Global evidence on scaling nature-based solutions for compound hazards

Abstract

Nature-based solutions (NbS) are increasingly promoted for climate adaptation to compound hazards, but systematic evidence of their effectiveness is still scarce. We address this gap with a systematic review of 38 studies. Each study was sorted by what its measurement design can support, not by what its authors claim. This produced four groups. Direct studies attribute a measured hazard reduction to the intervention, proxy studies infer protection from ecosystem condition, prioritization studies map suitable locations without estimating performance, and qualitative studies describe effectiveness without measuring it. We detect trends in geographic distribution, hazard combinations, intervention types, methodological approaches, and strength of evidence. Six studies directly measured attributable hazard reduction, and the remaining 32 studies fell into the proxy, prioritization, or qualitative classes, none of which can separate an ecosystem that buffers a hazard from one that receives it. The evidence concentrates on coastal flooding, with urban heat second. The most common hazard combination, compound drought and heat, yielded no direct evidence of hazard reduction, and only one of the five studies addressing cascading hazards did so. Research priorities include direct assessment of vegetation-based NbS under compound drought and heat, attributable measurements for cascading hazards, and long-term monitoring of hazard outcomes. As NbS are scaled, confidence in them should be scaled with the strength of the evidence base.