Climate change made record-breaking downpours in   9% more intense 

• Heavy downpours  caused damaging floods in Mississippi, Arkansas, Missouri, Illinois, Indiana, Kentucky, Tennessee, and Alabama.
•   Similar downpours are expected to occur on average about once a century in today’s climate with 1.3°C of warming.
•  If warming reaches 2.6°C (4.7°F), which is expected by 2100  spells of rainfall are expected to become a further 7% more intense and twice as likely.

 The floods that killed at least 15 people across eight US states at the start of April were caused by rainfall made about 9% more intense by human-caused climate change, rapid analysis by a team of leading scientists found. The study by World Weather Attribution highlights how forecasting and early warnings helped limit deaths during the region’s heaviest four days of rainfall on record. However, it also notes that layoffs at the National Weather Service (NWS) could impact efforts to keep the public safe in future extreme weather.

The analysis found:

● Climate change made the rainfall about 9% more intense. Uncertainty in climate models means this result is likely an underestimate.

● The warm sea temperatures in the Gulf of Mexico which fed the storm moisture were about 14 times more likely due to climate change.

● If countries fail to speed up the shift away from fossil fuels causing warming to reach 2.6°C (4.7°F), similar rainfall events are expected to increase in intensity by about a further 7% .

 ● Forecasting and early warning has a vital role to play to protect people from increasingly dangerous extreme weather events in the US.

Ben Clarke, Researcher at the Centre for Environmental Policy, Imperial College London, said: “This study shows how severely the US is being hit by climate change. “The four days of downpours, falling across a vast region covering eight states, were the heaviest on record by a large margin. Overall, we found that climate change made the rainfall 9% heavier, which could have caused massive increases in flood damages. “Fossil fuel warming is clearly driving more intense, and increasingly costly, extreme weather across the US.”

The floods occurred after a powerful storm swept across the US, dropping huge volumes of rainfall. From April 3-6, in the Central Mississippi Valley The ensuing floods killed at least 15 people, wrecked homes, swept away vehicles and caused power outages that impacted hundreds of thousands.

 The study found the four days of rainfall was the heaviest ever recorded for the region analysed. A combination of weather patterns, including the collision of two air masses, created a storm that stalled and subjected the Central Mississippi Valley to days of severe weather. Based on historical data, similar downpours are expected to occur on average about once a century in today’s climate with 1.3°C of warming. While the event is unusual, the burning of fossil fuels – which releases planet-heating emissions – made it more intense and more likely to happen.

 Four-day spells of rainfall have become 13-26% heavier and 2-5 times more likely, weather observations indicate. The scientists then analysed climate models to understand if, and how much of the increase is due to climate change. This analysis found that climate change made the rainfall about 9% heavier and 1.4 times more likely. However, due to uncertainty in the climate models, the scientists warn this is likely an underestimate and the role of climate change could be as high as the weather data suggests. The region will get hit by even heavier downpours without a rapid switch from fossil fuels to renewable energy.

 If warming reaches 2.6°C (4.7°F), which is expected by 2100 under current policies, four-day spells of rainfall are expected to become a further 7% more intense and twice as likely, the study found. The results agree with a huge body of scientific evidence showing that a warmer atmosphere can hold more moisture, leading to heavier rainfall and more dangerous floods. The analysis also investigated the hot seas in the Gulf of Mexico that fed moisture to the storm. Overall, climate change made the high surface temperatures 1.2°C (2.2°F) hotter and 14 times more likely.

The study highlights the critical role forecasting and early warning plays in keeping Americans safe from extreme weather. The downpours were well forecast and warnings were issued up to a week before and throughout the floods by the NWS. While these efforts likely reduced the death toll, especially for an event of this magnitude, the scientists warn that increasingly heavy rainfall highlights. Nearly half of NWS field offices are believed to have vacancy rates of 20% or more – double the level of short-staffing from ten years ago. A recent open letter from former NWS leaders has also warned that strained office workforces could lead to a “needless loss of life.” The study was conducted by 15 researchers as part of the World Weather Attribution group, including scientists from universities and meteorological agencies in the United States, United Kingdom, France and Netherlands.

Bernadette Woods-Placky, Chief Meteorologist, Climate Central, said: “Even though the flooding sadly took lives, strong forecasts and early warnings from the National Weather Service and other emergency services helped avoid a larger disaster.” “Staff in local National Weather Service offices worked around the clock to provide life-saving information and services. “This is an example of how critical these employees are and why recent workforce cuts risk undermining their ability to keep people safe and prepared.”

 Shel Winkley, Weather & Climate Engagement Specialist, Climate Central, said: “Another round of storms and flooding last week shows the non-stop reality of climate impacts in the US. “We’re dealing with floods, droughts, wildfires, and heatwaves – many times all at once – and science keeps confirming they’re getting more dangerous as the planet heats up. “Understanding precisely where and when these unnatural extreme events will strike is vital for protecting public safety.”

Friederike Otto, Senior Lecturer in Climate Science at Grantham Institute – Climate Change and the Environment, Imperial College London, said: “These floods didn’t make front pages, but they should have. At least 15 people died, homes were ruined and farmland turned into swamps. “For decades, we’ve known exactly what is driving the dangerous changes to extreme weather – burning oil, gas and coal. Transitioning to renewable energy will help create a safer, more stable world for all. “In an increasingly dangerous world of extreme weather, a well-resourced forecasting workforce is essential. The recent layoffs at the National Weather Service workers will put lives at risk.”

 Scientists globally agree that climate change, caused by burning oil, coal and gas and deforestation, is making rainfall heavier in many regions of the world. To quantify the effect of human-caused warming on the heavy downpours that caused damaging floods in Mississippi, Arkansas, Missouri, Illinois, Indiana, Kentucky, Tennessee, and Alabama, researchers analysed weather data and climate models using peer-reviewed methods to compare how these types of events have changed between today’s climate, with approximately 1.3°C of global warming, and the cooler pre-industrial climate.

The study analysed four-day rainfall events and focused on a region of the Central Mississippi valley that experienced severe impacts. Previous analyses on US events A World Weather Attribution analysis found that climate change worsened the Los Angeles wildfires, which killed 30 people, by reducing rainfall, drying out vegetation, and increasing the overlap between flammable drought conditions and strong Santa Ana winds.

 World Weather Attribution World Weather Attribution is an international collaboration that analyses and communicates the possible influence of climate change on extreme weather events, such as storms, extreme rainfall, heatwaves, and droughts. The group has completed more than 100 studies on a range of extreme weather events around the world. The Intergovernmental Panel on Climate Change included research by World Weather Attribution to provide evidence that human-caused climate change is already intensifying weather extremes in every region of the world in its Sixth Assessment Report published in March, 2023.