Global warming pollution has doubled the likelihood of the extreme levels of rain that hit central Europe in September, according to a study.
Researchers have found that four days of heavy rain that caused deadly floods in countries from Austria to Romania was exacerbated by global warming.
A study by World Weather Association (WWA) found that climate change has increased rainfall by at least 7%, resulting in the town being inundated with water at half the rate that would have been the case if humans hadn’t been warming the planet.
“The trend is clear,” said Bogdan Chojnicki, a climate scientist at Poznan University of Life Sciences and co-author of the study. “The situation will become even more severe if humanity continues to fill the atmosphere with fossil fuel emissions.”
Storm Boris stalled over Central Europe in mid-September, dumping record-breaking rainfall on Austria, the Czech Republic, Hungary, Poland, Romania and Slovakia. The heavy downpour turned calm streams into raging rivers, causing floods that destroyed homes and killed more than 20 people.
The researchers said adaptation measures had reduced the death toll compared to similar floods that hit the region in 1997 and 2002. They called for improvements in flood prevention, warning systems and disaster response plans, and warned against continued rebuilding in flood-prone areas.
Storm Boris hits northern Italy, causing severe flooding – VIDEO
“These floods show how costly climate change can be,” said Maja Varberg, technical adviser at the Red Cross Red Crescent Climate Centre and co-author of the study. “Despite days of preparation, the floods devastated towns, destroyed thousands of homes and prompted the European Union to pledge 10 billion euros in aid.”
Rapid attribution studies, which use established methods but are published before going through a lengthy peer review process, look at how human influences affect extreme weather events in the immediate aftermath of a disaster.
The scientists compared rainfall recorded in Central Europe over four days in September with rainfall simulated for a world 1.3 degrees cooler – the level of warming caused to date by burning fossil fuels and destroying nature – and attributed “a doubling in the likelihood of occurrence and a 7% increase in intensity” to human influence.
But the results are “conservative,” the scientists wrote, because the model does not explicitly model convection, which could lead to underestimating rainfall. “We want to emphasize that while the direction of change is quite clear, the rate is not.”
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Physicists have shown that for every degree rise in temperature, there is a 7% increase in the amount of moisture in the air, but whether this actually happens depends on the availability of water. Rain in Central Europe occurs when cold air from the Arctic meets warm, moist air from the Mediterranean and Black Seas.
Miroslav Trnka, a climate scientist at the Global Change Institute who was not involved in the study, said warming oceans increase the hydrological cycle’s rainfall but tend to dry out parts of land, which “could lead to intensified flooding” if conditions were right.
Trnka likened the drivers of extreme rainfall to a lottery, saying the increased risk from global warming was like buying more tickets, playing for a longer period and changing the rules to make more combinations of numbers winning.
“If you bet long enough, you have a good chance of hitting the jackpot,” Trnka said.
The study found that if the world were to warm by 2°C above pre-industrial levels, more intense four-day rain events would occur, with roughly 5% more intensity and 50% more likely to occur than today.
Other factors, such as a wavy jet stream, could exacerbate this trend, which some scientists believe is becoming more common as a result of global warming, trapping weather systems in one place. A study published Monday in Nature Scientific Reports predicts that we’ll see an increase in such blocked systems under moderate and worst-case emissions scenarios.
“Cut off from the jet stream, these massive storms could become stuck in one place and bring huge amounts of rain due to the increased moisture and energy coming from record-breaking hot ocean waters,” said Hayley Fowler, a climate scientist at Newcastle University who was not involved in the study.
“These ‘blocked’ slow-moving storms are becoming more frequent and are projected to become more frequent as the climate warms,” she added. “The question is not whether we should adapt to an increase in these types of storms, but whether we can adapt.”
The WWA described the week after Typhoon Boris as “highly active” because 12 disasters occurred around the world, triggering analysis criteria more than any other week in the organization’s history.
The study did not say how much global warming has increased damage from rain, but the researchers said even small increases in precipitation caused disproportionate increases in damage.
“Almost everywhere in the world we see situations where a small increase in rainfall leads to a similar increase in flood magnitude,” said Friederike Otto, a climate scientist at the Grantham Institute at Imperial College London and co-author of the study, “but it translates into a large increase in damage.”
