Climate changes are affecting the water balance in the world's largest cities, shows a fresh report from the organization Wateraid, which has gone through the development over the last 42 years.
In many places, floods risk knocking out sewage systems and contaminating drinking water, while in other places, droughts risk knocking out water supply.
"Tossed back and forth"
In 17 of the surveyed cities, which consist of the world's 100 largest and twelve additional ones, people are now more often affected by both.
Over the last 20 years, both extremes have become more common than in the previous 20-year period. When it also often goes quickly from long periods of drought to heavy rainfall, it is extra difficult to protect oneself. Researchers describe the phenomenon as a climate whip effect.
People living there are literally tossed back and forth between two extreme states, says Katerina Michaelides, professor of hydrology at the University of Bristol and one of the report authors.
They can have periods of extreme flooding followed by extreme drought – and there is basically nothing in between, she says.
Stockholm is the only Swedish city that participated in the study and is not one of the affected cities. Yet.
Stockholm is one of the cities that are developing towards a whip effect. In the coming years, it may reach the criteria, she says.
Some cities winners
Among the affected cities are, for example, Canberra, Bangkok, and Dallas.
In East Africa, Nairobi, Addis Ababa, and Kampala are affected. There, drought has led to water and electricity shortages. Then, large rainfall amounts have poured in and overloaded the cities' sewage systems. This has led to floods that have forced people to flee, damaged roads, and spread waterborne diseases.
As usual, it is the poorest, often living in slum areas, who are affected the worst.
This means enormous challenges since people in slums often have unclean water and are the first to feel the effects, says Michaelides.
Eleven cities in the world are somewhat winners in the wake of climate change. For them, the effect is the opposite, with the risk of flooding and drought decreasing.
What I think is surprising is how different it looks. There is no single way to characterize how the climate expresses itself worldwide, she says.
When human emissions of greenhouse gases raise the temperature on Earth, the atmosphere can hold more moisture. This tends to draw moisture from land areas. If the winds blow away the air masses, the moisture can fall as rain somewhere else, and thus lead to both drought and heavy rainfall.
Since several climate and weather systems (such as winds and seasonal variations) interact, the global warming hits very differently, with large regional differences as a result.
Wateraid has, with the help of researchers, investigated how wet and dry months the world's 100 largest cities, as well as twelve other cities where Wateraid is active, have been over the 42 years up to 2023.
15 percent of the cities are affected by both more drought and more flooding.
20 percent experience a change from dry to wet, or vice versa.
Nearly ten percent of the cities have got a more stable climate, with fewer extreme weather conditions. These include, among others, Nagoya and Tokyo in Japan, as well as London.
