Getting the message right on nature- based solutions to climate change

Nature- based solutions (NbS)— solutions to societal challenges that involve working with nature— have recently gained popularity as an integrated approach that can address cli mate change and biodiversity loss, while supporting sustainable development. Although well- designed NbS can deliver multiple benefits for people and nature, much of the re cent limelight has been on tree planting for carbon sequestration. There are serious con cerns that this is distracting from the need to rapidly phase out use of fossil fuels and protect existing intact ecosystems. There are also concerns that the expansion of forestry framed as a climate change mitigation solution is coming at the cost of carbon rich and biodiverse native ecosystems and local resource rights. Here, we discuss the promise and pitfalls of the NbS framing and its current political traction, and we present recom mendations on how to get the message right. We urge policymakers, practitioners and researchers to consider the synergies and trade- offs associated with NbS and to follow four guiding principles to enable NbS to provide sustainable benefits to society: (1) NbS are not a substitute for the rapid phase out of fossil fuels; (2) NbS involve a wide range of ecosystems on land and in the sea, not just forests; (3) NbS are implemented with the full engagement and consent of Indigenous Peoples and local communities in a way that respects their cultural and ecological rights; and (4) NbS should be explicitly designed to provide measurable benefits for biodiversity. Only by following these guidelines will we design robust and resilient NbS that address the urgent challenges of climate change and biodiversity loss, sustaining nature and people together, now and into the future.

inequality across the globe and are severely undermining the development gains of the 20th Century (IPBES, 2019;WEF, 2020aWEF, , 2020b. There is a growing realization that these challenges are interlinked and cannot be addressed independently (IPCC, 2019b;Turney et al., 2020).
As evidence builds that the natural systems on which we depend are deteriorating beyond a point of no return (IPCC, 2018;Rockström et al., 2009;Steffen et al., 2015), it is clear that larger scale and more coherent approaches to tackling global challenges are needed.
Nature-based solutions (NbS)-solutions to societal challenges that involve working with nature-have recently gained popularity as an integrated approach that could address the twin crises of climate change and biodiversity loss , while supporting a wide range of sustainable development goals (Gómez Martín et al., 2020;Maes et al., 2019). NbS are actions that are broadly categorized as the protection, restoration or management of natural and semi-natural ecosystems, sustainable management of working lands and aquatic systems, or the creation of novel ecosystems (Figure 1).
Although more research is needed, a rapidly growing evidence base Hanson et al., 2020) demonstrates that welldesigned NbS can deliver multiple benefits . For example, protecting and restoring habitats along shorelines or in upper catchments can contribute to climate change adaptation by protecting communities and infrastructure from flooding and erosion, at the same time as increasing carbon sequestration and protecting biodiversity . Meanwhile, increasing green space and planting trees in urban areas can help with cooling and flood abatement while mitigating air pollution, providing recreation and health benefits and sequestering carbon (Alves et al., 2019;Brink et al., 2016; Figure 1).
The simple logic of 'working with and enhancing nature to help address societal challenges'  F I G U R E 1 Conceptual diagram of nature-based solutions. Nature-based solutions (NbS) 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. They are actions that are underpinned by biodiversity (Section 3.1) and are designed and implemented with the full engagement and consent of Indigenous Peoples and local communities (Section 3.2). People and nature, together (yellow circle), co-produce a variety of outcomes (ecosystem services or Nature's Contributions to People, blue band) which benefit society; these benefits can, in turn, support ecosystem health (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, functional and biodiverse if such benefits are to be provided in the long term (Section 3.1). Hence, to qualify as an NbS, an action must sustainably provide one or more benefits for people (such as reducing flood risk or storing carbon) while causing no loss of biodiversity or ecological integrity (or preferably a gain) compared to the pre-intervention state. Although actions with only one societal benefit could be classified as NbS, an intervention in nature usually has multiple interlinked effects on the climate and the social-ecological system. By identifying all of these effects, interventions can be designed to build synergies and to be resilient to future climate and socio-economic change Agroforestry, including silvoarable and silvo-pasture The practice of planting trees on farmland, including as rows between crops, or as shelter for livestock. Torralba et al. (2016) Agro-ecology, conservation agriculture and organic agriculture Various approaches to sustainable agriculture that aim to protect soil health. Warren et al. (2008) Forest and landscape restoration (FLR) A process that aims to regain ecological integrity and enhance human wellbeing in a deforested or degraded forest landscape. Maginnis and Jackson (2012) Reduced emissions from deforestation and degradation+ (REDD+) Reducing Emissions from Deforestation and forest Degradation, and fostering conservation, sustainable management of forests, and enhancement of forest carbon stocks in developing countries.
REDD+ 'rulebook', also known as the Warsaw Framework for REDD (UNFCCC, 2016); Paris Agreement (Article 5); (UNFCCC, 2015) Natural climate solutions (NCS) or Nature-based Climate Solutions (NbCS) et al., 2019) has facilitated understanding and engagement across diverse sectors while the breadth of the concept has drawn together disparate communities of researchers, policymakers and practitioners across climate change, biodiversity and development (Cohen-Shacham et al., 2019;van Ham & Klimmek, 2017). In uniting nature-based approaches within a single framework (Table 1), and enabling a flexible, integrated approach to tackling different challenges, NbS can-if properly designed and implemented-enable synergies and minimize trade-offs between actions to achieve different goals. This has encouraged extensive uptake of the concept by governments (Table 2) and the private sector (Table 3;  non-governmental organizations, as well as private sector institutions (Seymour, 2020).
Although the simplicity and breadth of the NbS concept is a strength, it has also led to confusion. Much work has been done to improve the conceptualization of NbS (Eggermont et al., 2015), including recent development of a Global Standard for NbS by the International Union for the Conservation of Nature (IUCN; Cohen-Shacham et al., 2019;IUCN, 2020). Nonetheless, there is still uncertainty as to what 'counts' as an NbS and the extent to which NbS represent a departure from existing concepts and practices.
In the context of climate change, concerns have been raised that NbS are being used to excuse business-as-usual consumption of fossil fuels Edwards, 2020); that there is an over-emphasis on tree planting as a 'silver-bullet' solution to climate change Seymour, 2020) and that this is distracting from the urgent need to protect and connect a wide range of intact ecosystems across landscapes and seascapes (Solan et al., 2020;Watson et al., 2018). These issues arise partly from uncertainties in the underlying science, such as the limited set of contexts in which the broader benefits of NbS have been demonstrated . They also arise as a result of miscommunications about the mitigation potential of working with nature, such as the recent meme that NbS can provide '30% of the climate solution'. There are also concerns that where rights are weak, especially around land tenure, NbS may be implemented in the absence of community consent or cause adverse social consequences. Such rights infringements can impede the success and sustainability of interventions (Ramprasad et al., 2020;Scheidel & Work, 2018;Vidal, 2008

| ORI G IN S AND DEFINITI ON S OF NATURE-BA S ED SOLUTIONS
How NbS have been framed, identified and implemented has evolved over time. Local societies have been working with nature to cope with the impacts of natural disasters and climate variability for millennia (Berkes et al., 2000;Ruiz-Mallén et al., 2013). For example, there is a long documented history of interventions such as the restoration of mangroves to boost local livelihoods or provide flood protection (Kairo et al., 2001). It is only in recent years that such practices have been given scientific names (see Table 1 for examples) and, even more recently, classed as NbS. Through the establishment and recognition of the NbS concept, global interest in these types of practices has grown rapidly and NbS has moved up political agendas at municipal, national and international levels .
The first publication to focus on NbS was a report by the World Bank in 2008 detailing the climate change mitigation and adaptation benefits of the Bank's investments in biodiversity conservation (Mackinnon et al., 2008). NbS were then adopted by the IUCN which promoted them 'as a way to mitigate and adapt to climate change, secure water, food and energy supplies, reduce poverty and drive economic growth' in a position paper for the United Nations Framework All the positive contributions, or benefits, and occasionally negative contributions, losses or detriments that people obtain from nature. Díaz et al. (2018) Nature's contribution to adaptation (NCA)-formerly referred to as adaptation services Properties of ecosystems that provide options for future livelihoods and adaptation to transformative change. Colloff et al. (2020)

Grain for Green Program
Chinese Government (1999Government ( -2018 29 Mha of trees planted across China to reduce severe soil erosion and land degradation (Xian et al., 2020  Investment in NbS such as restoration and protection of forests, grasslands and wetlands, as a form of offsetting for fuel use by customers at about 1400 fuel stations. The investment in NbS will go beyond the initial 3 years, for example, they aim to plant 1 million trees over 5 years in Scotland. This is part of Shell's plan to reach net-zero emissions by 2050: 65% by emission reduction and 35% by offsetting, including the NbS programme (but see Section 6.1).

Unilever
Climate and Nature Fund (€1 billion) Ecosystem restoration, protection and water security projects. This is in addition to committing to deforestation-free supply chains by 2023, and net-zero emissions for all products by 2039. Unilever (2020)  age, it could also be classified as an NCS. Meanwhile, depending on the specific actions involved, such an intervention could also be termed ecological restoration or ecological engineering. A major advantage of applying the NbS concept is that it encourages recognition of a wider range of outcomes of a given intervention than these more specific terms. Referring to a restoration project as NbS rather than NCS or eco-DRR avoids the implication that the sole purpose and outcome of the project is either storing carbon or reducing floods and landslides.
By considering the full range of potential outcomes, the NbS concept helps practitioners to design and implement interventions in nature that provide multiple benefits and to manage any trade-offs.

| B I OD IVER S IT Y AND PEOPLE AT THE FOUNDATI ON OF NATURE-BA S ED SOLUTIONS
Well-designed NbS are place-based partnerships between people and nature. Here we discuss the two key elements that underpin successful, sustainable NbS: biodiversity and people.

| Biodiversity underpins the benefits delivered by NbS
Biodiversity is the diversity of life from the level of gene to the level of the ecosystem (CBD, 2009). In this paper, we use this term to refer to ecologically appropriate levels of diversity needed to support healthy, well-functioning ecosystems that support local habitats and species, bearing in mind that some ecologically valuable ecosystems naturally host fewer species than others.
There has been some confusion about the relationship between biodiversity and NbS. Some definitions do not explicitly reference biodiversity (e.g. European Commission, 2015), and concerns have been raised that some interventions badged as NbS may ultimately be harmful for biodiversity. Here we argue that because biodiversity is essential to secure the flow of ecosystem services now and into the future (Cardinale et al., 2012;IPBES, 2019;Seddon et al., 2016), NbS must deliver benefits for biodiversity, as well as people. This is in line with the IUCN definition and the Global Standard for NbS (Cohen-Shacham et al., 2019;IUCN, 2020), and it clearly distinguishes NbS from actions that exploit nature but can damage biodiversity, such as certain types of agriculture, BioEnergy Carbon Capture and Storage (BECCS), commercial forestry and recreational activities that harm sensitive habitats or species.
Actions that support biodiversity underpin societal benefits in two ways: they boost the delivery of many ecosystem services in the short term, and they support the health and resilience of ecosystems in the long term, that is, their ability to resist or quickly recover from perturbations. In the short term, more biodiverse ecosystems have greater productivity and, in general, a higher level of ecosystem service provision (Cardinale et al., 2012;Tilman et al., 2012).
For example, coral reef fish diversity (which can be enhanced by establishing marine-protected areas) has a strong positive relationship with fish biomass and productivity (Benkwitt et al., 2020) while soil biodiversity (which can be improved using agro-ecological practices) can increase crop yields (Bender & van der Heijden, 2015;Vignola et al., 2015). Cultural ecosystem services are also enhanced: more species-rich green spaces have been shown to support greater personal wellbeing (Aerts et al., 2018), and more visitors are attracted to protected areas with more habitat types and threatened species (Siikamäki et al., 2015) and/or higher bird species richness (Naidoo & Adamowicz, 2005).
Diversity is not always associated with higher delivery of shortterm benefits. For example, high-yielding monoculture crops or plantations can produce more food or wood per hectare for a few years compared to a mixed species system .
However, diversity is essential for long-term sustainability, as functional resilience to stressors such as climate change, invasive species and new pathogens is strongly determined by ecosystem connectivity and biodiversity at multiple trophic levels (Oliver et al., 2015).
Connectivity of similar ecosystems across landscapes enables recovery of disturbed habitats by facilitating dispersal from surrounding intact areas. Connectivity also allows allow species to track their preferred ecological niches across the landscape in response to changing environmental conditions (Biggs et al., 2012). Meanwhile, the diversity of species, ecological traits and genes contained within communities of plants, animals, fungi and bacteria buffers ecosystems against perturbation via 'insurance effects', that is, spatial and temporal complementarity in ecological functions, as well as by functional redundancy among multiple taxa (Alvarez et al., 2019; Biggs et al., 2020;Cardinale et al., 2012;Tuck et al., 2016;Yachi & Loreau, 1999). For example, natural forests and mixed species forest plantations have more stable carbon stores during climate extremes compared to species-poor plantations (Hutchison et al., 2018;Osuri et al., 2020), as do high diversity grassland plots compared to low diversity plots (Isbell et al., 2017). Compared to low diversity plantations, biodiverse natural forests and areas allowed to regenerate naturally also have higher resilience to fires, pests and diseases (Barlow et al., 2007;Jactel et al., 2017). In marine ecosystems, greater species turnover among reefs (β-diversity) increases community stability (Mellin et al., 2014) and possibly also resistance to disturbance (Mellin et al., 2016). Therefore in order to maintain healthy, resilient ecosystems that can continue to deliver benefits to people over the long term, NbS must be explicitly designed to protect or enhance biodiversity ( Figure 1).

| NbS with and for people
To deliver effective, resilient, legitimate and equitable outcomes, all  (Appadurai, 2018;Chaterjee, 2020). If external 'experts' undermine or ignore local knowledge, this could result in poor and ineffective land management decisions (Leach & Mearns, 1996 (Fedele et al., 2018). In contrast, a lack of alignment with local perspectives can deter active participation and disempower local communities, which, in turn, can compromise local support for NbS, jeopardizing their success ; see Section 6.3), while also constraining local adaptive capacity (Woroniecki, 2019).

| Growth of research on NbS and recent global syntheses
Research into NbS has grown very rapidly in recent years (Hanson et al., 2020;Keesstra et al., 2018;Raymond et al., 2017;. The Special Report on global warming of 1.5°C above pre-industrial levels (IPCC, 2018) concluded that 1.5°C warming will be surpassed within a few years unless transformational change reduces emissions at an unprecedented rate. The four IPCC pathways that stay within In summary, the concept of NbS has expanded from an initial focus on ecosystem-based adaptation to encompass urban green infrastructure, climate change mitigation, sustainable agriculture and many other concepts ( Table 1). The potential of NbS is now recognized by all the major international scientific bodies working on climate change and biodiversity, and there is a growing consensus around key caveats concerning the limits of NbS for climate mitigation, the potential adverse impacts of some actions on biodiversity and food security, and the need to accompany NbS with deep cuts to fossil fuel emissions.

| Governmental and non-governmental interest in nature-based solutions
Over the last few years, numerous national, intergovernmental and

| Private sector interest in naturebased solutions
Appreciation of the critical importance of healthy, functioning ecosystems to human wellbeing and economic activity has also grown in the private sector in recent years (Dasgupta, 2020 Forum) has a cross-industry corporate alliance and intends to be the 'pinnacle for corporate leadership in this space'.
As well as these pro-nature commitments, there has been a sharp rise in major funding pledges for NbS from the private sector (Table 3). While many of these pledges refer to a wider range of NbS options than just tree planting, there is very little publicly available information to determine the extent to which these pledges have been implemented, nor on key details such as the type of NbS, species selected (i.e. native or non-native) and the previous use of the land.

| THE PROMIS E OF NATURE-BA S ED SOLUTIONS
NbS offer multiple benefits for people and nature ( Figure 1). However, much recent attention has focused on their potential for addressing climate change in particular. Here we provide an overview of the strength of the evidence supporting a role for NbS in both climate change adaptation and mitigation.

| Adaptation benefits of nature-based solutions
NbS can reduce the vulnerability of the social-ecological system (i.e. the interconnected ecological and socio-economic systems) to environmental shocks and changes in three ways: by reducing exposure to climate hazards; reducing sensitivity to adverse impacts; and building adaptive capacity .
There is now a substantive evidence base demonstrating that NbS can reduce exposure to climate impacts such as flooding, erosion, water scarcity and reduced agricultural productivity . resilience of food supplies to pests, diseases and climatic extremes (Altieri et al., 2015;Tamburini et al., 2020;Vignola et al., 2015); and urban NbS can make a key contribution to flood mitigation (Stefanakis, 2019) and cooling cities (Kabisch et al., 2016;Marando et al., 2019).
NbS can also reduce the degree to which individuals, communities and societies are actually affected by the climate impacts they experience, that is, their social sensitivity (e.g. Valenzuela et al., 2020). In particular, NbS secure the delivery of a wide range of benefits that sustain diverse sources of food and income, which can provide nutritional and financial security when crops or usual sources of income fail in the face of climate extremes (Ahammad et al., 2013;Waldron et al., 2017). This is particularly important in the Global South where dependency on local natural resources for food and income is high (Uy et al., 2012). For example, in Vanuatu, marine protected areas act as a reservoir of resources that can be temporarily opened to fishing as a source of food and income for the local communities, when terrestrial-based livelihoods are reduced due to drought from El Niño (Eriksson et al., 2017).
Estimates also vary because they differ in the extent to which they consider constraints on deployment of NbS related to economic and political feasibility, land rights and local needs, and safeguards for food security and biodiversity . The models developed by Griscom et al. (2017), for example, only include reforestation in areas ecologically appropriate for forests; this excludes boreal systems, where the albedo effect may lead to net warming (Betts & Ball, 1997) and afforestation of native non-forest habitats such as savannahs .
The total mitigation potential of improvements in the land-  Griscom et al. (2017). Given the wide range of assumptions involved in running these models, these estimates should be regarded as rough approximations at best. Moreover, while the models include coastal ecosystems (mangroves, saltmarshes and seagrass) they exclude marine systems such as coral reefs, phytoplankton, kelp forests and marine fauna, that is, calcifiers (shellfish, zooplankton), krill and teleost fish, for which data remain sparse and estimates highly uncertain (Howard et al., 2017;Siikamäki et al., 2013).
Despite these sources of uncertainty, an influential oft-cited statement regarding NbS has been circulating in business and policy discourse: decreasing sources and increasing sinks of GHGs through NbS have the potential to provide around 30% of the cost-effective climate mitigation needed through to 2030 to achieve the targets of the Paris Agreement (CBD, 2020). However, this statement is not always accompanied by the essential caveat that this potential can only be achieved in tandem with the decarbonization of the global econ-  A number of high emitting industries are now proposing to use NbS to offset their greenhouse gas emissions, including airports (Heathrow Airport Limited, 2018), airlines (Delta, 2020) and oil and gas companies (Shell, 2019b Standard for NbS to ensure the quality of offset projects (IUCN, 2020).

| Over-emphasis on tree planting rather than a wide range of NbS
Although NbS span a wide range of actions, from protection and restoration of terrestrial and marine ecosystems to sustainable agriculture and urban green infrastructure (see Table 1), funds are currently being channelled mainly towards tree planting ( Table 2). The simple and powerful narrative of 'plant a tree to save the planet' is universally appealing, but over-reliance on tree planting as a climate solution raises a number of concerns (Chazdon, 2020; (Hong et al., 2020). This suggests that the widely used method of estimating soil carbon from a fixed ratio with vegetation biomass overestimates carbon sequestration from afforestation (Hong et al., 2020). Afforestation on peaty soils can lead to losses of soil carbon that outweigh that sequestered as the trees grow (Brown, 2020;Brown et al., 2014;Friggens et al., 2020;Sloan et al., 2018).
Third, afforestation can also reduce ecosystem resilience and thus long-term carbon storage and sequestration. For example, fireadapted savannah and dryland grassland ecosystems hold large carbon stores below ground. They readily recover from the relatively cool and frequent grassland fires, which do not destroy soil carbon, but afforestation risks much greater carbon losses during intensely hot plantation fires (Bennett & Kruger, 2015), and can also increase the risk of fires on peatland in temperate regions (Davies et al., 2013;Wilkinson et al., 2018).
Fourth, tree-planting schemes must be carefully designed if they are to deliver the intended benefits. For example, mangroves can only thrive in particular conditions of soil, climate, tidal fluctuations and wind velocity (Singh, 2006;Thivakaran et al., 2016).
Compensatory offsets and afforestation schemes that ignore these factors have often resulted in slow and stunted growth (Srivastava & Mehta, 2017). Many investments badged as NbS are for commercial plantations, which do not provide permanent carbon stores (Lewis, Wheeler, et al., 2019). Although harvested timber can lock up carbon in long-lived products such as timberframed buildings or furniture, the carbon stored in these products has been over-estimated (Harmon, 2019). A high proportion of harvested wood is used for paper, card and short-lived products such as MDF furniture, which soon end up in landfill or incineration, releasing carbon back to the atmosphere so that the net result could even be a carbon loss (Hudiburg et al., 2019;Lewis, Wheeler, et al., 2019).
Fifth, trees in the wrong place can also cause trade-offs between ecosystem services. More research is needed into the dynamics of these trade-offs, but current evidence shows that, for example, single-aged, low diversity, intensively managed plantations deliver wood products but may cause water pollution from soil disturbance and agrochemical use (Drinan et al., 2013) and reduce water availability in arid regions . The Grain-for-Green program in China succeeded in rapidly increasing tree cover to restore degraded agricultural soils, but used mainly fast-growing non-native species that have reduced water supply, and also resulted in a decrease of 6% in native forest cover as farming was displaced to new areas Hua et al., 2018;Xian et al., 2020).
Finally, and critically, the current focus on planting trees is distracting from the urgent need to effectively protect remaining intact ecosystems. Indeed, in the United States, the Trump administration signed up to the World Economic Forum's Trillion Trees initiative while also opening up previously protected forests for logging (Frazin, 2020). Less than 1% of tropical, temperate and montane grasslands, tropical coniferous forests, tropical dry forests and mangroves are classed as intact, that is, having very low human influence (Riggio et al., 2020). Not only are these intact ecosystems hotspots for biodiversity, but intact old-growth forests are particularly important for carbon storage and sequestration  while also protecting people from climate change impacts (Martin & Watson, 2016). Yet, many of the world's remaining intact ecosystems lack effective protection or are poorly managed (Soto-Navarro et al., 2020;Tan et al., 2020); including marine-protected areas where dredging takes place (McVeigh, 2020). Degradation of terrestrial habitats (e.g. through logging, drainage, infrastructure development) significantly reduces carbon storage (Maxwell et al., 2019;Tan et al., 2020) and increases vulnerability to climate-related hazards such as fire (Barlow et al., 2007). Freshwater, coastal and marine habitats face similar issues due to water pollution, temperature increases, sea-level rise, over-fishing, the spread of invasive species and, in some cases, inappropriate management (Elliott & Lawrence, 1998).
A balanced NbS approach would give greater priority to protecting these remaining intact ecosystems, as well as restoring partially degraded forests (Philipson et al., 2020), and other approaches such as 'proforestation'-leaving forests to grow to their full potential, with minimal intervention , and natural regeneration of native ecosystems, where appropriate Guariguata et al., 2019;Holl, 2017;Holl & Aide, 2011;Meli et al., 2017;Molin et al., 2018).
In summary, a more holistic approach is needed which protects, restores and connects a wide range of ecosystems across landscapes and seascapes, including native woodlands, shrublands, savannas, wetlands, grasslands, reefs and seagrass, as well as sustainable agriculture and urban green infrastructure. This will identify which ecosystems are appropriate to suit the local ecological and climate context, and will balance local needs for food and materials with the need to support biodiversity, climate change adaptation and other sustainable development goals. To support investment in a diverse range of habitats, we also need to extend current metrics and standards beyond those used for forest carbon to include other carbon-rich habitats such as wetlands and grasslands.

| Potential adverse impacts on local communities
Despite the fact that Indigenous Peoples and local communities (IPLCs) can play a key role in tackling the biodiversity and climate crises (Section 3.2), they are often excluded from land-use decisions involving ecosystem protection and management, their rights disrespected (Bayrak & Marafa, 2016).
Where regulatory frameworks are weak, this can facilitate 'green grabbing', that is, appropriation of land and resources for environmental ends (Vidal, 2008), displacing and marginalizing poor and vulnerable communities through securitization of resources (Scheidel & Work, 2018;Veldman et al., 2019).
Failure to involve IPLCs can ignore cultural links that communities have with local ecosystems, as a source of livelihoods and identity (Srivastava & Mehta, 2017;Sullivan, 2009). Some conservation or planting programmes have alienated local communities by using them simply for labour, while restricting their access to what were previously common-pool ecosystem resources Srivastava & Mehta, 2017). This forces communities to find alternative fishing or hunting areas and can lead to negative impacts on stocks and biodiversity (Mora & Sale, 2011).
As is the case with most development initiatives, NbS programmes evolve in locally specific ways contingent on social, economic and political forces as well as the relative power of various stakeholders (Woroniecki, 2019). Procedural aspects involving local people in decision-making can sometimes be reduced to programmatic formalities and box-ticking exercises (Newell et al., 2020), rather than providing space at the negotiation table to influence the decision-making process. Local communities are often labelled as ignorant and in need of training or capacity building (Li, 2007) rather than being recognized as agents with extensive local knowledge that are capable of exercising choice and making decisions.
Experiences with such poorly implemented 'offset' projects labelled as NbS and their detrimental ecological and social side effects can lead to push-back against NbS from local communities and conservation practitioners. They are also wary of trade-offs that may be generated by NbS where some people benefit at the expense of others. This may arise in situations where there are benefits to project participants but costs to non-participants or where a project benefits all members of a community locally but imposes costs to communities elsewhere . For example, an urban shelterbelt in China protects city-dwellers from dust storms, but Uighur communities downstream suffer because the heavy irrigation demand of the shelterbelt is drying out the native riparian forests on which they depend (Missall et al., 2018).
This problem is exacerbated by the misuse of the concept of NbS as a quick 'ecological fix' for the crisis generated by unsustainable patterns of production and consumption (Castree, 2008;Dempsey & Suarez, 2016). For example, using biochar (charcoal) to lock carbon into the soil can be seen as implying that unsustainable practices in one place (fossil fuel emissions) can be repaired by sustainable practices in other places Leach et al., 2012). Such activities, if not accompanied by larger structural changes in production and consumption including reduced fossil fuel use, may lead to unsustainable outcomes and can marginalize the poor who become committed and reliant on these NbS practices but who have little to no voice in deciding and shaping these systems.
Transitions and finance mechanisms underlying NbS programmes need to be 'just', putting the needs and livelihoods of the most vulnerable at the centre of policy and implementation. For example, communities may need financial support during any lag time taken for NbS to start delivering benefits. Affected communities must be fully included in the decision-making processes, not merely used for labour , and social differentiation (ethnicity, caste, class, gender, ableism) needs to be factored in to ensure that all voices count in the decision-making process.
Distributive (who gains and who loses), procedural (who decides for whom) and recognition justice (understanding plural notions of value) need to be incorporated into accountability and regulatory frameworks with compliance being monitored through regular social audits (involving local communities) and third-party actors (including the judiciary). In this way, NbS pathways can disrupt unequal systems of power and enable fair futures for the marginalized and vulnerable groups who are at the frontline of climate change and its impacts.

| Failure to ensure benefits for biodiversity
Protecting intact terrestrial, freshwater, coastal and marine ecosystems, restoring degraded habitats to their natural state and managing working lands more sustainably can deliver significant biodiversity benefits (Bustamante et al., 2019;Coetzee et al., 2014; IPBES, 2019; Solan et al., 2020). However, as discussed in Section 6.2, investments and policy support are currently being directed largely towards created ecosystems, especially tree plantations.
The outcome of these initiatives for biodiversity will depend on many factors, including the species used, the state of the landscape prior to the intervention, the management regime and the scale at which outcomes are measured. For example, agroforestry is likely to have biodiversity benefits compared to conventional arable, pasture or forestry, but this depends on the variety, abundance and ecological suitability of the tree species used (Torralba et al., 2016).
Establishing plantations of non-native trees in a highly degraded landscape might benefit biodiversity locally if the trees enable native vegetation to regenerate  or regionally if plantations take pressure off native biodiverse forest (Ghazoul et al., 2019). Conversely, if non-native tree plantations replace intact native ecosystems such as ancient grasslands, peatlands or woodlands, the outcomes for biodiversity will be poor (Balthazar et al., 2015;Barlow et al., 2007;Bond, 2016;Bremer & Farley, 2010;Stephens & Wagner, 2007). Native biodiversity can also suffer if exotic species used in plantations become invasive (García-Palacios et al., 2010), over-dominant (Yu et al., 2012) or reduce water supplies (Missall et al., 2018).
Complex trade-offs can arise, however, which require more research. For example, a modelling study by Ohashi et al. (2019) concluded that although afforestation and BECCS can cause local biodiversity loss in certain regions (Europe and Oceania), it can achieve net biodiversity benefits at the global level through its contribution to mitigating climate change, which is a major driver of biodiversity loss. Yet, if biomass production for BECCs mainly takes place in developing tropical countries where productivity is highest and costs are low, this could exacerbate global biodiversity loss.
Several studies have investigated whether NbS can provide a win-win for biodiversity and climate change mitigation. There can be biodiversity benefits from REDD+ and PES schemes that protect forests for carbon storage, as high-carbon ecosystems often overlap with biodiversity hotspots (Larjavaara et al., 2019). Although conservation priorities sometimes lie in lower carbon ecosystems (Budiharta et al., 2014), research increasingly shows how NbS can effectively support both mitigation and biodiversity goals . For example, conservation actions in areas rich in both carbon and biodiversity were recently estimated to secure nearly 80% of the potential carbon stocks and 95% of the potential biodiversity benefits that would be achievable if either carbon or biodiversity were prioritized alone . Meanwhile, restoring 15% of agricultural and pastoral lands in areas across several biomes that are high priority for both biodiversity and climate mitigation could result in 60% fewer expected species extinctions and sequester nearly 300 Gt of CO 2 (Strassburg et al., 2020). NbS that involve sustainable management of natural or modified ecosystems, such as adding organic matter to soils to improve carbon storage and water retention, would also be expected to have benefits for biodiversity (in this case soil biota).
NbS for climate change adaptation can also have biodiversity benefits. A recent systematic map of 376 peer-reviewed studies  showed that most (73%) of the 91 cases that reported on 'ecological outcomes' showed benefits, such as an increased number of species, functional diversity, or higher plant or animal productivity (e.g. Barsoum et al., 2016;Liquete et al., 2016) with only 1% showing exclusively negative effects, and 24% reporting mixed or unclear effects. Of the cases with positive ecological outcomes, 47 were reported to also have benefits for adaptation (none were negative, four were mixed).
However, in general, few studies of NbS include explicit monitoring of biodiversity outcomes and many of the current pledges for NbS (Table 2)  for governments, jobs for local communities and fibre, food or fuel resources, which also reduces the likelihood that the forest will be illegally cleared after establishment (Dave et al., 2019;Guariguata et al., 2019). However, they are not the only type of nature-based intervention that can deliver social benefits and thereby gain local support; those that conserve and restore natural habitats can deliver win-wins for biodiversity and people ).
Yet the imbalance in funding leaves few resources for protection or natural regeneration of diverse ecosystems (Heilmayr et al., 2020) and risks damaging biodiversity. There are concerns that low-diversity plantations of non-native species may be replacing important carbon-rich and biodiverse ecosystems including native forests (Curtis et al., 2018;Heilmayr et al., 2020;Scheidel & Work, 2018), ancient grasslands and savannahs Kumar et al., 2020), heather moorland and peat bogs (Brown, 2020;Friggens et al., 2020;Sloan et al., 2018). The 'Atlas of Forest Restoration Opportunities' that supports the Bonn Challenge identifies two billion hectares of 'deforested and degraded' land as potentially suitable for tree planting (Laestadius et al., 2011(Laestadius et al., , 2015 WRI, 2014) but this includes natural grasslands and savannahs that support endangered populations of large mammals (Veldman et al., 2019). Similarly, research in north-west India shows that although afforestation activities can lead to an aggregate increase in forest cover, in most cases it results in loss of diversity and promotes monocultures (Singh, 2006;Srivastava & Mehta, 2017).
Even NbS based on protecting or restoring natural habitats carry a risk that impacts (such as deforestation) could simply shift to unprotected areas to satisfy demand for food or livelihoods (Mekuria et al., 2015). Reforestation must therefore be accompanied by protection of nearby areas of intact forest, to avoid displaced deforestation (Heilmayr et al., 2020), especially as avoided deforestation offers 7.2-9.6 times as much potential low-cost climate change mitigation as reforestation overall (Busch et al., 2019).
There are cases where use of non-native species or modified species compositions may be beneficial (Harris et al., 2006); for example, if they are better adapted to current or future climates (Gray et al., 2011;Hewitt et al., 2011), if they establish more readily in harsh conditions (Yu et al., 2012) or if land is too degraded to restore to a natural state (Murcia et al., 2014;Suding et al., 2004).
If non-native species are being introduced, it is important to assess and mitigate the associated risks (Sáenz-Romero et al., 2016;Simler et al., 2019;Weeks et al., 2011). Even when restoration involves only native species, there can be trade-offs that need to be managed to enhance biodiversity outcomes, as some species may benefit at the expense of others (Biel et al., 2017;Porensky et al., 2014), or species abundance could increase at the expense of species richness (Lennox et al., 2011).
In summary, NbS need to be designed explicitly to demonstrate how they will deliver measurable benefits for biodiversity. Although the optimum strategy is case-specific, and much more research is needed, a good strategy is likely to involve choosing a diverse mix of native species where possible, avoiding destruction of existing species-rich habitats, conducting initial baseline assessments, setting quantitative targets, monitoring progress and managing any unintended negative consequences (IUCN, 2020). Understanding the spatial scales and timeframes over which nature-based interventions can deliver benefits for biodiversity as well as support climate change mitigation and adaptation should be a key focus for future research.

| One clear voice on successful, sustainable NbS
As nations and businesses begin to incorporate NbS in their climate and biodiversity strategies, it is crucial to reach a consensus on what constitutes successful and sustainable NbS. Practitioners and decision-makers need clear and coherent principles and standardized evidence-based frameworks (Cohen-Shacham et al., 2019).
This will enable NbS to be designed and implemented using the best evidence-based criteria and will allow commitments on NbS for both climate change and biodiversity to be aligned, tracked and improved over time.
To this end, we worked with a consortium of conservation and development organizations and research institutions to develop four high-level guidelines on how to develop successful NbS that avoid the pitfalls described in Section 6, which we sent to the President of the upcoming CoP26 (NbSI, 2020; Table 4 A key question is how to ensure compliance with these standards and guidelines. NbS should be subject to rigorous assessment and validation, including monitoring of multiple environmental, social and economic outcomes over the long term. However, companies have failed to comply with previous voluntary agreements (NYDF Assessment Partners, 2019), and this is likely to continue unless there is an independent regulator capable of enforcing these standards. Accountability and regulatory frameworks supported by government policy are essential to ensure NbS support transformational pathways, and this is an important area for further work.

TA B L E 4
Four high-level guidelines for successful, sustainable nature-based solutions agreed on by a large community of researchers and conservation and development practitioners in the UK (www.nbsgu ideli nes.info)

Guideline Context
Guideline 1: NbS are not a substitute for the rapid phase out of fossil fuels and must not delay urgent action to decarbonize our economies.
NbS play a vitally important role in helping to mitigate climate change this century, but their contribution is limited by a finite land area and is relatively small compared to what can be achieved by the rapid phase out of fossil fuel use. Furthermore, unless we drastically reduce GHG emissions, global heating will adversely affect the carbon balance of many ecosystems, turning them from net sinks to net sources of GHGs.
Guideline 2: NbS involve the protection and/or restoration of a wide range of naturally occurring ecosystems on land and in the sea.
All ecosystem types hold opportunities for NbS to enhance provision of ecosystem services to people. Management at the landscape scale, accounting for and utilizing interactions between ecosystems, can maximize long-term benefits. It is especially urgent to prevent inappropriate tree planting on naturally open ecosystems such as grasslands, savannahs and peatlands, or in areas with native forests. NbS must be valued in terms of the multiple benefits to people, rather than overly simplistic metrics such as numbers of trees planted. Robust social safeguards must be applied, to recognize, respect and reinforce human rights (including land/ecological and cultural rights), and support livelihoods. Just institutions will support larger scale, sustainable and more resilient NbS, at a crucial moment for the global response to climate change.
Guideline 4: NbS sustain, support or enhance biodiversity, that is, the diversity of life from the level of the gene to the level of the ecosystem.
Biodiversity plays a vital role in the healthy functioning and resilience of ecosystems. It secures the flow of essential services now and into the future, reduces trade-offs among them (e.g. between carbon storage and water supply) and helps to build human capacity to adapt to climate change in urban and rural areas.  (Neßhöver et al., 2013). Genuine collaboration between researchers and research users increases the legitimacy, ownership and accountability of the solutions (Mauser et al., 2013).
Second, NbS should be integrated into a multifunctional landscape (Kremen & Merenlender, 2018) or seascape approach that takes account of the interconnections between habitats and the needs of different beneficiaries. On land, depending on the local context, this might include a balanced mix of habitats, including sustainably managed working forests and farmland together with wetlands, grasslands, native forest, heath and scrub. A diversity of land uses supports a diversity of livelihoods and thereby provides more security of income during times of environmental or socio-economic stress. Spatial planning can target the right land use in the right place.
For example, the most biodiverse habitats could be integrated into a connected network that allows animal and plant species to shift their ranges in response to climate change (Brancalion & Chazdon, 2017;Lavorel et al., 2020). Sustainable agriculture and agroforestry could be prioritized on the most productive land while hydrological models could identify the optimum areas for new native woodland to reduce flood and erosion risk, avoiding naturally open habitats and organic soils. Where plantations are needed to meet demand for wood products, more sustainable forest management practices could reduce their adverse impacts, such as planting a mix of native species (which can be more productive than a monoculture), lengthening rotation times (Law et al., 2018), leaving strips of native vegetation and practising selective logging rather than clear-felling (Griscom, & Cortez, 2013;Hartley, 2002;Putz et al., 2012). In the marine context, planning of NbS needs to take into account the interdependencies between habitats. For example, the storm protection service of an interconnected reef-seagrass-mangrove seascape is greater than for a single coastal habitat on its own (Barbier & Lee, 2014;Sanchirico & Springborn, 2011).
Third, it is important to evaluate the full range of potential benefits, as well as actively identifying, managing and mitigating tradeoffs and conflicts in an equitable way. Focusing on a narrow range of benefits, such as carbon sequestration or timber production, can lead to avoidable adverse impacts such as biodiversity loss or water scarcity. Currently, few of the studies reporting adaptation outcomes of NbS also consider mitigation and broader social outcomes, and biodiversity outcomes in particular are often only implied or rudimentarily studied . Robust monitoring and evaluation of the multiple benefits of NbS across landscapes and societies demand a transdisciplinary approach to research that can capture environmental, economic and social impacts (Hoffmann et al., 2019;Scholz & Steiner, 2015) and this must be tailored to local value systems and perspectives (Sterling et al., 2017). Integrated valuation can promote more equitable and inclusive governance of NbS (Liquete et al., 2016;Pascual et al., 2017); scenario analyses can help to identify policies that minimize trade-offs (Metzger et al., 2017) and quantification of trade-offs can be used support participatory approaches for dealing with conflicts (King et al., 2015).
Finally, NbS should form part of an integrated sustainability strategy across sectors. They should not be seen as an alternative to technological solutions and must not be framed as a 'fix all' solution.
We need both nature-based and technological approaches to many of the challenges we face. For example, to build coastal resilience often requires a combination of nature-based and man-made flood defences (Vuik et al., 2016); to restore landscapes, mixing in productive 'nursery trees' for selective logging can provide an income source while native species regenerate (Amazonas et al., 2018;Brancalion et al., 2020). We have discussed how NbS must be accompanied by rapid reductions in fossil fuel emissions. And, critically, NbS will work best in the framework of a green and circular economy. Shifting to a circular economy with less waste, a more plant-based diet and less over-consumption of resources would free up land for carbon storage and biodiversity (Chaudhary et al., 2017;Poore & Nemecek, 2018;Strassburg et al., 2020). For example, more re-use and recycling of wood products would reduce the demand for wood from plantations and allow rotation lengths to be increased, with benefits for carbon sequestration (Hudiburg et al., 2019

| Mobilizing and targeting finance for sustainable NbS
There is a huge funding gap in investments in nature: for preserving and restoring ecosystems alone, the required investment is esti- To address these issues, a system needs to be in place to restrict verification of the benefits of investment in NbS as a carbon offset to those entities that meet stringent criteria for ambitious and verifiable emissions reductions through their operations and supply chains (Section 6.1). In addition, companies and banks should adopt standards for monitoring and evaluation of NbS, such as the IUCN Global Standard for NbS and the upcoming revision of the UN's System of Environmental Economic Accounting (SEEA); together, these can facilitate accounting and help ensure the quality of NbS, including encouraging the funding of multiple types of actions beyond tree planting (Section 7.1).
Since much of the funding currently available for NbS requires documented increases in carbon stocks, there is also much need for improved quantification of the GHG stocks and fluxes of a greater diversity of habitats, moving beyond the current metrics for tree planting and peatland restoration. By combining these data with high-quality information on biodiversity and the value of ecosystems for local communities, we can develop more granular metrics for assessing and verifying the return on NbS investments over time. Such metrics, together with a robust typology that clarifies the benefits and trade-offs of different NbS options for different social groups, will allow investors to identify appropriate projects, and help NbS practitioners identify willing funders.

Formation of intermediary bodies which help link good investors
with high-quality NbS projects (Freireich & Fulton, 2009) can also facilitate the transition to large-scale funding of successful, sustainable NbS.
Businesses have a critical role to play in creating a sustainable world where nature and people thrive together, but funding NbS is only one part of this role-fundamental changes in the functioning of businesses and the economy more broadly are also urgently needed. Governments can incentivize the sustainable management of resources through measures such as carbon and resource taxes, and regulation to reduce environmental externalities such as pollution while providing financial support for sustainable investments. Companies must adopt regenerative and circular economy models, and must appropriately embed natural capital into accounting procedures (Reed et al., 2007). Natural capital accounting aims to measure the extent and condition of ecosystems and their potential to provide services in years to come, not just the current flow of services, to ensure that natural capital stocks are not depleted by over-exploitation and habitat degradation. firms, governments and regulatory bodies, think tanks and consortia, the TNFD will publish guidelines on measuring and reporting dependences and impacts on nature (TNFD, 2020). Blended public-private finance can also support NbS, where governments underwrite the risk to companies of investing in unproven technologies. Achieving the transition to a sustainable economy will require unprecedented collaboration between private and public sector actors, economists, and NbS researchers and practitioners.

| CON CLUS ION
Nature-based solutions emerged from the major paradigm shift that took place in the late 2000s, that involved a move away from conserving nature for its own sake to conserving nature for people's sake, and from 'regarding people as passive beneficiaries of nature to active protectors and restorers' (Mace, 2014). A decade later, NbS could play a key role in enabling another and even more fundamental paradigm shift that is being 'fast-tracked' by the current coronavirus pandemic. This is the transformation of a destructive global economic model centred around GDP and infinite growth, that ignores nature's value to people and its intrinsic value, to one where a healthy economy is defined by the social and ecological well-being it brings (Raworth, 2017).
For NbS to support this transformation, it is vital to get the message right about what the concept of NbS comprises. Successful NbS are co-designed and implemented with local communities, to optimize the equitable delivery of multiple benefits and manage undesirable trade-offs. They are biodiversity-based and explicitly designed to deliver biodiversity benefits, and occur as part of a holistic framework of sustainability policies including the rapid phase out of fossil fuels. For NbS to be part of a 'just transition', we need to challenge the structural features and inequities of human society which drive biodiversity loss and climate change, and hold companies and governments to account for the environmental and social damage they cause or permit. To implement NbS at scale and avoid simply displacing environmental impacts, land must be freed up from other uses, through a shift towards plant-based diets and widespread adoption of a circular economy to reduce demand for raw materials. By following these guidelines, we can design robust and resilient NbS that address the urgent challenges of climate breakdown and biodiversity loss, sustaining nature and people together both now and into the future. Woroniecki for his help with Figure 1, and for comments on the manuscript. Finally, we are most grateful to Robin Chazdon, Beatriz Luz and an anonymous reviewer for comments that helped to improve this paper.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing not applicable to this article as no datasets were generated or analysed or analysed during the current study.