How Did Cape Town (Almost) Get to Day Zero?

Nature Climate and Atmospheric Science published a paper written by AOES assistant professor Natalie Burls and colleagues on worsening droughts in South Africa. Coauthors of the paper include AOES scientists Ben Cash, Erik Swenson, Abdullah al Fahad, and David Straus. The paper grew out of discussions around the department’s monthly Current Climate Conversations.

In 2018, drought brought Cape Town (Dr. Burls previous home) perilously close to “Day Zero”, meaning that reservoir levels would fall so low that the city would have to shut off water flowing to residents’ taps. Burls and coauthors examined multi-decadal meteorological records to examine whether the unprecedented flood was part of a long-term trend. They found a subtle, but troubling, pattern.

Much of the rain in the Cape Town region is associated with weather fronts, relatively sharp gradients in temperature often associated with stormy weather. While the total number of rainy days shows a clear decline since 1980 (see figure, purple data points), the number of days on which a front occurred (black data points) did not show a statistically significant decline. Nevertheless, the number of days with both a front and rain did also have a statistically significant decrease. The increase may be due to fronts generating less rainfall than in previous decades.

Burls et al. link the changing condition to an expanding Hadley Cell. The Hadley Cells are large-scale wind patterns in which air rises near the equator and falls a few thousand kilometers to the north and south. The downwelling parts of the Hadley Cells account for the presence of desert regions in subtropical regions. Expansion of the Hadley Cells is predicted as a side effect of global heating due to society’s emissions of carbon dioxide and other gases. Thus South Africa may be showing signs of desertification due to human-caused climate change. If that is the case, Cape Town’s 2018 approach to Day Zero may not be its last.