University of Oxford
Biodiversity – the diversity of life from the level of gene to the level of ecosystem – is declining precipitously across the globe. Recent global syntheses report steep declines in the abundance and diversity of plant and animal species across multiple trophic levels, and of natural ecosystems locally and globally, on land and in the oceans1,2 and 3. The failure to slow biodiversity loss, or indeed to address the deeply related issue of climate change4, demands we quickly find more ambitious and more coherent approaches to tackling these challenges.
Nature-based solutions (NbS) are one such family of approaches that has recently become prominent in international policy and business discourse5. Broadly defined as actions that involve working with nature to address societal goals (Figure 1), NbS are being widely hailed as a win-win for addressing biodiversity loss and climate change, whilst also supporting sustainable development6. This recognition is based on the understanding that natural and semi-natural ecosystems support human societies in multiple ways from providing food, clean water and shelter, to storing carbon and protecting us from the impacts of extreme events such as floods, droughts and heatwaves1 and 6. The NbS concept is also grounded in the knowledge that biodiversity loss and climate change share some of the same drivers and hence share some of the same solutions. In particular, land-use change is both the biggest driver of biodiversity declines (accounting for approximately 30% declines in global terrestrial habitat integrity1) and the second biggest source of GHG emissions (accounting for 23%)7. Protecting or enhancing carbon stores through the restoration of natural ecosystems can therefore, in theory, both reduce warming and slow biodiversity declines. However, this win-win scenario is not guaranteed. Some NbS — particularly those involving planting trees in naturally treeless habitats — can have negative outcomes both for biodiversity and for local people (discussed in 6 and 8). There are also critical questions around the timeframes over which NbS can help tackle the biodiversity and climate crises given the impacts of warming on the health of the biosphere and its capacity to draw down and store carbon.
Here, I will outline the potential of working with nature to address the causes and consequences of climate change and discuss why NbS must be designed, implemented and adaptively managed by or in partnership with local communities to provide measurable benefits for biodiversity and ecosystem health.
Figure. 1 | Nature-based solutions involve the protection, restoration or management of natural and semi-natural ecosystems; the sustainable management of aquatic systems and working lands such as croplands or timberlands; or the creation of novel ecosystems in and around cities or across the wider landscape. People and nature, together (beige band), co-produce outcomes which benefit society (blue band) and, in turn, the ecosystems on which people depend (blue arrows). While the ultimate goal of NbS is to support sustainable development, including human health and wellbeing, the ecosystems that provide NbS must be healthy and functional if these benefits are to be sustained over the long-term. Hence, to qualify as a NbS, an action must sustainably provide one or more benefits for people while causing no loss of biodiversity or ecological integrity compared to the pre-intervention state. To be sustainable and equitable, NbS must also be designed, implemented, managed and monitored by or in partnership with Indigenous peoples and local communities through a process that fully respects and champions local rights and knowledge, and generates local benefits. Figure reproduced and legend adapted with permission from Global Change Biology8.
Nature-based climate change mitigation
Estimating the global mitigation potential of scaling up the protection, restoration and sustainable management of our lands and seas is hugely challenging and estimates need to be understood with respect to a large number of important caveats. They are strongly influenced by 1) the carbon saturation point of mature forests, which ranges from 50-100 years to several centuries; 2) future trends in demand and supply within the land and sea sectors, which reflects demand for meat/fish and whether we can sustainably and ethically increase agricultural output; 3) impacts of climate change on ecosystem functioning; and 4) a wide range of socioeconomic constraints on the implementation of NbS, including safeguards for biodiversity and food security, land rights and tenure and financial and socio-political feasibility9, 10 and 11. Meanwhile the price of carbon, which heavily depends on global climate ambition, determines the fraction of the total mitigation potential of NbS which society decides to realise. According to the most recent study—based on the model presented in Griscom et al.9 albeit more tightly constrained—the most significant contribution for cost-effective avoided emissions of CO2 comes from protecting intact forests, wetlands and grasslands (4 Gt CO2 yr-1), while the greatest potential contribution to the global carbon sink comes from managing working lands (4 Gt CO2 yr-1 from 4.1 billion hectares of timberlands, croplands and grazing lands), followed by restoring native ecosystems (2 Gt CO2 yr-1 from 678 million hectares)11.
The total mitigation potential of NbS in the land use sector is therefore around 10 Gt CO2 yr-1. This amounts to reducing global warming by 0.1°C if warming peaks mid-century at 1.5°C since pre-industrial times4 and 11. However, if we overshoot the 1. 5°C temperature goal of the Paris Agreement and warming peaks around 2075 at 2°C, there would be more time for the benefits of NbS to accrue and, if scaled up to the maximum extent possible, they could reduce peak warming by 0.3°C11. Whilst this might seem like a small contribution to global cooling, the contribution of land-based NbS becomes hugely significant when viewed in the context of the severe social, economic and ecological impacts of 1.5 degrees of warming12. Nonetheless, the total potential of NbS is much smaller than what can be achieved through the decarbonisation of the global economy10 and 11. Furthermore, unless we rapidly phase out the use of fossil fuels the mitigation potential of NbS won’t be realized because climate warming will undermine the health of the biosphere and its capacity to draw down and store carbon or provide any other benefits to society13 and 14.
Nature-based climate change adaptation
Ambitious climate change mitigation action that combines rapid phasing out of fossil fuel use with rapid scaling-up of robust, sustainable NbS will significantly reduce the severity of impacts on societies and ecosystems. However, even if humanity successfully limits the global temperature increase to within 1.5°C, feedbacks and inertia in the global climate system mean that phenomena such as sea‐level rise will continue to increase, making adaptation essential. The most established approaches to addressing climate change impacts generally involve engineered interventions15. However, nature-based interventions in a range of ecosystems can work with and sometimes improve upon these approaches and can generally do so at lower economic cost6 and 15. If properly implemented, NbS can support human adaptation to climate change in three different ways6. First, NbS can reduce exposure to the immediate impacts of climate change: e.g. restoring and protecting coastal ecosystems can defend against coastal flooding and storm surges; restoration and protection of forests and wetlands can reduce risk of floods, soil erosion and landslides brought about by extreme weather events; and green infrastructure can cool cities during heatwaves and help to abate floods (reviewed in 16).
Image caption: Aerial view of Ras al Khaimah over the mangroves and the creek in the United Arab Emirates at sunrise. Mangrove forests bring multiple benefits to people, from protecting communities and infrastructure from the impacts of storm surges and preventing coastal erosion, to sequestering large amounts of carbon dioxide, protecting biodiversity and supporting livelihoods. According to the Global Commission on Adaptation, the benefits of protecting and restoring mangroves outweigh implementation costs by a factor of 10.
Second, NbS can reduce social sensitivity to climate impacts by supporting diversification of sources of food and income and thereby providing nutritional and financial security when crops or usual sources of income fail during climate extremes. This is particularly important in lower income nations where dependency on nature for food and income is high. Third, NbS can reduce vulnerability to climate impacts by building adaptive capacity through the process of designing, implementing and managing nature-based interventions. This process can empower local communities and equip them with knowledge and other resources to address future climate impacts. Such adaptive capacity, in turn, can enhance awareness of the value of nature and hence incentivise stewardship of ecosystems to ensure the continued supply of benefits from nature17.
The importance of NbS for adaptation globally and regionally has been quantified using various metrics including the number of people affected, jobs created, the monetary value of avoided damage to infrastructure from climate impacts, or the market value of resources such as timber or fish. For example, the protection of coastal ecosystems could benefit upwards of 500 million people globally, bringing benefits of over $100 billion per annum8. Meanwhile, for every $1 million invested in habitat restoration for coastal defence in the USA, around 40 new jobs would be created, compared to 19 for investment in the aviation industry, seven for finance, and five for oil and gas18.
Ultimately, however, the effectiveness of NbS for adaptation can only be meaningfully understood at a local level. Research shows that effectiveness depends on many local biophysical, ecological and socioeconomic factors that influence the exposure, sensitivity and adaptive capacity of the social-ecological system. Effectiveness can also only be understood in relation to the needs of different sectors of society impacted by climate change and more research is urgently needed to clarify how the benefits of NbS can be more equitably distributed. There are also substantial gaps in the evidence base on the cost‐effectiveness of nature-based interventions compared to alternatives and the extent to which they support short versus long term economic recovery after COVID-19. Such research is urgently needed in lower income tropical nations which are very vulnerable to the impacts of climate change and pandemics, and support high levels of biodiversity16.
Biodiversity and people at the foundation of successful, sustainable NbS
For NbS to deliver sustained benefits to people, the ecosystems involved must be healthy and resilient, i.e. their ecological functions must be able to resist and/or recover from climate change19. There is now good empirical evidence that functional resilience is strongly determined by ecosystem connectivity, heterogeneity and/or genetic, functional (trait) and species richness at multiple trophic levels19 and 20. For example, natural forests and mixed species forest plantations have more stable carbon stores during climate extremes compared to species-poor plantations21, 22 and 23, as do high diversity grassland plots compared to low diversity plots24. The precise mechanisms linking biodiversity to climate resilience are not fully understood and the relationship between biodiversity and resilience is context specific, complex, and not completely resolved20. However, the generally higher stability of naturally biodiverse ecosystems is thought to be mediated by functional redundancy among multiple taxa and by so-called “insurance effects”, i.e. spatial and temporal complementarity in ecological functions25 and 26. A number of recent studies demonstrate that forests allowed to regenerate naturally harbour higher biodiversity that supports a wider range of ecosystem services, with fewer trade-offs between them, compared to plantations (reviewed in 16). Higher diversity also safeguards the evolutionary potential of ecosystems, allowing for ecological adaptation to climate change (often in the form of phenological changes). In short, if we don’t ensure that biodiversity is supported as we design and implement NbS, then nature won’t be able to provide any solutions at all.
The extent to which NbS sustainably support biodiversity varies among the different types of intervention (Figure 1). Clearly, protecting intact ecosystems or restoring degraded landscapes to their natural state can deliver significant benefits to biodiversity while protecting and enhancing carbon sinks8. Indeed, recent global analyses show that conservation actions in areas rich in both carbon and biodiversity would secure nearly 80% of the potential carbon stocks and 95% of the potential biodiversity benefits that would be achievable were either carbon or biodiversity prioritised alone27; meanwhile, restoring 15% of agricultural and pastoral lands across several biomes could result in 60% fewer expected species extinctions and sequester nearly 300 GT of CO228.
However, the outcomes for biodiversity of creating a new ecosystem will depend on the species used, the state of the landscape prior to the intervention, and the scale at which biodiversity outcomes are measured. For example, establishing plantations of non-native trees in a highly degraded landscape might have net positive benefits for biodiversity locally if the trees enable native forest to regenerate29, regionally if plantations take pressure off natural biodiverse forest, or globally if they help to mitigate climate change(e.g. 30). Conversely, if non-native tree plantations replace intact native ecosystems such as ancient grasslands, peatlands or woodlands, the outcomes for biodiversity will generally be poor. There is evidence that the latter is happening in many places. In Chile, for example, government-subsidised plantation forestry in 1986-2011 caused a 13% decline of native Nothofagus forests and hence a loss of native biodiversity, yet only achieved a 2% increase in carbon storage31. Meanwhile, in Cambodia, a 34,007ha commercial Acacia monoculture was established with the aims of supporting climate change mitigation and livelihoods, yet replaced mature native forest, causing a decline in biodiversity, no net carbon benefits and the displacement of local people32.
This last example highlights a major challenge around the implementation of NbS. In regions where regulatory frameworks are weak and land and resources easily appropriated for environmental ends, plantations and other nature-based interventions can be established without taking the livelihoods, rights or knowledge of local communities into account34. Local people might be used for labour, but are then restricted from what were previously common-pool ecosystem resources. Yet Indigenous Peoples and local communities often have considerable knowledge on how best to work with nature and are playing a key role in tackling the biodiversity and climate crises35. They must therefore be included in land-use decisions and have their rights and cultural links to the natural world fully respected. In ignoring the cultural links that communities have with local ecosystems, as a source of livelihoods and identity, such initiatives are not only deeply unethical, they are not sustainable over the long-term4.
Image caption: The Amazon rainforest, Brazil. The world’s remaining intact ecosystems and biomes, especially old-growth tropical rainforests such as those found in Amazonia, are hotspots for both biodiversity and carbon storage, while also supporting livelihoods and protecting people from climate change impacts. As such, their protection should be prioritised when it comes to policy and funding for nature-based solutions.
Getting the message right on NbS
NbS are place-based partnerships between people and nature and there is no one intervention that can be applied in a top-down fashion at scale8. However, there are broad principles that can guide investments in successful, sustainable NbS and their federation across land- and seascapes36. In particular, there is a deepening consensus about the critical importance of protecting, restoring and connecting biodiverse natural or semi‐natural habitats across multifunctional landscapes for the broad range of benefits they bring and of ensuring that NbS are designed and implemented by or in partnership with local communities8. There is also broad agreement that NbS are not an alternative to keeping fossil fuels in the ground8, 9, 10, 11, 36 and 37.
The problem is that many high emitting industries are investing in tree-planting programmes to offset their greenhouse gas emissions8. This has created a ‘moral hazard’ wherein the simple but ultimately misguided narrative that planting trees will stop climate change is encouraging businesses and individuals to conduct business as usual rather than drastically scale back use of fossil fuels8 and 10. Moreover, these programmes tend to involve commercial plantations of exotic tree species established in naturally treeless habitats8. While these are often labelled as NbS, they do not qualify: such plantations tend to offer only short term high-risk carbon storage (many harvested products quickly release carbon back into the atmosphere)8. It is therefore critical to hold to account those claiming to invest in NbS to ensure they a) have ambitious, credible and verifiable action plans to phase out use of fossil fuels, and b) only support projects that are people-led and biodiversity based36. To support this process, the IUCN has developed a “Global Standard for NbS”, a set of clear and coherent principles and standardized evidence-based framework for investors and practitioners that will be refined with feedback over time37.
NbS in 2021 and beyond
The upcoming global biodiversity and climate summits — the United Nations Convention on Biological Diversity (CBD) CoP15 and the United Nations Framework Convention on Climate Change (UNFCCC) CoP26 — are a major opportunity to catalyse the transformative change needed to address biodiversity loss and climate change. Research clearly shows that these crises are deeply interlinked, that they cannot be addressed in isolation from one another and that biodiversity can support climate action through NbS. It is therefore vital that policymakers and practitioners consider the impacts on biodiversity of interventions to address climate change, and vice versa, and that targets for NbS are fully aligned across different international policy processes, especially the UNFCCC, CBD, UN Convention to Combat Desertification and the Sustainable Development Goals. We need to see evidence-based targets for NbS in the Nationally Determined Contributions of Paris Agreement signatories that harmonise action for climate change mitigation and adaptation without compromising biodiversity goals, and we need to see targets in the post-2020 global biodiversity framework that take climate outcomes into account. Closer collaboration between the communities of researchers addressing these challenges, in particular the Intergovernmental Panel on Climate Change and the Intergovernmental Panel on Biodiversity and Ecosystem Services would help with this. Encouragingly, the draft of the post-2020 global biodiversity framework includes a target to contribute to climate change mitigation, adaptation and disaster-risk reduction through NbS.
The problem is, even if pledges are effectively targeted and global environmental policy processes and goals are integrated and aligned, the number of businesses and nations making pledges for NbS and, in particular, the scale of funding committed, are inadequate to realise the full potential of NbS8. What is needed, ultimately, is transformative change in how businesses function, economies are run and individuals behave. For example, to enable large-scale ecological restoration of degraded ecosystems, land must be freed up from other uses; this requires just transitions towards plant-based diets, elimination of harmful agricultural subsidies and strong support for meeting the remaining demand for food though higher-yielding systems wherever possible38. It also necessitates rapid and widespread adoption of a circular and regenerative economy with reduced demand for raw materials, use of green energy and clean supply chains39. While such changes cannot occur in synchrony, every step towards sustainability and prioritisation of nature in decision-making will catalyse further change. Only in this way will we stabilize the climate and secure the biodiversity that underpins all that is valuable to humanity, now and for future generations.