Nature-Based Solutions in Mountain Catchments Reduce Impact of Anthropogenic Climate Change on Hydrological Drought Severity

Attributing the extent to which Nature-based Solutions (NbS) can offset emerging risk from anthropogenic climate change is a test of effectiveness for climate change adaptation, but has rarely, if ever, been done. Here we show that a widely applied NbS in South Africa – invasive alien tree clearing – can reduce the impact on hydrological drought severity due to human influence on climate, using the Cape Town “Day Zero” drought as an example. Using a multi-model “joint-attribution” framework of coupled climate and landscape states, we find that 12–29% of the hydrological drought severity can be attributed to anthropogenic climate change, compared to 7–15% for reduced rainfall and 1.6-2% for increased reference evapotranspiration. Attributable human influence on meteorological drought was amplified by 5–14% as it propagated through the hydrological system. Clearing invasive alien trees and preventing spread significantly reduced the attributable human influence on hydrological drought severity (2–15% for moderately and 10–27% for fully invaded catchments) but was not sufficient to remove the climate change impact completely. The relative contribution of NbS to drought severity reduction will decrease as climate change progresses. NbS will continue to be important but should be combined with other adaptation options to manage increasing hydro-climatic risk.