According to the World Economic Forum (2020) Global Risks Report, failure to mitigate and adapt to climate change presents the greatest risk to the global economy in terms of severity of impact. Meanwhile, extreme weather – which is exacerbated by climate change (National Academies of Sciences, Engineering, and Medicine, 2016) – is listed as the risk most likely to damage the economy. Identifying and implementing robust climate change adaptation approaches that are cost-effective and build resilience across a range of potential future climates is therefore critical. The prevailing approach across the world has involved a mix of direct, engineered (or ‘grey’) interventions such as sea walls, levees or irrigation infrastructure, and indirect (or ‘soft’) interventions such as early warning systems (Enríquez-de-Salamanca et al., 2017). However, there is widespread recognition that nature-based (or ‘green’) solutions (NbS) can complement these approaches in both rural and urban contexts (Global Commission on Adaptation, 2019; Hobbie & Grimm, 2020; Royal Society, 2014).
A specific type of NbS targeting human adaptation to climate change is widely referred to as ecosystem-based adaptation (EbA). This is defined by the Convention on Biological Diversity (CBD) as “the use of biodiversity and ecosystem services … to help people adapt to the adverse effects of climate change” (Secretariat of the Convention on Biological Diversity, 2009). Examples include: protecting natural wetlands and forests in upper catchments to reduce the impacts of flooding downstream; restoring mangroves and salt marshes to protect communities and infrastructure from storm surges and to reduce coastal erosion; and planting trees amongst crops or crops within forest to maintain or even enhance yields in drier, more variable climates. For specific examples of NbS for climate change adaptation, or EbA, see Box 1 (adapted from Seddon et al., 2020). EbA is often described as an alternative to ‘grey’ engineering. However, in reality, there is a spectrum of interventions, some including components of both (i.e., hybrid or “grey–green” approaches) (Browder et al., 2019; Royal Society, 2014). Reports from projects implemented by non-governmental organizations and United Nations (UN) institutions suggest that EbA can provide low-risk, low-cost protection from a range of climatic impacts, whilst also delivering other vital ecosystem services (e.g., Reid et al., 2019; Rizvi, 2014; Osti et al., unpublished data). However, although project-specific accounts of the benefits of EbA are increasingly backed up by more systematic research, there is a need for robust scientific synthesis (Seddon et al., 2020).
Box 1. Examples of nature-based solutions for climate change adaptation (taken from Seddon et al., 2020 with permission of Royal Society Publishing).
Protection from soil erosion
Ethiopia: Farmer-managed natural regeneration of 2728 ha of degraded native forests with living tree stumps in Humbo reduced soil erosion and flash flooding and increased groundwater recharge, which was associated with higher crop productivity. In 2006–2036, the project will remove an estimated ~870,000 tonnes of CO2 equivalent, while diversifying livelihoods (Brown et al., 2011).
China: A combination of afforestation, reforestation and conservation of existing natural forests over 25 years in the Poyang Lake basin halved heavy soil erosion while increasing net carbon sequestration five-fold and net income for local farmers six-fold (Huang et al., 2012). Meanwhile, restoration of natural herbaceous and shrubland vegetation on the Loess Plateau reduced soil erosion to a comparable or significantly greater extent than low-diversity tree plantations across a range of soil erosion indices. Compared to afforested slopes, these naturally re-vegetated slopes also had 1.3–2.0 times higher soil water content (Jia et al., 2017).
Protection from inland flooding
Europe: Restoration of all but one of six rivers reduced flood damage and was associated with increased agricultural production, carbon sequestration and recreation, with a net societal economic benefit over unrestored rivers of €1400 ± 600. Interventions included floodplain re-wetting, restoration of riparian vegetation, assisting upstream fish migration and the re-meandering and re-connection of channels (Vermaat et al., 2016).
Canada: Reforestation in the headwaters of a river basin significantly reduced peak stream flows compared to an adjacent deforested basin, offering greater protection against flooding during spring snow melt (Buttle, 2011).
USA: Natural regeneration of mixed-species hardwood watersheds following forest clear-cutting reduced flood risk in lowland areas, reducing stream flows during periods of high precipitation by >104 L/ha/day (Kelly et al., 2016).
Buffering natural resources against drier and more variable climates
Panama: Agroforestry systems yield up to 21% higher economic returns than farm mosaic approaches (i.e., where trees and crops are on separate parcels), including under a climate change scenario of more frequent droughts, in models that account for market and climate uncertainty (Paul et al., 2017).
Europe: Agroforestry has reduced erosion, increased soil fertility, increased precipitation and reduced temperatures, with greatest effects in hotter, drier regions such as the Mediterranean basin (which is suffering from soil damage through increasing aridity under climate change) (Torralba et al., 2016).
Protection from coastal hazards and sea-level rise
Global: Natural coastal habitats significantly reduce wave heights, with coral reefs and salt marshes being most effective, causing a reduction of 70%, followed by seagrass and kelp beds (36%) and mangroves (31%). Across 52 sites harnessing these habitats in coastal defence projects, nature-based solutions were two to five times more cost-effective at lowering wave heights and at increasing water depths compared to engineered structures (Narayan et al., 2016). Globally, mangroves protects 15 million people from flooding every year and provide over US$65 billion in flood protection services (Menendez et al., 2020).
Gulf of Mexico: Construction of ‘living shorelines’ by aiding the natural recruitment of oyster reefs can reduce vegetation retreat by 40% compared to unprotected sites, stabilizing the shoreline from the effects of waves and erosion and increasing the abundance and diversity of economically important species (Scyphers et al., 2011).
Moderating urban heatwaves and heat island effects
Global: Green spaces are on average 0.94°C cooler in the day than urban spaces, with stronger effects the larger the green space, according to a meta-analysis of 47 studies comparing the cooling effects of green spaces in cities (parks, areas with trees) with those of purely urban areas (Bowler et al., 2010).
Managing storm-water and flooding in urban areas
Italy: The establishment of wetlands and green recreational space has been effective at reducing flood risks, with a 10% higher reduction of downstream flooding and 7.5% higher reduction of peak flow compared to potential grey infrastructure alternatives. Nature-based solutions also outperform grey infrastructure in terms of water purification and provide greater social-ecological benefits such as recreation and habitat for biodiversity (Liquete et al., 2016).
2. Nature-based solutions for adaptation in the Paris Agreement
As the evidence base for the efficacy of NbS strengthens, so ecosystems are receiving attention in international climate change policy fora. Of particular importance, the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC) recognizes the importance of ecosystems for mitigation and adaptation. It calls on all Parties to acknowledge “the importance of the conservation and enhancement, as appropriate, of sinks and reservoirs of the greenhouse gases,” and to “note the importance of ensuring the integrity of all ecosystems, including oceans, and the protection of biodiversity.” It then includes in its Articles several references to ecosystems, forests and natural resources (Seddon et al., 2019a). For example, Article 5.2 encourages Parties to adopt “…policy approaches and positive incentives for activities relating to reducing emissions from deforestation and forest degradation, and the role of conservation and sustainable management of forests and enhancement of forest carbon stocks in developing nations; and alternative policy approaches, such as joint mitigation and adaptation approaches for the integral and sustainable management of forests, while reaffirming the importance of incentivizing, as appropriate, non-carbon benefits associated with such approaches” (UNFCCC, 2016).
In order to determine the extent to which this has translated into high-level national intent, we conducted a comparative analysis of the prominence of NbS in the Nationally Determined Contributions (NDCs) that were submitted to the UNFCCC by signatories of the Paris Agreement, with a particular focus on the those 142 NDCs that included adaptation components. Full details of our methodology can be found in the Supplementary Materials, while all of the data are available to explore on an interactive web platform, the Nature-based Solutions Policy Platform (www.nbspolicyplatform.org). We focused on the NDCs, rather than other policy documents (e.g., National Adaptation Plans), because the Paris Agreement has considerable political momentum, meaning that NDC targets are often in the limelight and under scrutiny. Furthermore, unlike any other policy processes, the Agreement has an inbuilt ratchet mechanism for increasing ambition: every 5 years, progress towards targets set out in the NDCs must be reported on, monitored and compared with other nations. We focused on adaptation because this has not been fully addressed in prior research on NbS in the NDCs. Instead, most studies to date have examined the extent to which nations have incorporated forestry, agriculture and/or forest landscape restoration (FLR) for the purpose of increasing sinks and reducing sources of greenhouse gas emissions (Forsell et al., 2016; Grassi et al., 2017).
In our detailed analysis of the text of the adaptation components of the NDCs, we make a distinction between nature-based ‘visions’, ‘actions’ and ‘targets’ for NbS. A vision was defined as a high-level pledge or statement of recognition of the importance of NbS for adaptation. An action was defined as tangible, locally relevant action or intervention in a particular habitat or the development/implementation of a specific and relevant policy or process that is being implemented or planned for. We considered an action to be broadly ‘nature-based’ if it referred to the protection, restoration or management of ecosystems, including assisted natural regeneration, reforestation and afforestation. We defined a target as either a time-bound or quantitative target linked to an adaptation action that could, in theory, be tracked over time. We make a distinction between nature-based interventions aimed at delivering direct positive adaptation outcomes and other socioeconomic benefits through ecosystem services (i.e., ecosystem-based adaptation (EbA)) and those aimed at delivering positive outcomes for species or habitats (which, for simplicity, we refer to as conservation). For full details, see Supplementary Materials.