Western-Mediterranean heatwave ‘almost impossible’ without climate change

The extreme heatwave that swept across Spain, Portugal, Morocco and Algeria in April 2023 was “at least 100 times more likely” due to climate change, according to a new “rapid attribution” study.

​​During the last week of April, temperatures surged up to 20C higher than normal across large parts of the western Mediterranean, reaching an intensity more typical of mid-summer.

Temperatures exceeded 40C in parts of Morocco and Algeria. And, in Spain, temperatures reached 38.8C at Córdoba airport – not only breaking the previous April record by nearly 5C, but also setting a new April record for all of Europe.

In its most rapidly produced study to date, the World Weather Attribution service, which launched in 2014, finds that the heatwave was up to 3.5C hotter than it would have been in a world without climate change.

The study adds that, even in today’s climate, the heatwave was unlikely, with roughly a 0.25% chance of happening in a given year.

Mediterranean heatwave

​​During the final week of April 2023, a wave of “superheated air” from the Sahara desert swept northward towards the Mediterranean. Temperatures surged up to 20C higher than normal across large parts of Morocco, Algeria, Spain and Portugal, with many local April temperature records broken by up to 6C.

“People in this region are no strangers to high temperatures,” Roop Singh – a climate risk adviser at the Red Cross Red Crescent Climate Centre and co-author on the study – told a press briefing. However, she added that it is “really exceptional to have temperatures near 40C near the end of April”. 

Mortality data from the heatwave is not yet available. However, Dr Fatima Driouech – an associate professor at the Mohammed VI Polytechnic University in Morocco and co-author on the study – told the press briefing that “early-season heatwaves tend to be particularly deadly due to a lack of acclimatisation in the population and lower preparedness for heat”.

For many countries around the western Mediterranean, the “summerlike” heat served to exacerbate an intense multi-year drought, which was already threatening crop yields and water security. 

Spain is on track to see its driest April on record, having recorded below-average levels of rainfall for at least the past 36 months. Even before the most intense heat struck, 27% of Spain was already classified as in drought “emergency” or “alert” and water reserves were at 50% of national capacity.

SImilarly, average dam storage in Morocco was only 33% by the end of April following its many consecutive years of below-average rainfall.

Dr Frederieke Otto, a senior lecturer in climate science at the Grantham Institute for Climate Change and the Environment at Imperial College London and co-author of the study, told the press briefing that the Mediterranean region is a ”hotspot” for worsening compound hot and dry extremes.

April extreme

The hottest three-day period in the Mediterranean heatwave was 26-28 April. The map below compares the maximum daily temperature over this period with the 1991-2020 average temperature for the region. The red shading shows how much hotter these days were than the 1991-2020 average.

The black box shows the region analysed in the study, which includes southern Spain and Portugal, large areas of Morocco and the northwest part of Algeria.

To put the heatwave into its historical context, the authors analysed a time series of temperature data over the Mediterranean region. They find that, even in today’s climate, the heatwave is unusual, with a “return period” of one in 400 years. This means that a heatwave of this magnitude has roughly a 0.25% chance of happening in any given year.

Dr Sjoukje Philip – a researcher at the Royal Netherlands Meteorological Institute and co-author on the study – told a press briefing that this was the fastest attribution study conducted by the team to date.

There is an “ongoing question” of how quickly the study can be completed and the results shared, Philip told the press briefing. She added that, in this case, “we decided to do a very rapid analysis simply because people were waiting for the results”.

When undertaking a rapid attribution analysis, the WWA usually uses observation data from weather stations to assess the temperature during the extreme. However, Philip says that as this study was conducted so quickly, observation data was not available yet, so temperature forecasts were used instead.

She stresses, though, that this should not make a big difference to the results of the study.

Attribution

Attribution is a fast-growing field of climate science that aims to identify the “fingerprint” of climate change on extreme-weather events, such as heatwaves and droughts. In this study, the authors investigated the impact of climate change on the western Mediterranean heat extremes over the three-day period of 26-28 April.

To conduct attribution studies, scientists use models to compare the world as it is today to a “counterfactual” world without human-caused climate change.

The authors find that climate change made the heatwave at least 100 times more likely, with temperatures up to 3.5C hotter than they would have been without climate change. They add that the event would have been “almost impossible” without climate change. 

They add that if temperatures continue to rise, reaching 2C above pre-industrial temperatures, such a heatwave would have been another 1C hotter. However, they note that this result may be an underestimate, as the models have been “conservative” in this respect.

(These findings are yet to be published in a peer-reviewed journal. However, the methods used in the analysis have been published in previous attribution studies.)