A new study has shown that the 2023/24 storm season in UK and Ireland was made 20% heavier by climate change – and warns that it will continue to cause an increase in extreme winter storms combining strong winds and heavy rainfall.

Image :Storm Debi Ⓒ Crown copyright, Met Office, Satellite data: EUMETSAT, Background data: NASA Earth Observatory

Scientists from the United Kingdom, Ireland, the Netherlands, Sweden and Germany, including scientists from each of the National Meteorological Services in the Western Europe storm naming group, collaborated to assess to what extent human induced climate change and the North Atlantic Oscillation (NAO) influenced the average storm severity, using the wind-based Storm Severity Index (SSI) over a wide region encompassing the United Kingdom and Ireland.

The study finds that the 2023/24 storm season, studied by stormy day wind severity, associated rainfall, and accumulated seasonal rainfall in October-March, has brought deaths, flooding, transport disruptions and power outages, among other impacts, to the UK and Ireland. The flooding had a significant impact on food production.

During the winter half-year of 2023/2024, western Europe experienced a series of damaging storms. These storms led to disruptions and the associated precipitation caused exacerbated flood risks. The study says that the jet stream was stronger than normal, which likely contributed to how strong the storms became. Impacts of individual storms can be worsened when the soils are already very wet due to preceding sustained rainfall or a succession of storms over a similar area, leading to saturation, increased run-off and risk of flooding.

The 2023/24 storm season is the ninth season since the founding of the Western Europe storm naming group. The initiative began in 2015, when the Met Office and Met Éireann, Ireland’s national meteorological service, officially started to identify and name storms that have the potential to cause medium or high impacts, and expanded to include the Royal Netherlands Meteorological Institute (KNMI) in 2019.

The study also investigated the influence of climate change on the average precipitation on stormy days from October 2023 to March 2024, which was one of wettest Oct-Mar periods on record for the UK and the third on record for Ireland, and the wettest over the region south of 54N studied. The study uses peer-reviewed methods to assess changes in storm severity, associated precipitation and precipitation accumulated over the storm season.

The study finds successive floods have compounded impacts on the agriculture and housing sectors, leading to cascading impacts on socioeconomic and psychosocial health, and eroding people’s coping capacity, particularly low-income groups. Combined with the cost-of-living crisis, the successive flood events are another layer of disruption at a time when people’s financial resilience is already being tested.

In today’s climate with 1.2C of warming, stormy days with winds as intense as in the 2023/24 season occur about every 4 years. The associated precipitation is expected to occur about once every 5 years. The seasonal precipitation of the October-March period was more extreme, expected to occur about once every 20 years.

The average precipitation on stormy days are observed to have become approximately 30% more intense, compared to a 1.2C cooler pre-industrial climate. Models agree on the direction of change, combining observations and models indicate that average precipitation on stormy days increased by about 20% due to human induced climate change, or equivalently the 2023/24 level has become about a factor of 10 more likely.

The study says that models indicate that the trends in average precipitation on stormy days and seasonal precipitation continue into the future, in a climate that is 0.8C warmer than now. Average precipitation on stormy days becomes about another factor of 1.6 times more likely, or 4% more intense, and seasonal precipitation becomes about a factor of 1.5 more likely or 2% more intense.

Comprehensive flood risk management is required in the UK and Ireland that encompasses legislative frameworks, strategic planning, and substantial funding.

Royal Netherlands Meteorological Institute researcher Sarah Kew one of the report’s authors, commented:

“Autumn and winter rainfall has become much heavier, bringing more damaging and sometimes deadly floods to urban and agricultural areas.

“Until the world reduces emissions to net zero, the climate will continue to warm, and rainfall in the UK and Ireland will continue to get heavier.”