Monday, August 6, 2018

Climate Change and Cape Town

Cape Town is South Africa’s second largest city. This past spring after three years of persistent drought, the city was almost out of water. The government was forced to limit water use to 50 liters per person per day (that's 13 gallons a day) in hopes of preserving water supply until the rains or be forced to turn off most of its taps to preserve water for hospitals and other essential and urgent needs.

In fear of what the South Africans called "Day Zero” – when the city will no longer have running water, the citizens cut back. The rainy season this winter (recall that Cape Town is in the Southern Hemisphere) started early in their winter and fell at rates closer to the long term average than in the previous three years. The city’s dams are reported to be half full. Cape Town officials hope to relax water restrictions in the next few months. Residents of the Washington DC metropolitan area use about 300 liters of water per day per person- personally it is hard to imagine going months using one sixth of my usual amount of water.

The early winter rains easing the drought have given local officials and politicians the time they need to review the water augmentation schemes launched at the height of the drought crisis to establish the best mix. City of Cape Town Deputy Mayor Ian Neilson said in the South African “News24”: "Now that we have navigated our way through the immediate drought crisis, it is necessary that we review our water supply strategy and augmentation plans to ensure that what was devised in a time of crisis is appropriate for longer-term sustainability and resilience."

According to the city government the water crisis appears to have been caused by a combination of climate change- shifting weather patterns and city mismanagement. Municipal water comes primarily from surface reservoirs that rely entirely on rainwater, and were designed to withstand up to three years of lower-than-average rainfall. But with the growing population and a reported increased likelihood of drought, the city needs to have major backup or augmentation resources.

World Weather Attribution (WWA) says that their computer model using proxy data found that changing climate has made the recent drought three times more likely than in the past. WWA is a four year old international effort designed to sharpen and accelerate the scientific community’s ability to analyze and communicate the possible influence of climate change on extreme-weather events such as storms, floods, heat waves and droughts. WWA seeks to identify the human fingerprint on individual extreme-weather events. This has been the goal of the scientific community for more than a decade. Right now, studies of the attribution of extreme events such as those in the Bulletin of the American Meteorological Society (BAMS) take months to complete and are published long after the event.

A team brought together by WWA used the available southern Africa rainfall records to run several climate models. Rainfall records from Cape Town do not go back very far. The oldest rainfall records in the western cape region of Africa go back to 1930, but most are not that old. The WWA team used the data they had to run their climate models without the 1 degree C increase in temperature the earth has experienced since 1900 and then with the 1 degree C increase. The team concluded that changing climate tripled the risk of such a severe drought and if the planet warms a further degree it will triple again.

Cape Town needs to make its water supply sustainable and resilient to survive more frequent and longer droughts. With the increased demand from a growing population and economy, and with climate change models predicting a drier Cape Town, the City has realized that it cannot rely only on rainfall for future water supply.

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