This includes using observational data and two sets of computer simulations, one that models the world as it is, about 2 degrees Fahrenheit (1.1 degrees Celsius) warmer than it was before widespread emissions began in the late 19th century, and a hypothetical world in which global warming never happened.
The finding that the likelihood of such an extreme rain event has increased with global warming is consistent with many other studies of individual events and broader trends. A major reason for the increase is that as the atmosphere warms, it can hold more moisture.
The study noted that from a meteorological perspective, a storm that has a 1-in-20 chance of occurring in any given year, while not common, is hardly a rare event. So the researchers looked at other factors that could have contributed to the disaster’s high toll in deaths and damage.
Among these, they wrote, were legacies of policies instituted during the apartheid era. In 1958, for example, the Durban City Council adopted a measure that forced nonwhites into less desirable and, in many cases, more flood-prone, areas.
The researchers also cited the rise of makeshift settlements as a result of rapid urban growth and a lack of affordable housing. About 22 percent of Durban’s population, or 800,000 people, live in such settlements, which usually lack services and proper infrastructure. In the April flooding, the study noted, about 4,000 of the 13,500 houses that were damaged or destroyed were along riverbanks in these types of settlements, and most of the deaths were in these areas as well.
“Again we are seeing how climate change disproportionately impacts the most vulnerable people,” said Friederike Otto, a founder of World Weather Attribution and a climate scientist at Imperial College London.