An “interplay” of climate change exacerbated heavy rainfall during the first two weeks of September 2023 in several countries across the Mediterranean, caused by low-pressure systems forming around a blocking high centred over the Netherlands.

This is according to World Weather Attribution (WWA), which notes that not all extreme weather events are caused or worsened by climate change.

But the researchers found that an event as extreme as the one observed over Libya has become up to 50 times more likely and up to 50% more intense compared with a 1.2C cooler climate, they said.

For the large region including Greece and parts of Bulgaria and Turkey, human-induced climate change made an event as extreme as the one observed up to 10 times more likely and up to 40% more intense, they said.

WWA is a non-profit group of researchers from institutions such as Imperial College London, The Royal Netherlands Meteorological Institute, the Laboratoire des Sciences du Climat et de l’Environnement and the Red Cross Red Crescent Climate Centre.

They found that severe flooding in Spain, Greece, Turkey, Bulgaria and Libya was caused by very heavy rainfall that fell, in the case of Spain in less than 24 hours, whereas it lasted 24 hours in Libya and up to four days over Greece and Türkiye.

WWA noted that on 3 September torrential rain fell within a few hours in Spain, followed by very heavy rain and floods caused by the low-pressure system “Daniel” in Greece and Bulgaria between 4 and 7 September, and devastating floods in Libya after very extreme rainfall during the 10th.

All three individual rainfall events caused severe flooding, submerging settlements and leaving thousands homeless and killing at least four people in Bulgaria, six in Spain, seven in Turkey and 17 in Greece.

Further, 3,958 casualties have been confirmed in the Libyan city of Derna alone, and an additional 170 fatalities elsewhere in the country, while more than 10,000 people are still missing after two major dams broke, said WWA.

The uncertainty in these estimates is high, they said, and encompass the possibility of no detectable change, “but there are multiple reasons we can be confident that climate change did make the events more likely”, they said. From theory we know that an increase in rainfall intensity of around 10% would be expected given current warming levels, so we could only report that there has been no change if there was a well-known dynamic process counteracting this effect, which there is not.