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Human-caused warming has doubled the chances that southern Brazil will experience extreme, multiday downpours like the ones that recently caused disastrous flooding there, a team of scientists said on Monday. The deluges have killed at least 172 people and displaced more than half a million residents from their homes.
Three months’ rain fell in a two-week span of April and May in the southern state of Rio Grande do Sul. After analyzing weather records, the scientists estimated that the region had a 1 percent chance each year of receiving so much rain in so little time. In the cooler climate of the 19th century, before large-scale emissions of greenhouse gases, such colossal downpours were far rarer, the researchers said.
Brazil’s south is one of the country’s rainiest regions. As the world gets warmer, the areas of high atmospheric pressure that occasionally form over the Atlantic coast of South America are becoming larger and longer lasting. That pushes more warm, moist air toward the south, where it can fall as rain.
When the latest rains hit, Rio Grande do Sul was still recovering from floods that killed at least 54 people late last year. Three of the four largest floods ever recorded in the state’s capital, Porto Alegre, have now occurred in the past nine months, said Regina Rodrigues, a professor of physical oceanography at the Federal University of Santa Catarina and one of the scientists who worked on the new analysis.
“While significant floods have occurred in the state of Rio Grande do Sul in the past, they are becoming increasingly strong and widespread,” Dr. Rodrigues said at a news conference.
The report was produced by World Weather Attribution, an international scientific collaboration that examines the influence of human-induced climate change on extreme weather. The analysis of the floods in Brazil has not undergone academic peer review, though it relies on widely accepted methods.
To conduct their analysis, the researchers looked at historical records of four-day and 10-day rainfall events in Rio Grande do Sul. They used models of the global climate to estimate how the probability of rain levels as high as the ones experienced this year had changed over the past two centuries as human-induced warming took place.
They estimated a significant increase in likelihood, which is consistent with a basic finding about climate change and rainfall: Because warmer air can hold more moisture, whenever it rains, it is likely to rain more intensely.
Another factor behind the recent flooding, the researchers found, was El Niño, the cyclical weather pattern in the Pacific. El Niño tends to cause large areas of high atmospheric pressure to form over central Brazil. These systems channel moist air southward from the tropics, leading to heavy rains in southern Brazil, Uruguay and northern Argentina.
Brazil is no stranger to rain-driven catastrophes. In 2011, floods in the state of Rio de Janeiro killed almost 1,000 people and displaced tens of thousands. The tragedy prompted Brazil’s national government to create a center for monitoring natural disasters and issuing early warnings.
This year, the center warned of potential flooding in Rio Grande do Sul almost a week before the rains started. Still, it’s unclear how many people the warnings reached, or how well people understood the dangers and how to respond, said Maja Vahlberg, a risk consultant for the Red Cross Red Crescent Climate Center who also contributed to the new analysis.
“The early warning is very important, but it’s not sufficient,” Dr. Rodrigues said.
The floods damaged roads, bridges, airports and transmission lines. They ruined soy, rice and wheat crops, which are essential to Brazil’s food supply and economy. Schools closed for weeks, leaving hundreds of thousands of students out of the classroom.
Experts and business groups have estimated that the damages could add up to upward of $30 billion, which might make it Brazil’s costliest disaster on record. Some economists have compared the floods with Hurricane Katrina, which caused comparable financial losses to the U.S. Gulf Coast, relative to the size of the American economy.
Years of scientific studies have warned that climate change would lead to more intense rainfall and flooding in southern Brazil. But politicians still struggle to accept and act on future climate risks, said Natalie Unterstell, the head of Talanoa, a climate policy research institute in Brazil.
“The willingness to listen to scientific information hasn’t translated into decision and investments based on long-term considerations,” she added.
