Human-caused warming has doubled the odds that southern Brazil will experience extreme, multi-day downpours like those that recently caused disastrous floods, a team of scientists said Monday. The floods have killed at least 172 people and forced more than half a million people to flee their homes.
Three months of rain fell in the span of two weeks in April and May in the southern state of Rio Grande do Sul. After analyzing weather data, scientists estimated that the region had a 1% chance each year of receiving this much rain in such a short time. In the cooler climate of the 19th century, before large-scale greenhouse gas emissions, such colossal downpours were much rarer, the researchers said.
Southern Brazil is one of the rainiest regions in the country. As the world gets warmer, the areas of high atmospheric pressure that occasionally form on the Atlantic coast of South America are becoming larger and more long-lasting. This pushes warmer, moister air southward, where it can fall as rain.
When the last rains arrived, 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 capital, Porto Alegre, have 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 project . new analysis.
“Although significant flooding has occurred in the state of Rio Grande do Sul in the past, it is becoming increasingly stronger and more widespread,” Dr. Rodrigues told a news conference.
The report was produced by World Weather Attribution, an international scientific collaboration examining the influence of human-induced climate change on extreme weather. The analysis of floods in Brazil has not been subjected to academic peer review, although it refers to widely accepted methods.
To conduct their analysis, the researchers examined historical records of four- and 10-day rainfall events in Rio Grande do Sul. They used global climate models to estimate how the probability of high rainfall levels like those experienced this year has changed over the past two centuries due to human-induced warming.
They estimated a significant increase in probability, which is consistent with a basic finding about climate change and precipitation: Because warmer air can hold more moisture, whenever it rains, it is likely to rain harder.
Another factor behind the recent floods, 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 funnel moist air from the tropics southward, causing heavy rain in southern Brazil, Uruguay and northern Argentina.
Brazil is no stranger to rain disasters. In 2011, floods in Rio de Janeiro state killed nearly 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 nearly a week before the rains began. However, it's not clear how many people received the warnings, or how well people understood the dangers and how to respond, said Maja Vahlberg, a risk advisor for the Red Cross and Red Crescent Climate Center who also contributed to the new analysis .
“Early warning is very important, but it is not enough,” Dr. Rodrigues said.
Floods damaged roads, bridges, airports and transmission lines. They ruined soy, rice and wheat crops, essential to Brazil's food supply and economy. Schools closed for weeks, leaving hundreds of thousands of students out of classrooms.
Experts and business groups have estimated that damages could amount to more than $30 billion, which could make Brazil the biggest disaster. costliest disaster ever recorded. Some economists they compared the floods to Hurricane Katrina, which caused financial losses comparable to the U.S. Gulf Coast, relative to the size of the American economy.
Years of Scientific studies have warned that climate change will lead to heavier rainfall and flooding in southern Brazil. But politicians still struggle to accept future climate risks and act accordingly, said Natalie Unterstell, director of Talanoa, a climate policy research institute in Brazil.
“The willingness to listen to scientific information has not translated into decisions and investments based on long-term considerations,” he added.
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