Environmental Change Institute, University of Oxford

  • 27 September 2019

Storm Imelda twice as likely, ten to fifteen percent stronger, due to climate change

Irina K / Adobe Stock

Just like Hurricane Harvey, the extreme rainfall and flooding caused by Tropical Storm Imelda was made more likely and intense due to global warming concludes a rapid analysis from international science partnership World Weather Attribution.

In mid-September 2019 torrential rainfall from Tropical Storm Imelda caused large-scale flooding in Southeast Texas. Authorities have linked five deaths to the floods and over one thousand people had to be rescued, making this the worst storm in the area since Hurricane Harvey.

An international team of scientists, including researchers from Texas A&M University at Galveston, Princeton University, the Dutch Met Office (KNMI), the Environmental Change Institute at the University of Oxford and the Red Cross Red Crescent Climate Centre conducted a rapid analysis to determine whether and to what extent the probability of these rainfall and flooding events had changed as a result of climate change.

The analysis was led by World Weather Attribution, an international effort to analyse the possible influence of climate change on extreme weather events such as heatwaves, cold spells, floods and droughts.

Combining established climate models and observational data, the researchers found that the extreme rainfall seen during two-day Tropical Storm Imelda was around two times (1.6 to 2.6) more likely due to human-caused climate change, or 9% to 17% more intense.

The researchers carried out a similar analysis for the record-breaking and devastating Hurricane Harvey in 2017, finding that it was roughly three times more likely and 15 percent more intense due to man-made climate change (van Oldenborgh et al, 2017). By making use of the same models and datasets to analyse precipitation caused by Storm Imelda, the scientists were able to carry out the analysis on a much faster timescale than typical academic studies.

The results are aligned with multiple scientific analyses of flooding events carried out over the last few years.

Geert Jan van Oldenborgh, climate scientist at the Dutch Met office (KNMI), said: "The probability of extreme precipitation is clearly increasing along the US Gulf Coast, both in observations and in climate models. However, I had not expected another disastrous event like Imelda so soon."

Professor Gabriel Vecchi from Princeton University said: "As the atmosphere and oceans warm, increased atmospheric water vapor helps make extreme rainfall more likely along the US Gulf Coast, providing an added challenge to the already complex issue of flood mitigation in this region."

Since Hurricane Harvey, significant steps have been taken to reduce the vulnerability of southeast Texas to coastal flooding, including increased funding, training, infrastructure and new laws on flood risk disclosure. Tropical Storm Imelda was a stark reminder of the need for these mitigation measures and the continued exposure of the area to severe rainfall and flooding.

Dr Antonia Sebastian, Associate Research Scientist at Texas A&M University at Galveston said: "Coupled with improved flood hazard models and new communication platforms, this new legislation will go a long way towards providing homeowners with a better understanding of their flood risk."

The Tropical Storm Imelda study is the third rapid analysis carried out this summer by World Weather Attribution. In August 2019, the team found that July's record-breaking European heatwave was made up to 100 times more likely due to climate change.

Dr Friederike Otto, acting director of the Environmental Change Institute, University of Oxford, said: "The impacts of climate change on Imelda might seem small compared to the impacts of global warming on other events such as heatwaves. But they are particularly crucial because Texas is an area of rapidly increasing exposure, with population growth, urban expansion and sea-level rise putting many more people at risk."

Read the full report here.

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