Scientists said Monday that human-caused warming has made days of intense, downpours like those that recently caused devastating floods in southern Brazil, killing at least 172 people and forcing the evacuation of more than half a million residents.
In the southern state of Rio Grande do Sul, three months' worth of rain fell in a two-week period between April and May. After analyzing weather records, scientists estimated that there is a 1% chance each year that this much rain will fall in such a short period in the region. Such downpours would have been much rarer in the cooler climate of the 19th century, before massive greenhouse gas emissions began, the researchers say.
Southern Brazil is one of the country's wettest regions. As the world warms, pockets of high pressure that form along South America's Atlantic coast are becoming larger and lasting longer, pushing more warm, moist air south, where it can fall as rain.
When the recent rains hit, Rio Grande do Sul state was still recovering from floods late last year that killed at least 54 people. Three of the four biggest floods recorded in the state capital, Porto Alegre, occurred in the past nine months, said Regina Rodrigues, a professor of ocean physics at Santa Catarina Federal University and one of the scientists involved in the analysis.
“Rio Grande do Sul has experienced major floods before, but these are becoming more intense and widespread,” Dr Rodrigues told a news conference.
The report was produced by World Weather Attribution, an international scientific collaboration that studies the impact of human-induced climate change on extreme weather events. Its analysis of the Brazilian floods was based on widely accepted methodologies but has not been peer-reviewed.
To conduct their analysis, the researchers looked at historical records of four- and 10-day rainfall in the state of Rio Grande do Sul. They used a global climate model to estimate how the probability of high rainfall events like this year's has changed over the past two centuries amid increasing human-induced warming.
They estimated a large increase in the probability of occurrence, which is consistent with a fundamental finding about climate change and precipitation: Warmer air can hold more moisture, so whenever it rains, it's more likely to be intense.
The researchers found that another contributing factor to the recent flooding is El Niño, a cyclical weather pattern in the Pacific Ocean that tends to create a wide area of ​​high pressure over central Brazil. These systems bring moist air south from the tropics, bringing heavy rains to southern Brazil, Uruguay and northern Argentina.
Rain is no stranger to devastating disasters in Brazil: In 2011, floods in Rio de Janeiro state killed nearly 1,000 people and forced tens of thousands to evacuate. That tragedy prompted the Brazilian government to set up a center to monitor and issue early warnings about natural disasters.
This year, the center warned of possible flooding in Rio Grande do Sul almost a week before the rains set in. Still, it's unclear how many people received the warning or how well they understood the danger and how to respond, said Maja Varberg, a risk consultant at the Red Cross Red Crescent Climate Center who contributed to the analysis.
“Early warning is very important, but it's not enough,” Dr Rodriguez said.
Floods damaged roads, bridges, airports and power lines. They destroyed harvests of soybeans, rice and wheat that are vital to Brazil's food supply and economy. Schools were closed for weeks, leaving hundreds of thousands of students unable to attend class.
Experts and business groups estimate that damages could reach more than $30 billion, making it Brazil's costliest disaster on record. Some economists have compared the floods to Hurricane Katrina, which caused similar economic losses on the U.S. Gulf Coast relative to the size of the U.S. economy.
Scientific studies have warned for years that climate change will bring more intense rainfall and flooding to southern Brazil, but politicians are still struggling to accept and address future climate risks, said Natalie Unterstel, director of the Brazilian climate policy research institute Talanoa.
“A willingness to listen to scientific information has not translated into decisions and investments based on long-term considerations,” she added.
