Picture of a cumulonimbus cloud from recent Cape Town thunderstorm

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I was asked to write a short piece for an independent Russian (!) media outlet—to be translated—to contribute to answering the title question from a climatological perspective. I’ve been asked to post this to the blog here, having made adapted it slightly for a more South African audience:

It is important to distinguish between the water crisis experienced in Cape Town and drought conditions over the Winter Rainfall Zone (WRZ) of South Africa, within which Cape Town and its water supply dams lie. Clearly drought and the water crisis are closely related. Climate data are my personal area of interest, so I’ll look first at that aspect.

Cape Town and its immediate surroundings experience a Mediterranean-type climate, receiving most of its rainfall from cold fronts which extend northwards from the Southern Ocean and move over the WRZ in winter (generally April-October). Since Spring 2014 (August-October), the WRZ has seen fewer rainy days in the winter half-year, which also on average have yielded less rainfall per event (based on analysis by the Agricultural Research Council (ARC), presented in their Umlindi newsletter, and independent research by Piotr Wolski). The ARC (in the Umlindi newsletters) suggests that this has been due to fewer land-falling frontal systems impacting the WRZ, which have also tended to be less intense. This is not in itself unprecedented, however, as multi-year drought periods and wet periods occur relatively frequently in the WRZ; in fact, natural “quasi-cyclical” variability has resulted in previous droughts sometimes lasting for 11 years in some regions (1926-1936), while decade long periods that are mostly wetter (e.g., 1950s, 1980s, 1990s) or drier (e.g., 1890s, 1970s, 2000s) than normal are common. However, although the severity of the meteorological drought (there are various types of drought) is not spatially uniform, the period between October 2016 and September 2017 (was the driest such period in about 100 years of record keeping at most places that have been assessed. In their response to Helen Zille, the South African Weather Service conclude that 2017 was the driest year over a region including Cape Town and its main supply dams dams. While 2016 was 14th driest, 2015 was the 2nd driest. This is consistent with Piotr Wolski’s assessment, which found 2015-2017 to be the driest 3-year period in the dam catchment area in records going back to the 1930s and that such periods would naturally tend to occur only about once every few hundred years.

So, is climate change an important cause of the drought? This is a very difficult part of the question. Many politicians (including the mayor, premier and deputy-president) have said that the drought is “clearly” a consequence of climate change. Climate models suggest that climate change should lead to drying across the Mediterranean climate zones of the SH, and this is already apparent in southern and south-western Australia. When looking at data from the last 40-years or so, during which there are satellite data to help understand the causes of rainfall variability and change, it seems clear that the WRZ has experienced significant drying. If one looks at the longer-term (more difficult, for many reasons), most of the WRZ has seen slight wetting, not drying. It becomes clear that the apparent recent drying is part of long-term natural variability, referred to previously. However, if one looks only at the mountainous region from which the dams are fed, the pattern is a bit different, showing slight drying even in the long-term. It is not clear why there are these different patterns, or whether either is actually meaningful. It means we really don’t know whether and to what extent climate change is influencing the current drought; more research is clearly needed.

There is also a complex interplay between climate and other drivers of the water crises; for example, both agricultural and formal residential water consumption tend to increase during periods of low rainfall and high temperatures, as happened during the first summer of the drought (2014/15), before water restrictions were imposed, when water consumption was about 2x as high as now. In fact there were two summers with normal to above normal usage after the start of the drought. Different data sources are not entirely consistent, but average water consumption in Cape Town has risen only slightly since around 2000, due in part to the city management’s programs to limit consumption. Since 2002, the city has been warned (by engineering consultants, among others) that it would require additional water sources to be able to ensure consistent supply to the expanding metropole. In response, a new dam was built in 2009. However, in part because usage by other towns drawing water from the same source and agricultural usage were increasing more quickly, the city was warned that new sources would probably be needed before 2020 (the exact time differs by source). Serious work on developing such sources has only started towards the end of 2017, it seems. Consequently, different levels of government (controlled by different political parties) have been blamed for the water crisis, as have researchers and forecasters, farmers, residents, poor communities and urbanisation. This article does a reasonable job of discussing these claims, although their take on the farming component is rather one-sided.

In summary: this drought is very severe, has lasted longer than 3 years, but is not necessarily strongly linked to climate change. There are political, social and infrastructure management components to the water crisis, which are also complex. It is unlikely that Cape Town is unique in its exposure to these risk factors. The crisis serves as a reminder of our vulnerability to natural rainfall variability, which may in many places be exacerbated by climate change.

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