A global study on NbS effectiveness for climate change adaptation

Collecting evidence on the effectiveness of different types of NbS for climate change adaptation

Many countries’ intentions to implement NbS for climate change adaptation have not yet been fully translated to actions on the ground, partly due to the limited evidence available on the effectiveness of different types of NbS compared to conventional adaptation measures (Seddon et al., 2020). A large number of NbS projects have been conducted in various sites worldwide, and a great deal of scientific evidence can be obtained by compiling and analysing the outcomes of these individual NbS projects in terms of their effectiveness.

A global analysis of NbS project outcomes

A recent study led by Oxford University (Chausson et al., 2020) systematically reviewed 376 peer-reviewed articles which analysed the effectiveness of nature-based interventions on climate impacts. The authors compiled and analysed the results of these 376 individual articles, with the objective of providing evidence and guidance for policymakers and scientists on the effectiveness of NbS for climate change adaptation. This work represented the largest global study of NbS effectiveness to date.

Climate change adaptation impacts of NbS

Figure 2 shows the main results of the study in terms of the effectiveness of NbS in addressing several climate change impacts, including: loss of food production, loss of biomass cover, freshwater flooding, loss of timber production, soil erosion and reduced water availability.

Multiple climate change impacts were reported for 30% of the cases of NbS (n = 89). From these cases, the co-benefits and trade-offs of NbS on climate change impacts were analysed. Of these 89 cases, 68 reported adaptation co-benefits of interventions (i.e. NbS had positive effects on multiple climate impacts, and no negative/mixed effects), while 21 reported trade-offs between climate impact outcomes (i.e. positive effect on one climate impact and negative or mixed effect on another climate impact). Most effectiveness assessments reported that NbS implemented in natural or semi-natural ecosystems reduced multiple climate change impacts (thus supporting people's adaptation to climate change). In non-natural ecosystems, however, interventions involving ecosystem creation (often using non-native species to reduce soil erosion and/or increase biomass cover) were sometimes associated with reduced water availability (which could be a negative impact in areas with water shortage).

Mitigation, social, and ecological impacts of NbS

Figure 3 shows the main results of the study in terms of the non-adaptation related co-benefits (positive effects) and trade-offs (negative effects) found for these NbS. As most of the NbS involve conserving or enhancing plant biomass cover, the large majority were found to have greenhouse gas mitigation benefits. Most NbS also led to positive social and ecological outcomes. Notably, no cases of purely negative social outcomes were identified in the study (although some reported mixed results). Negative or mixed results for ecological outcomes were found in a few cases, mainly for interventions involving creating new ecosystems (sometimes using non-native species).

Knowledge gaps on the effectiveness of NbS for climate change adaptation

The study found that the number of studies on NbS effectiveness was rapidly increasing in recent years, but most studies were being conducted in high (n = 213) and upper middle income countries (n = 103), despite the great potential (and traditional use) of NbS in developing countries. Thus, further studies on the effectiveness of NbS in developing countries are needed to strengthen the evidence base.

Main points for planners and policymakers

• NbS have shown to be effective in reducing climate change impacts on people in the majority of cases found in the scientific literature, highlighting their potential as a strategy for climate change adaptation.
• NbS implemented in natural or semi-natural ecosystems (e.g. through activities to protect, sustainably manage, or restore these ecosystems) tend to have few negative impacts, while NbS involving creating new ecosystems (e.g. through afforestation) may entail trade-offs, particularly reduced water availability.
• The co-benefits of NbS in terms of GHG mitigation, social impacts and ecological impacts largely outnumber the trade-offs, so NbS can generally be counted on to provide multiple benefits in addition to climate change adaptation. This may make them preferable over alternative adaptation options involving hard infrastructure.