Human-caused warming made heavy rainfall up to 10 times more likely in Greece, Bulgaria and Turkey and up to 50 times more likely in Libya. Building in flood plains, poor dam maintenance and other local factors turned extreme weather into a humanitarian disaster.
Heavy rainfall, which caused devastation in large parts of the Mediterranean in early September, was made more likely to happen by climate change resulting from greenhouse gas emissions, according to a rapid analysis by an international team of climate scientists from the World Weather Attribution (WWA) group.
The study is called ‘Interplay of climate change-exacerbated rainfall, exposure and vulnerability led to widespread impacts in the Mediterranean region’. It was conducted by researchers from universities and research centres in Greece, the Netherlands, the United Kingdom and the United States. WWA is an international collaboration that analyses and communicates the possible influence of climate change on extreme weather events, such as storms, extreme rainfall, heatwaves, cold spells, and droughts.
Scientists compared current climate with historical data
Results showed the destruction caused by the heavy rain was much greater due to factors that included construction in flood-prone areas, deforestation, and the consequences of the conflict in Libya.
In early September, a cut-off low which affected Spain and a low-pressure system named Storm Daniel, which formed in the Eastern Mediterranean, brought large amounts of rain over a 10-day period to several countries, including Spain, Greece, Bulgaria, Turkey and Libya. The heavy rain led to massive floods across the region, killing four people in Bulgaria, five in Spain, seven in Turkey, and 17 in Greece.
The greatest disaster occurred in Libya, where the floods caused the collapse of two dams. While the exact number of casualties is still not clear, there are currently 3,958 confirmed deaths in Derna alone and 170 people elsewhere in Libya, with over 10,000 people still missing.
To quantify the effect of climate change on the heavy rain in the region, scientists analysed climate data and computer model simulations to compare the climate as it is today, after about 1.2°C of global warming since the late 1800s, with the climate of the past, following peer-reviewed methods.
Libya extreme weather event was extremely unusual
The scientists divided their analysis in three regions: Libya, where the analysis focused on the northeast part of the country, where most of the rainfall fell; Greece, Bulgaria and Turkey, where the analysis looked at maximum rainfall over four consecutive days; and Spain, where most of the rain fell in just a few hours.
For Libya, the scientists found that human-caused climate change made the event up to 50 times more likely to happen, with up to 50% more rain during the period, as a result of human-caused greenhouse gas emissions. The event is still extremely unusual, and can only be expected to occur around once in 300-600 years, in the current climate.
Breaking point for early warning system design
For Greece, Bulgaria and Turkey, the analysis showed that climate change made the heavy rain up to 10 times more likely to happen, with up to 40% more rain, as a result of human activities that have warmed the planet. For this large region, which encompasses parts of the three countries, the event is now reasonably common, and can be expected about once every 10 years, meaning it has a 10% chance of happening each year.
For central Greece, where most of the impacts took place, the event is less probable and only expected to happen once every 80-100 years, equivalent to a 1-1.25% chance of happening each year.
Central Greece has experienced a so-called 100-year flood, as the authors of the study calculated that the event has a 1% to 1.25% chance of happening each year
“The extreme rainfall amounts that affected central Greece and their devastating effects are a breaking point in the way we should re-organise the early warning systems towards impact-based alerts, the Civil Protection response capacity, and the design of resilient infrastructures in the era of climate change,” said Vassiliki Kotroni, one of the authors and the research director at the National Observatory of Athens.
In Spain, where most of the rain fell in just a few hours, the scientists estimated that such heavy rainfall is expected once every 40 years, but they could not conduct a full attribution analysis as the available climate models poorly represent heavy rainfall on timescales shorter than a day.
Mathematical uncertainties due to relatively small size of affected areas
These findings have large mathematical uncertainties, as the events occurred over relatively small areas, and most climate models do not represent rainfall on these small scales well.
While the scientists cannot completely rule out the possibility that climate change has not affected the likelihood and intensity of events like these, they are confident that it did play a role for several reasons: increased temperatures generally lead to heavier rainfall and studies project heavier rain in the region as temperatures rise; they could find no evidence of factors that might be making heavy rain less likely and balancing the influence of climate change; and weather station data in the region shows a trend towards heavier rain.
In the affected area in central Greece, most of the cities and communities and a large part of the infrastructure are located in flood-prone areas
Because of the limits in the models, the scientists did not give a central estimate of the influence of climate change, as they have done in previous studies, instead giving an upper-bound of the effect.
A key finding of the study is that the very large impacts observed in some of the regions were caused by a combination of high vulnerability of the population and their exposure to the event. In the affected area in central Greece, most of the cities and communities and a large part of the infrastructure are located in flood-prone areas.
In Libya, a combination of several factors including long-lasting armed conflict, political instability, potential design flaws and poor maintenance of dams contributed to the disaster. The interaction of these factors, and the very heavy rain that was worsened by climate change, created the extreme destruction.
Hotspot of climate change-fueled hazards
“The Mediterranean is a hotspot of climate change-fueled hazards. After a summer of devastating heatwaves and wildfires with a very clear climate change fingerprint, quantifying the contribution of global warming to these floods proved more challenging. But there is absolutely no doubt that reducing vulnerability and increasing resilience to all types of extreme weather is paramount for saving lives in the future,” co-author Friederike Otto said. She is a senior lecturer in climate science at Grantham Institute – Climate Change and the Environment, at Imperial College London.
Otto: There is absolutely no doubt that reducing vulnerability and increasing resilience to all types of extreme weather is paramount for saving lives in the future
Julie Arrighi, director of the Red Cross Red Crescent Climate Centre, said designing infrastructure for the future climate, strenghtening emergency management and other practical solutions can prevent disasters from becoming routine.
Previous studies by WWA include research that found that climate change exacerbated floods in Nigeria and other parts of West Africa last year. WWA studies have also shown that last year’s drought in the Northern Hemisphere was made more likely by climate change and that it increased the rainfall that led to Pakistan’s deadly flooding, but that it was not the main driver in Madagascar’s 2021 food crisis.