Increasing risk of another Cape Town "Day Zero" drought in the 21st century
Significance The Cape Town "Day Zero" drought was caused by an exceptional 3 y rainfall deficit. Through the use of a higher resolution climate model, our analysis further constrains previous work showing that anthropogenic climate change made this event five to six times more likely relative to the early 20th century. Furthermore, we provide a clear and well supported mechanism for the increase in drought risk in SSA through a dedicated analysis of the circulation response, which highlights how as in 2015 2017 a reduction in precipitation during the shoulder seasons is likely to be the cause of drought risk in southwestern South Africa in the 21st century. Overall, this study greatly increases our confidence in the projections of a drying SSA. Three consecutive dry winters (2015 2017) in southwestern South Africa (SSA) resulted in the Cape Town "Day Zero" drought in early 2018. The contribution of anthropogenic global warming to this prolonged rainfall deficit has previously been evaluated through observations and climate models. However, model adequacy and insufficient horizontal resolution make it difficult to precisely quantify the changing likelihood of extreme droughts, given the small regional scale. Here, we use a high resolution large ensemble to estimate the contribution of anthropogenic climate change to the probability of occurrence of multiyear SSA rainfall deficits in past and future decades. We find that anthropogenic climate change increased the likelihood of the 2015 2017 rainfall deficit by a factor of five to six. The probability of such an event will increase from 0.7 to 25% by the year 2100 under an intermediate emission scenario (Shared Socioeconomic Pathway 2 4.5 [SSP2 4.5] and to 80% under a high emission scenario (SSP5 8.5) These results highlight the strong sensitivity of the drought risk in SSA to future anthropogenic emissions.
Proceedings of the National Academy of Sciences