The unusually intense, prolonged and extensive flooding that devastated southern Brazil was at least twice as likely to have been caused by human burning of fossil fuels and trees, a study has shown.
The record disaster killed 169 people, destroyed homes and ruined harvests, and was worsened by deforestation, investment cuts and human shortages.
The international team of scientists behind the research predicted that disasters of this scale – the worst to hit the region – will become more common in the future unless there is a sharp reduction in planet-warming greenhouse gas emissions.
Hundreds of thousands of people in Rio Grande do Sul state and nearby Uruguay are still trying to rebuild their lives after a month of torrential rains that displaced 80,000 people and left more than a million without basic services such as electricity and drinking water.
At the peak of the rains on May 1, the city of Santa Maria broke the 24-hour rainfall record of 213.6 mm. The state’s capital, Porto Alegre, was submerged in two months’ worth of rain in just three days, with roads turning into rivers, football stadiums into lakes, and the city’s international airport so badly damaged that it remained closed.
The economic cost is expected to exceed $1 billion (£780 million) and the severe impact on agriculture is expected to increase the prices of rice (Rio Grande do Sul typically produces 90% of Brazil’s crop) and dairy products across the country.
The cost of the region’s focus on agriculture was high. In recent years, natural flood protection areas such as riverside forests and swamps have been converted into plantations, often in violation of poorly enforced environmental regulations, the study’s authors said.
The disaster in Porto Alegre was worsened by poor flood defense systems that were supposed to withstand 6 meters of water but reportedly began to fail at 4.5 metres.
In recent years, municipal governments have cut investment in these protections, despite warnings that this low-lying, deforested area at the confluence of five major rivers would become increasingly vulnerable to flooding as a result of man-made climate disruption. In addition to failing to stop rising waters, the state capital’s flood barriers also trapped floodwaters, slowing the drying and recovery process.
Scientists from the World Weather Attribution group confirmed strong human influence in the floods, which were the fourth to hit Brazil’s southernmost state in the last year and a half.
They analyzed four-day and 10-day flood periods by combining weather observations with results from climate computer models. They found that human-caused climate change has increased the likelihood of extreme precipitation by two to three times, making it about 6% to 9% more intense. This effect was similar to the natural effect of the El Niño phenomenon.
In addition to increasing the frequency and intensity of heavy precipitation, global warming has pushed the tropics further south; This region acts as a wall in the middle of Brazil, blocking cold fronts from Antarctica. As a result, flooding that was once more common north of Santa Catarina is now more likely to occur in Rio Grande do Sul. More than 90% of the state, covering an area the size of the United Kingdom, was affected.
Global warming is also increasing the frequency of El Niño and La Niña events, which are linked to extreme weather conditions, the authors said. There hasn’t been a neutral year in the last decade without either.
Looking at a world that will get hotter as a result of emissions from cars, factories and deforestation, they found that if global warming increased from the current level of 1.2, such a disaster would be 1.3 to 2.7 times more likely in Rio Grande do Sul. C to 2C, which is increasingly likely. “These events will become more frequent and severe,” the article concludes.
Lincoln Alves, a researcher at Brazil’s National Institute for Space Research, said the climate in Brazil has already changed: “This attribution study confirms that human activities are contributing to more intense and frequent extreme events and highlights the country’s vulnerability to climate change. “It is very important that decision makers and society recognize this new normal.”
To minimize the potential impact of future disasters, the authors recommend more comprehensive urban planning, greater investment in flood defenses and greater attention to equitable social development; because flooding can create a “poverty trap” with low-income communities in the most vulnerable areas.
Researcher Regina Rodrigues from the Federal University of Santa Catarina said priorities should be to protect and strengthen natural barriers such as forests and swamps. “Land use changes have directly contributed to widespread flooding by removing natural protection and may exacerbate climate change by increasing emissions.”
But as always, the most important measure is to rapidly reduce the burning of trees and fossil fuels, which are causing more and more carnage around the world.