 ].

3.2. Observational Studies

Cross-sectional observational studies have shown evidence of positive associations between exposure to nature, higher levels of physical activity, and lower levels of cardiovascular disease. Increasingly, longitudinal observational studies have started to examine the long-term effects of exposure to nature on depression, anxiety, cognitive function, and chronic disease. Below, we summarize the key findings on mental health, physical activity, obesity, sleep, cardiovascular disease, diabetes, cancer, mortality, birth outcomes, asthma and allergies, and immune function.

3.2.1. Mental Health

A recent systematic review found limited evidence suggesting a beneficial association with mental well-being in children and depressive symptoms in adolescents and young adults [ 21 ]. However, access to green space has been linked with improved mental well-being, overall health, cognitive development in children [ 46 ], and lower psychological distress in teens [ 47 ]. A study that examined the restorative benefits associated with frequency of use of different types of green space among US-based students found that students who engaged with green spaces in active ways ≥15 min four or more times per week reported a higher quality of life, better overall mood, and lower perceived stress [ 48 ]. Research in the U.S.-based Growing Up Today Study (GUTS) found that increased exposure to greenness measured around the home was associated with a lower risk of high depressive symptoms cross-sectionally (as measured with the McKnight Risk Factor Survey) and a lower incidence of depression longitudinally [ 49 ]. The investigators observed stronger associations in more densely populated areas and among younger adolescents [ 49 ]. Similarly, a study in four European cities (Barcelona, Spain; Doetinchem, The Netherlands; Kaunas, Lithuania; and Stoke-on-Trent, UK) that evaluated childhood nature exposure and mental health in adulthood showed that adults with low levels of childhood nature exposure had, when compared with adults with high levels of childhood nature exposure, significantly worse mental health, assessed through self-reports of nervousness or depression [ 50 ]. Another study of approximately one million Danes over 28 years of follow-up found that high levels of continuous green space presence during childhood were associated with lower risk of a wide spectrum of psychiatric disorders later in life [ 51 ]. A study based in the UK tracked individuals’ residential trajectories for five consecutive years and showed that individuals who moved to greener areas had better mental health than before moving [ 52 ]. Collectively, these studies suggest that implementation of environmental policies to increase urban green space may have sustainable public health benefits.

Novel research has examined green outdoor settings as potential treatment for mental and behavioral disorders, such as attention-deficit/hyperactivity disorder (ADHD). One study demonstrated associations between green space exposure and improvement in behaviors and symptoms of ADHD and higher standardized test scores [ 46 ]. A recent systematic review found significant evidence for an inverse relationship between green space exposure and emotional and behavioral problems in children and adolescents [ 21 ]. Research has also shown that more and better quality residential green spaces are favorable for children’s well-being [ 53 ] and health-related quality of life [ 54 ]. Furthermore, the quality of green space appears to be more important as children age, as associations between green space quality and well-being are stronger in 12–13 year-olds compared with 4–5 year-olds [ 53 ]. In addition, natural features near schools, including forests, grasslands, and tree canopies, are associated with early childhood development, preschoolers’ improvement in socio-emotional competencies [ 55 ], and a decrease in autism prevalence [ 56 ].

Exposure to nature during adulthood also appears to be important for mental health. A study of 94,879 UK adults indicated a consistent protective effect of greenness on depression risk that was more pronounced among women, participants younger than 60 years, and participants residing in areas with low neighborhood socioeconomic status or high urbanicity [ 57 ]. Other innovative studies are starting to examine quantifiable time of exposure to evaluate the duration of time spent in nature that is associated with mental health benefits. For example, using a nationally representative sample of American adults, Beyer et al. [ 58 ] found that individuals who spent 5–6 or 6–8 h outdoors during weekends had lower odds of being at least mildly depressed, compared with individuals who spent less than 30 min outdoors on weekends. Another study from the UK suggested that lower levels of depression were associated with spending five hours or more weekly in a private garden [ 59 ]. Other studies are focused on uncovering which characteristics of green space are the determinants of mental health benefits. A UK study examined neighborhood bird abundance during the day and found inverse associations with prevalence of depression, anxiety, and stress [ 60 ].

The collective results from these studies suggest that nearby nature is associated with quantifiable mental health benefits, with the potential for lowering the physical and financial costs related to poor mental health. Most of these studies are cross-sectional, and reverse causation is possible. However, researchers are employing novel designs to examine the relationship between green space and mental health. For example, in a study of twins enrolled in the University of Washington Twin Registry, increased greenness was associated with decreased risk of self-reported depression, stress, or anxiety; however, only the results for depression were robust in within-twin pair analyses, suggesting the effect of green space on depression cannot be explained by genetics alone [ 61 ]. Finally, it is important to note that technological advancements have yielded improvements in assessments of exposure to nature and mental health. For instance, one study among adults 18–75 years of age used smartphones equipped with ecological momentary assessment applications to track location, physical activity, and mood for consecutive days, and found positive associations with feeling happy and restored or relaxed within 10 min of exposure to natural outdoor environments [ 62 ]. More novel studies such as these will bolster the evidence behind exposure to nature and mental health among children and/or youth.

3.2.2. Physical Activity

An extensive body of literature documents the impacts of access to green spaces or surrounding greenness on physical activity in children and adults. Proximity to green spaces may promote physical activity by providing a space for walking, running, cycling, and other activities. Although the bulk of the literature is cross-sectional, most studies (in both children and adults) have observed higher levels of physical activity in areas with more access to green space. For example, a study in Bristol, UK, evaluated associations between accessibility to green space and the odds of respondents achieving a recommended 30 min or more of moderate activity five times a week; respondents who lived closest to the type of green space classified as a formal park were more likely to achieve the physical activity recommendation [ 63 ]. Another study of adults in the UK found that people living in greenest compared with least-green areas were more likely to meet recommended daily physical activity guidelines [ 64 ]. However, another UK-based study did not find associations between road distance to nearest green space, number of green spaces, area of green space within a 2-km radius of residence, or green space quality and physical activity [ 65 ].

Almanza et al. [ 66 ] used GPS and accelerometry data among 208 children in California and found that greenness was associated with higher odds of moderate to vigorous physical activity, when comparing those in the 90th and 10th percentiles of greenness. Additionally, they found that children with >20 min daily green space exposure had nearly 5 times the daily rate of moderate to vigorous physical activity compared with those with nearly zero daily exposure [ 66 ]. Another study of Australian children illustrated that time spent outdoors at baseline positively predicted the amount of physical activity three years later [ 67 ]. In a review of youth health outcomes related to exercising in nature (i.e., “green exercise”), the results of fourteen studies (5 in the UK, 5 in the U.S., 2 in Australia, and 1 in Japan) indicated little evidence that green exercise is more beneficial than physical activity conducted in other locations, although any physical activity was beneficial across settings [ 68 ].

More recent studies have employed more sophisticated study designs to determine whether exposure to greenness increases physical activity. In studies that objectively assessed physical activity via accelerometers, individuals exposed to more greenness tended to be more physically active. For example, in a study of 15-year-olds in Germany, increases in greenness around the home address were associated with increased moderate-to-vigorous physical activity among youth in rural, but not urban, areas [ 69 ]. Another study of children in the UK evaluated momentary green space exposure based on GPS-derived location and contemporaneous physical activity measured by an accelerometer and found higher odds of physical activity in green space (versus outdoor non-green space) for boys but not girls [ 70 ].

3.2.3. Obesity

Green space may influence overweight or obesity through a physical activity pathway [ 71 ]. Some studies have shown that exposure to green space is associated with lower rates of obesity in children [ 67 ] and adults [ 72 ]; however, the results are conflicting. As with physical activity, many early studies were cross-sectional, and findings were more mixed for children than for adults. Some studies reported U-shaped associations with obesity [ 73 ], while other studies reported no association after adjustment for respondent characteristics [ 63 ] or neighborhood socioeconomic status [ 74 ]. Some studies demonstrate effect modification by gender [ 72 ]. Further, one cross-sectional UK-based study found that living in the greenest areas was associated with an increase in risk of being overweight and obese [ 75 ].

In one study of U.S. children, increasing greenness was associated with lower BMI z-scores and lower odds of increasing BMI z-scores between two follow-up times [ 76 ]. Another study of schoolchildren in Spain found that greenness and forest proximity were associated with lower prevalence of being overweight or obese [ 77 ]. One study found that street tree density was associated with lower obesity prevalence in New York City (U.S.) children; however, no association was found with park areas [ 78 ]. In an Australian study, the prevalence of being overweight was 27–41% lower in girls and boys who spent more time outdoors at the study baseline than those who spent less time outdoors [ 67 ]. Another study found that greenness was associated with decreased risk of being overweight but only among those in areas with a greater population density [ 79 ].

3.2.4. Sleep

Exposure to green space may influence sleep duration and quality. For instance, surrounding greenness may serve as a buffer for noise, which would disturb sleep. To date, only a handful of studies have examined these associations, and to our knowledge, even fewer have explored this association in children. A recent systematic review provided evidence of an association between green space exposure and improved sleep quality among adults [ 80 ]. A study of Australian adults who lived in areas with greater than 80% green space demonstrated lower risk of short sleep duration, even after adjustment for other predictors of sleep [ 81 ]. Among U.S. adults participating in the Behavioral Risk Factor Surveillance System survey, natural amenities (e.g., green space, lakes, and oceans) were associated with lower reporting of insufficient sleep, and greenness was especially protective among men and individuals over 65 years of age [ 82 ]. In the Survey of Health in Wisconsin Study, increased tree canopy at the Census block group level was associated with lower odds of short sleep duration on weekdays and suggestive of an association with lower odds of short sleep duration on weekends, although there was no association between tree canopy and self-reported sleep quality [ 83 ]. A nationally representative study of Australian and German children and adolescents found no evidence of significant associations between residential green space and insufficient sleep or poor sleep quality [ 84 ].

3.2.5. Cardiovascular Disease

Exposure to green space may affect levels of physical activity, stress, and high blood pressure that drive cardiovascular disease risk. Recent reviews have found consistent evidence that exposure to residential green space is associated with decreased cardiovascular disease incidence [ 85 , 86 ]. Participants living in areas with lower greenness have higher levels of mortality following a stroke [ 87 ], higher cardiovascular disease mortality [ 88 , 89 ], and higher coronary heart disease [ 90 ]. A study from the UK found that associations between exposure to nature and cardiovascular outcomes differed by gender, where male cardiovascular disease and respiratory disease mortality rates decreased with increasing green space, and no associations were found for women [ 88 ]. Furthermore, the relationships between exposure to greenspace and cardiovascular outcomes may be modified by urbanicity. A recent Australian study showed significantly lower odds of high blood pressure among adults in an urban population when reported green space visits were an average of 30 min or more [ 91 ].

3.2.6. Diabetes

Although limited, evidence regarding the association between green space and type 2 diabetes highlights green space as a possible route for diabetes prevention. There are a few cross-sectional studies that have reported that green space is inversely related to type 2 diabetes among adults [ 92 , 93 ]. Few studies have examined the relationship between green space and diabetes in children. Cross-sectional studies of children found inverse associations between time spent in green spaces and fasting blood glucose levels [ 77 ] and insulin resistance [ 94 ]. A recent longitudinal study conducted on US children found no associations between residential exposure to green space and insulin resistance [ 95 ].

3.2.7. Cancer

Research on the link between green space and cancer is limited and may vary depending on the type of cancer. A recent case-control study examined whether residential green space exposure was related to prostate cancer incidence and found that higher residential greenness was associated with lower risk of prostate cancer [ 96 ], and a separate study of U.S. men demonstrated inverse associations between neighborhood greenness and lethal prostate cancer [ 97 ]. Another study examined the association between green space and several cancer types and found that green space was protective for mouth, throat, and non-melanoma skin cancers but was not associated with colorectal cancer [ 98 ]. A U.S.-based nationwide study of nurses found that residential greenness was inversely associated with breast cancer mortality [ 99 ]. Conversely, another systematic review that evaluated evidence on the association between residential green spaces and lung cancer mortality found no benefits of residential greenness [ 85 ].

3.2.8. Mortality

Many early mortality studies relied on cross-sectional data and could not estimate nature exposure over time [ 100 ], whereas others could not account for important potential confounding by race/ethnicity, individual-level smoking, and area-level socioeconomic factors, such as median home value [ 101 , 102 ]. A UK-wide ecological study found that all-cause mortality was higher in greener cities [ 89 ]. An analysis of greenness and mortality in male and female stroke survivors living in Boston (U.S.) found that greater exposure to greenness was associated with higher survival rates [ 87 ]. Another U.S.-based nationwide study of nurses found a consistent protective relationship between residential greenness and non-accidental mortality [ 103 ]. The greenness–mortality relationship was explained primarily by a mental health pathway, and the relationship was strongest among those who had high levels of physical activity [ 103 ]. A study of 4.2 million adults in the Swiss National Cohort assessed the relationship between residential greenness and mortality, while mutually considering socioeconomic status, air pollution, and transportation noise exposure, and found that higher exposure to green space was associated with lower rates of death from natural causes, respiratory disease, and cardiovascular disease [ 104 ]. Protective effects were stronger in younger individuals and in women and, for most outcomes, in urban (versus rural) and in the highest (versus lowest) socioeconomic quartile. Effect estimates did not change after adjustment for air pollution and transportation noise, suggesting that the protective effect of exposure to nature persists in the absence of pollution sources. Finally, a systematic review and meta-analysis of cohort studies on green space and mortality assessed findings from nine studies, comprising 8.3 million individuals from seven countries across the globe [ 105 ]. Seven of the nine studies demonstrated an inverse relationship between green space exposure and mortality, and the authors recommended wide-scale interventions to increase and manage green spaces in order to improve public health outcomes.

3.2.9. Birth Outcomes

The relationship between exposure to nature and birth outcomes has been studied extensively in analyses across multiple countries. Findings of positive associations between greenness and birth weight and decreased risk of low birth weight are consistent, with stronger associations observed among those of a lower socioeconomic status [ 106 ]. Banay et al. [ 107 ] reviewed studies that examined the association between greenness and maternal or infant health. While the majority of studies were cross-sectional, many studies found evidence for positive associations between greenness and birth weight. Fewer studies demonstrated consistent evidence for an association between greenness and gestational age, preeclampsia, or gestational diabetes. These studies also found that effects were stronger among those of a lower socioeconomic status. A more recent review highlighted the evolving literature showing that higher levels of residential greenness were associated with lower risk of preterm birth, low birth weight, and small gestational-age babies [ 108 ]. Akaraci et al. [ 109 ] conducted a systematic review and meta-analysis of 37 studies on residential green and blue spaces and pregnancy outcomes. Increases in residential greenness were associated with higher birthweight and lower odds of being small for gestational age; however, no significant associations between residential blue spaces and birth outcomes were found.

3.2.10. Asthma/Allergies

Several studies have examined the relationship between greenspace and atopic outcomes, including asthma and allergies. Mechanistically, trees and plants are a source of allergens and respiratory irritants [ 110 ]. However, the biodiversity created by green space could be protective against inflammatory conditions [ 111 , 112 ]. The literature reflects these contrasting hypotheses with mixed findings. Some studies have shown no association between the normalized difference vegetation index (NDVI) or tree canopy cover and asthma [ 113 ], while other studies have shown that living close to forests and parks was positively associated with allergic rhinoconjunctivitis and asthma [ 77 ]. Another study of greenspace and allergies in Germany demonstrated positive associations in urban areas and negative associations in rural areas [ 114 ]. The same investigators examined data from seven birth cohorts across Sweden, Australia, the Netherlands, Canada, and Germany and found that the relationship between residential NDVI and allergic disease was positive in some countries and negative in others [ 115 ]. A study in Spain found proximity to residential greenness to be protective of bronchitis in the Mediterranean region of Spain and protective of wheezing for children in the Euro-Siberian region of Spain [ 116 ]. One study conducted in China examined the relationship between exposure to greenness and parks and asthma and allergies among middle-school-aged children [ 117 ]. The researchers observed no associations between residential greenness exposure and self-reported doctor-diagnosed asthma, pneumonia, rhinitis, and eczema; however, living farther away from a park was associated with decreased odds of currently or ever having asthma. In sum, the relationship between exposure to nature and asthma and allergies is inconsistent, with associations varying in magnitude and direction by geography. One review of fourteen studies suggested an association between early life exposure to urban greenness and allergic respiratory diseases (e.g., asthma, bronchitis, allergic symptoms) in childhood; however, there were inconsistencies among study results, likely due to variability in study design, exposure assessment, outcome ascertainment, and geographic region [ 118 ].

3.3. Natural Experiments/Randomized Controlled Trials of Chronic Outcomes

Beyond smaller experimental studies of short-term outcomes and observational epidemiologic studies of chronic outcomes, there are a few natural experiments and randomized controlled trials that add substantial evidence to the relationship between exposure to nature and health. These quasi-experimental and randomized trials have lower potential for confounding bias to explain observed associations between nature and health. One important study capitalized on a natural experiment when an invasive tree pest, the emerald ash borer, killed over 100 million ash trees in the Midwestern United States [ 119 ]. The investigators found that living in a county infested with the emerald ash borer was associated with a 41% increased risk of cardiovascular disease, and these results were only consistent when looking in metropolitan areas where they could adjust for socioeconomic status. Another innovative study examined the greening of vacant lots in Philadelphia [ 120 ]. This citywide study used a three-arm randomized trial approach to randomize 110 vacant lots to either no intervention, cleaning but no greening, or cleaning and greening. The study found that those living around lots that were greened had substantial decreases in reports of depression, poor mental health, and feelings of worthlessness compared with lots that had no intervention. Those living around lots that were cleaned but not greened showed no difference compared with no intervention. Another ongoing longitudinal study in Sydney, Australia, is evaluating the effects of large-scale investment in green space (e.g., public access points, advertising billboards, walking and cycle tracks, BBQ stations, and children’s playgrounds) on physical activity, mental health, and cardiometabolic outcomes [ 121 ]. This natural experiment utilizes proximity to different areas of the Western Sydney Parklands to define treatment and control groups.

Looking to the future, there are a few randomized trials in progress that will provide fundamental evidence to understand whether adding green pace to cities benefits health. The Green Heart Project in Louisville, Kentucky, will assess risk of diabetes and heart disease, stress levels, and the strength of social ties in 700 participants [ 122 ]. The team will take baseline measurements of air pollution levels and will plant as many as 8000 trees, plants, and shrubs throughout Louisville neighborhoods to create an urban ecosystem that promotes physical activity while simultaneously decreasing noise, stress, and air pollution. During five years of follow-up, participants will receive annual check-ups to evaluate how the increasing greenery has affected their physical and mental health and social ties. A second randomized trial is the ‘Productive Green Infrastructure for Post-industrial Urban Regeneration’ or ProGIreg, a multi-city study examining the potential effects of green infrastructure [ 123 ]. This project is based in Dortmund (Germany), Turin (Italy), Zagreb (Croatia) and Ningbo (China) where Living Labs are hosted and nature-based solutions are developed, tested, and implemented. Although health is not the main focus of this study, researchers are hoping to incorporate health metrics into the study design to examine pre- and post-intervention outcome data. Collectively, these randomized trials, natural experiments, and pre-post study designs will establish crucial data on whether interventions to incorporate nature into cities can measurably improve health.

3.4. Effect Modification/Susceptible Populations

Inequitable distribution of green spaces could exacerbate health inequalities if people who are already at greater health risks (e.g., people with lower socioeconomic status) have limited access. Many studies have indicated that disadvantaged populations have decreased access to nature and greenspace [ 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 , 132 ]. At the same time, evidence suggests that exposure to nature disproportionately benefits disadvantaged populations, a phenomenon known as the equigenic effect of green space, which upends the expected association between lower socioeconomic status and greater risk of poor health outcomes [ 133 ]. Based on the theory of equigenic environments, one study showed that populations exposed to the greenest environments also had the lowest levels of health inequality related to income deprivation, suggesting that green space might be an important factor in reducing socioeconomic health disparities [ 89 ]. A review of 90 studies on green space and health outcomes demonstrated that individuals of lower socioeconomic status showed more beneficial effects than those of higher socioeconomic status; the authors found no significant differences in the protective effects of green space on health outcomes among different racial/ethnic groups [ 134 ]. The evidence is inconsistent, and more work is needed to elucidate potential mechanisms.

Conversely, improvements in access to green space may lead to “green gentrification,” an increase in property values that displaces low-income residents from their neighborhoods [ 129 , 135 , 136 , 137 ]. This process needs to be studied and understood so that its adverse effects can be prevented. Other cultural and contextual factors may affect nature preferences and experience of nature. For instance, there is evidence that the legacy of forced labor, lynching, and other violence may evoke deeply disturbing associations with trees, fields, and forests among some African Americans [ 138 , 139 ]. Similarly, some people may prefer open fields for sports, while others prefer picnic facilities for socializing.

4. Discussion

The purpose of this narrative review was to summarize recent experimental and observational literature on associations between nature exposure and health in adults and children/youth. While some associations between nature and health outcomes are well-studied, our review highlights the lack of studies, particularly experimental, among child/youth and other susceptible populations. We found evidence for associations between exposure to nature and improved cognitive function, brain activity, blood pressure, mental health, physical activity, and sleep. Results from experimental studies indicated protective effects of nature exposure on mental health and cognitive function. Cross-sectional observational studies provide evidence of positive associations between nature exposure, higher levels of physical activity and lower levels of cardiovascular disease. Observational studies, natural experiments, and randomized controlled trials are starting to assess the longitudinal effects of exposure to nature on depression, anxiety, cognitive function, chronic disease, and other health outcomes. Our review synthesizes recent literature, primarily from Western countries; thus, a limitation of this review is that we may not have captured all relevant literature from outside our publication range or across all geographic regions.

4.1. Data Gaps and Limitations

There are several limitations in the literature on exposure to nature and health. First, definitions of nature are inconsistent across studies. Further, the impacts of the quality of green space, duration of exposure to nature, frequency of exposure, or type of nature exposure on health outcomes are not well understood. Second, methods for measuring exposure to nature (e.g., percentage of residential greenness versus distance to the closest park) or defining the relevant geographic area of exposure (e.g., 500 m away from our home versus 1 km or 10 km) are inconsistent [ 140 , 141 ]. We must also develop methods to elucidate thresholds for dose and duration of nature exposure to achieve a given health effect. Although some studies have determined potential estimates of relevant doses [ 142 ], this area of research is nascent. In addition, standard approaches towards nature exposure assessment do not capture the variations in how people experience nature differentially (e.g., smell, touch, etc.) and have low reproducibility across studies (e.g., inconsistent land-use measures). Third, critical time windows of exposure during the life course that might have the greatest impact on health are also understudied (e.g., early life exposure, childhood exposure). Fourth, mechanistic pathways are understudied. Further, the dynamic relationship between green space, air pollution, noise, temperature, and neighborhood walkability also warrant further exploration, as these factors could be both mediators or moderators of the nature–health relationship [ 143 , 144 ]. We also know little about the potential harms of exposure to nature, most commonly observed in studies of asthma and allergies.

4.2. Future Directions

There are ample promising future directions for nature and health research. First, future research should employ rigorous study designs (e.g., longitudinal studies, randomized controlled trials) and investigate the underlying mechanisms of observed associations between exposure to nature and health outcomes. Although cross-sectional studies dominate the literature, there is increasing evidence emerging from prospective studies, which are essential to investigating causal relationships [ 108 ]. Novel designs, such as quasi-experimental studies and randomized trials, will provide further detail on how nature influences health [ 119 ]. Furthermore, studies should thoroughly evaluate potential biases, such as confounding by socioeconomic status, that may threaten the validity of studies on nature and health. Researchers should rigorously examine factors that may modify the effects of exposure to nature (e.g., socioeconomic status, gender, or race) to determine the subpopulations that might benefit most from exposure to nature. A life-course approach to examining associations between green space and health is also essential. We need to better understand vulnerable time windows in the early life-course where access or exposure to nature may have stronger impacts on health than in other time periods. Similarly, additional research assessing dose-response relationships (e.g., duration of time in nature or quantity of vegetation) is crucial to determine the minimum amount of exposure to green space needed to yield health benefits or if the relevant dosage varies across the life-course or across different countries/settings [ 142 ].

Second, future studies should make use of novel datasets and computational approaches that may provide rapid advances in exposure assessment. Emergence of advanced satellite and aerial photos combined with machine learning to develop tree canopy measures and other more specific metrics of nature provide information on specific species on the ground. Google Street View and other ubiquitous geocoded imagery, when combined with machine learning, also provide scalable approaches to estimate specific natural features from the on the ground perspective as human beings experience them [ 145 ]. Combined with geocoded residential addresses or GPS data and health or behavioral data, these approaches may unveil novel insights on how nature exposure affects health. Leveraging smartphones with GPS and accelerometry enable fine-scale information on exposure and physical activity. Ecological momentary assessment (EMA) or micro-surveys administered through smartphones can be used to ask about processes for how and why people interact with nature [ 62 ]. EMA can also be applied to estimate mental health outcomes in real time, and these responses can be geo-tagged and linked to spatial measures of natural environments. In addition, consumer wearable devices (e.g., FitBit) provide objective information on physical activity patterns, heart rate, sleep, and other biometrics down to the second level [ 146 ]. These data will prove crucial to better understand the behavioral mechanisms through which nature exposure impacts health. We should also capitalize on geo-located social media data (Flickr, Twitter, Facebook) and other data sources to understand exposure to nature [ 147 ]. Innovative metrics of mental health, such as skin conductivity, cortisol (stress), heart rate variability, brain activity through EEG, and functional MRI, can also provide information on stress processes when individuals encounter natural environments [ 148 ]. Such measures of nature exposure and time spent in nature should be incorporated into large federal data collection efforts, such as the Behavioral Risk Factor Surveillance System (BRFSS), National Health Interview Survey (NHIS), and National Health and Nutrition Examination Survey (NHANES) in the United States or the Health Survey for England (HSE) in the United Kingdom. These recommendations cannot be accomplished without also considering the impacts climate change is currently having and will have on exposures to nature, and how climate change may alter the relationship individuals have with nature.

Third, future studies on nature and mental health should focus more on positive health—happiness, purpose, flourishing—instead of just the absence of negative mental health outcomes. Further, more research is required on natural water features, or blue space [ 149 ], as well as other natural environments.

Fourth, the overwhelming majority of research on nature and health is on urban study populations in North America, Europe, and Australia. Researchers should also focus on different geographic areas, low-income and middle-income settings, and vulnerable or historically marginalized populations where nature benefits might be greatest. Researchers should also work together with communities as they conduct their research to ensure their work addresses the needs of community members.

Finally, we must also recognize the potential unintended consequences of adding green infrastructure in cities. Adding green amenities to cities may entice high-income populations, and the resulting increased property values shape a new conundrum, embodied in the exclusion and displacement associated with so-called green gentrification [ 135 ]. Results from this type of research should also be considered for policies, urban planning, and designing cities.

5. Conclusions

The purpose of this review was to examine recent literature on exposure to nature and health, highlighting studies on children and youth where possible. We assessed the strength of evidence from experimental and observational studies and found evidence for associations between exposure to nature and improved cognitive function, brain activity, blood pressure, mental health, physical activity, and sleep. Evidence from experimental studies suggested protective effects of exposure to natural environments on mental health outcomes and cognitive function. Cross-sectional observational studies provide evidence of positive associations between exposure to nature, higher levels of physical activity and lower levels of cardiovascular disease. Longitudinal observational studies are starting to assess the long-term effects of exposure to nature on depression, anxiety, cognitive function, and chronic disease. Limitations and gaps in studies of nature exposure and health include inconsistent measures of exposure to nature, knowledge of the impacts of the type and quality of green space, and the health effects of the duration and frequency of exposure among different populations (e.g., adults, children, historically marginalized). Future research should incorporate more rigorous study designs, investigate the underlying mechanisms of the association between green space and health, advance exposure assessment, and evaluate sensitive periods throughout the life-course.

Author Contributions

Conceptualization, M.P.J., N.V.D., J.E.H. and P.J.; methodology, M.P.J., N.V.D., J.E.H. and P.J.; writing—original draft preparation, M.P.J., N.V.D., E.G.E., J.E.S., G.E.W., J.E.H. and P.J.; writing—review and editing, M.P.J., N.V.D., E.G.E., J.E.S., G.E.W., J.E.H. and P.J.; supervision, J.E.H. and P.J.; project administration, N.V.D.; funding acquisition, P.J. All authors have read and agreed to the published version of the manuscript.

This research was funded by The National Geographic Society, and NIH grants R00 CA201542, R01 HL150119, T32 {"type":"entrez-nucleotide","attrs":{"text":"ES007069","term_id":"164015192","term_text":"ES007069"}} ES007069 , K99 AG066949, R01 ES028712 and P30 ES000002.

Institutional Review Board Statement

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Conflicts of interest.

The authors declare no conflict of interest.

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Pathologic maternal and neonatal outcomes associated with programmed embryo transfer

• Published: 27 March 2024

Cite this article

• Kirk P. Conrad   ORCID: orcid.org/0000-0003-4748-5765 1 ,
• Frauke von Versen-Höynck 2 &
• Valerie L. Baker 3  

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In this first of two companion papers, we critically review the evidence recently published in the primary literature, which addresses adverse maternal and neonatal pregnancy outcomes associated with programmed embryo transfer cycles. We next consider whether these pathological pregnancy outcomes might be attributable to traditional risk factors, unknown parental factors, embryo culture, culture duration, or cryopreservation. Finally, in the second companion article, we explore potential etiologies and suggest strategies for prevention.

Comprehensive review of primary literature.

The preponderance of retrospective and prospective observational studies suggests that increased risk for hypertensive disorders of pregnancy (HDP) and preeclampsia in assisted reproduction involving autologous embryo transfer is associated with programmed cycles. For autologous frozen embryo transfer (FET) and singleton live births, the risk of developing HDP and preeclampsia, respectively, was less for true or modified natural and stimulated cycles relative to programmed cycles: OR 0.63 [95% CI (0.57–0.070)] and 0.44 [95% CI (0.40–0.50)]. Though data are limited, the classification of preeclampsia associated with programmed autologous FET was predominantly late-onset or term disease. Other adverse pregnancy outcomes associated with autologous FET, especially programmed cycles, included increased prevalence of large for gestational age infants and macrosomia, as well as higher birth weights. In one large registry study, FET was associated with fetal overgrowth of a symmetrical nature. Postterm birth and placenta accreta not associated with prior cesarean section, uterine surgery, or concurrent placenta previa were also associated with autologous FET, particularly programmed cycles. The heightened risk of these pathologic pregnancy outcomes in programmed autologous FET does not appear to be attributable to traditional risk factors, unknown parental factors, embryo culture, culture duration, or cryopreservation, although the latter may contribute a modest degree of increased risk for fetal overgrowth and perhaps HDP and preeclampsia in FET irrespective of the endometrial preparation.

Conclusions

Programmed autologous FET is associated with an increased risk of several, seemingly diverse, pathologic pregnancy outcomes including HDP, preeclampsia, fetal overgrowth, postterm birth, and placenta accreta. Though the greater risk for preeclampsia specifically associated with programmed autologous FET appears to be well established, further research is needed to substantiate the limited data currently available suggesting that the classification of preeclampsia involved is predominately late-onset or term. If substantiated, then this knowledge could provide insight into placental pathogenesis, which has been proposed to differ between early- and late-onset or term preeclampsia (see companion paper for a discussion of potential mechanisms). If a higher prevalence of preeclampsia with severe features as suggested by some studies is corroborated in future investigations, then the danger to maternal and fetal/neonatal health is considerably greater with severe disease, thus increasing the urgency to find preventative measures. Presupposing significant overlap of these diverse pathologic pregnancy outcomes within subjects who conceive by programmed embryo transfer, there may be common etiologies.

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Pathologic maternal and neonatal outcomes associated with programmed embryo transfer: potential etiologies and strategies for prevention

Kirk P. Conrad, Frauke von Versen-Höynck & Valerie L. Baker

Natural vs. programmed cycles for frozen embryo transfer: study protocol for an investigator-initiated, randomized, controlled, multicenter clinical trial

Sheriza Baksh, Anne Casper, … NatPro Study Group

The impact of endometrial preparation for frozen embryo transfer on maternal and neonatal outcomes: a review

Jacqueline C. Lee, Martina L. Badell & Jennifer F. Kawwass

Data availability

Not applicable.

Because the use of true or modified NC was not consistently specified in the literature, we combined the two approaches for discussion.

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Investigations by KPC, VLB, and FVH were supported, in part, by a grant from the National Institutes of Health P01 HD065647: KPC Program Director/Principal Investigator, University of Florida; VLB Principal Investigator, Stanford University; FVH was also underwritten by a grant from the German Research Foundation DFG project number 273703678. VLB is currently supported by funding from the National Institutes of Health R01 HD100341 to study frozen embryo transfer protocols and FVH by DFG project number 507276351 to study decidualization.

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Kirk P. Conrad

Department of Obstetrics, Gynecology, and Reproductive Medicine, Division of Gynecologic Endocrinology and Reproductive Medicine, Hannover Medical School, Hannover, Germany

Frauke von Versen-Höynck

Division of Reproductive Endocrinology and Infertility, Department of Gynecology and Obstetrics, Johns Hopkins University School of Medicine, Baltimore, Lutherville, MD, USA

Valerie L. Baker

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KPC conceived and drafted the review; FVH prepared Figs. 1 and 2 ; KPC, FVH, and VLB revised, edited, and approved the final version.

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Conrad, K.P., von Versen-Höynck, F. & Baker, V.L. Pathologic maternal and neonatal outcomes associated with programmed embryo transfer. J Assist Reprod Genet (2024). https://doi.org/10.1007/s10815-024-03041-9

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Received : 20 November 2023

Accepted : 21 January 2024

Published : 27 March 2024

DOI : https://doi.org/10.1007/s10815-024-03041-9

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