Scientists from Italy, the Netherlands, Sweden, the UK, the European Commission and the US used published peer-reviewed methods to assess whether and to what extent climate change influenced the 12.month drought in the two islands (1) Sicily and (2) Sardinia.

There are several ways to characterise a drought: meteorological drought is defined only by low rainfall, while agricultural drought combines rainfall estimates with evapotranspiration or directly measures soil-moisture content. As increased evapotranspiration due to regional warming can play a major role in exacerbating drought impacts, we assess agricultural drought in this study by means of  the Standardised Precipitation Evapotranspiration Index (SPEI), which calculates the difference between rainfall and potential evapotranspiration to estimate the available water. The more negative the SPEI values are, the more severe the drought is classified. Figure 1 shows the SPEI for the 12 months between July 2023 and August 2024 over Italy, and the category it falls in according to the US Global Drought Monitor classification system.  

Main findings

• The main economic activities in Sicily, agriculture and tourism, both strongly depend on water availability. The economic consequences of this drought are thus catastrophic for many in the region and recovery will take time. In Sardinia, agriculture is economically less important, but of high cultural relevance; leading to challenges with water prioritisation for domestic and agricultural use on both islands. 
• Natural ecosystems also suffer. Agricultural expansion, especially in Sicily, has increased water demand, with a decrease of 62ha per year since 1990 in natural ecosystems, including wetlands. 
• In different observational datasets the extreme drought, defined by SPEI12 (fig. 1) is among the most severe droughts since records began. All data agree that this drought is not very rare in Sardinia in today’s climate that has warmed by 1.3°C primarily due to the burning of fossil fuels, with a return period of about 10 years (3 – 93 years). In Sicily, such a drought does not occur very often, with a best estimated return period of around 100 years (10- 200,000 years) in today’s climate. 
• Based on the US Drought Monitoring Classification system, the 1-in-10 year drought over Sardinia which is an ‘extreme’ (D3) drought now would be classified as a ‘severe’ (D2) drought without the effects of climate change, and with further warming would be a more severe ‘extreme’ drought (D3). The rare 1-in-100 year drought over Sicily which is also an ‘extreme’ (D3) drought would be a ‘severe’ (D2) drought without climate change, and with a further 0.7C of warming, will become an ‘exceptional’ drought (D4). For both islands, the likelihood of a drought as defined by SPEI12 from August 2023 to July 2024 has increased by about 50% due to human-induced climate change. 
• To understand the meteorological drivers of these changes in drought, we also assess the individual components making up the drought index, namely rainfall, potential evapotranspiration and temperature. We find that changes in rainfall are small and not statistically significant: on the contrary, both values observed for potential evapotranspiration and temperature as observed this year would have been almost impossible to occur without human-induced climate change. We thus conclude that this increase in drought severity is primarily driven by the very strong increase in extreme temperatures due to human-induced climate change. 
• It has been well-established that climate models tend to underestimate the increase in extreme temperatures in Europe; this in part accounts for the discrepancy between the observed change in drought frequency and intensity and those represented in climate models. 
• Unless the world rapidly stops burning fossil fuels, these events will become even more common in the future. In a world 2°C warmer than preindustrial, which could happen as soon as 2050 without large and rapid reductions in greenhouse gas emissions, droughts like the ones in Sicily and Sardinia will become more frequent. 
• Effective drought risk management in regions such as Sardinia and Sicily requires a sustained focus on long-term preparedness and adaptation. Investing in resilient infrastructure, water conservation strategies, and sustainable resource management is crucial to mitigating the impacts of drought.

The Event 

Attribution studies continue to show that human induced climate change is making heatwaves hotter and deadlier. In many regions, the influence of human induced climate change is so large that temperatures recorded during heatwaves would not be possible without warming caused by the burning of fossil fuels. This includes parts of the US, the Sahel, West Africa, the Philippines and other countries in East Asia  as well as the Mediterranean. We assess how rare the extreme July 2024 heat in the Mediterranean and examine its impacts, focusing on observational data instead of conducting a detailed attribution study, which would likely produce similar results to previous studies. The results are contextualised with findings from our previous attribution studies in the region and global analyses.

Key Messages 

• Heatwaves are the deadliest type of extreme weather, with hundreds of thousands of people dying from heat-related causes each year. The July heatwave caused at least 21 deaths in Morocco after temperatures reached 48°C. However, it is likely there were dozens or hundreds of other heat-related deaths in the countries affected that have not been reported and the full impact of a heatwave is rarely known until months afterwards, once death certificates are collected, or scientists can analyse excess deaths. Many places lack good record-keeping of heat-related deaths, therefore global mortality figures are a significant underestimate.
• In line with past climate projections and IPCC reports, extreme heat events like July 2024 in the Mediterranean are no longer rare events. Similar heatwaves affecting Greece, Italy, Spain, Portugal and Morocco  are now  expected to occur on average about once every 10 years in today’s climate that has been warmed by 1.3°C due to human-induced climate change. 
• Based on the data set ERA5, the extreme temperatures reached in July would have been virtually impossible if humans had not warmed the planet by burning fossil fuels. In addition, the 1 in 10 year extreme July heat would have been 3°C [2.5 – 3.3°C] cooler in a world without climate change. 
• These results are based on observational data and do not include climate models. However, the results are very similar to the studies published  in 2023 that analysed heatwaves in the same region and included climate models. For both the April and e July heatwaves in 2023, we found the events would have been virtually impossible without climate change, but are not rare today. We also found the heatwaves were made 1.7-3.5 ºC hotter when compared to a pre-industrial world. 
• The results for the 2023 events synthesised observations and climate models. However, the models are known to systematically underestimate extreme heat in Europe (van Oldenborgh et al., 2022, Vautard et al., 2023, Schumacher et al., 2024)  meaning the real world changes due to climate change are probably closer to the changes  shown in observations. 
• Furthermore, the previous studies use slightly different temporal and spatial definitions of heat, but the numerical results remain very similar. Given the 2024 event is very similar to the observed changes found in studies published 2023, they are a good indicator of how climate change is affecting extreme heat in the Mediterranean.
• Unless the world rapidly stops burning fossil fuels, these events will become hotter, more frequent and longer-lasting. 
• Studies show that elite Olympic athletes who are exposed to high temperatures and are not acclimated to them may see impacts such as a decline in performance, and increase in heat-related illness, such as heat cramps and exhaustion, having implications for the Paris Olympics that are currently ongoing (de Korte et al., 2021, Griggs et al., 2019). Measures to reduce exposure, ensure adequate hydration and cooling, acclimatisation, and emergency plans can help keep athletes safe during periods of extreme heat.  
• Heat action plans that reduce heat-related deaths are increasingly being implemented across the region, which is encouraging. However, there remains an urgent need for an accelerated roll-out of heat action plans in light of increasing vulnerability driven by the intersecting trends of climate change, population ageing, and urbanisation. Cities are hot-spots for heat risk, so urban planning needs to focus on measures to  reduce the urban heat island effect, such as increasing cooling green and blue spaces. 

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. 

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 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. 

Main findings

• • The 2023/24 storm season, studied here 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.
  • 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. 
  • The storm severity index (SSI) was used to define stormy days to study. The SSI considers both the strength of the wind and the area affected. In this analysis we looked at rainfall and wind speed on stormy days identified by the SSI.
  • 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.
  • Analyses of observations are used to determine whether a trend can be observed in these measures. To determine the role of climate change in these observed changes, we combine observations with climate models.
  • 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 observed precipitation across Oct-Mar has a strong trend, with a magnitude increase of about 25%. Climate models utilised in this study broadly agree on the direction of the change, and the combination of  observation and model results indicates an increase in magnitude of 6% to 25%, or equivalently the 2023/24 level has become at least a factor of 4 more likely.
  • 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.
  • Looking at average SSI on storm days, while some studies using other methods suggest an increase in storminess in a future climate, our analysis has shown a decreasing trend. Our results show that average SSI indices as observed this year became about a factor of 2 less likely. The synthesis of the models also shows a negative trend and, when combined with observations, the results indicate that  a stormy season as observed this year is nowadays a factor of about 1.4 less likely due to human induced climate change. 
  • This highlights the need for ongoing research into how climate change may influence the severity and frequency of windstorms in northern Europe.
  • NAO is a key driver of ‘storminess’ and has been accounted for in this analysis. However, the Oct-Mar 2023/24 averaged NAO was almost neutral.
  • Comprehensive flood risk management is required in the UK and Ireland that encompasses legislative frameworks, strategic planning, and substantial funding. Major UK cities are starting to integrate nature-based solutions into their designs. In Ireland, flood relief projects have been integrating nature-based solutions alongside traditional engineering solutions for over 20 years. Both the UK Met Office and Met Éireann are continuously improving their impact-based weather forecasting mechanisms to facilitate the translation of warning into action, in partnership with other government bodies to ensure their people’s safety. 

To assess to what extent human-induced climate change altered the likelihood and intensity of the heavy precipitation that caused these impacts researchers from the World Weather Attribution initiative, Maynooth University and Met Éireann undertook an attribution study on the event.

We focused on two key indicators to allow us to differentiate the role of climate change in two important characteristics of the event: the 2-day extreme precipitation in October, and the 3-month total accumulated precipitation from July – September, both averaged over County Cork.

Main findings

• On the 17th and 18th October 2023, Storm Babet brought record rainfall amounts to the south of Ireland leading to significant flooding, with the town of Midleton, County Cork severely impacted. The intense 2-day rainfall fell on soils saturated by over 3 months of above average rainfall.
• Peak river flows coincided with a spring low tide, meaning that the river was able to efficiently drain into the sea. Had the event occurred at high tide and/or with substantial storm surge, flooding could have been much more extensive.
• From hydrological modelling, we find that high river flows in October upstream of Midleton are principally driven by extreme rainfall over 2-days and above average preceding rainfall over a longer period (leading to soil saturation), with little evidence of a significant contribution from recent land use changes.
• In order to assess whether and to what extent human-induced climate change was a driver of the rainfall leading to this flood we combine observations-based data products and climate models to look at both the extreme 2-day October and 3-month July-September accumulations over County Cork.
• We find that 2-day October rainfall at least as high as occurred on 17-18^th October 2023 has more than doubled in likelihood and increased in intensity by around 13% due to global warming since pre-industrial levels. This result has high confidence with agreement between models and observations. At 2 degrees of warming, there is also high confidence of further increases in the likelihood and intensity of such events. However, these projected changes are challenging to quantify due to model uncertainties, and so the reported results of a 20% increase in likelihood and 2% in intensity have low confidence.
• On the other hand, the observed high antecedent rainfall from July-September may have become less likely by about 25% and the rainfall totals have reduced by around 5%. However, this result has very low confidence due to high uncertainties across all datasets and the disagreement in the direction of change between models (drying) and observations (wetting).
• In summary, forensic analysis of the drivers of flooding in Midleton show that changes in extreme rainfall due to anthropogenic climate change drove more intense flooding in October 2023, and such changes are likely to continue with further warming. Despite the substantial damages, Midleton proverbially ‘dodged a bullet’ of a far worse disaster thanks to the chance spring low tide at the time of the peak river flow.

While Fennoscandia is well accustomed to and prepared for winter conditions, very low temperatures have become less frequent in the recent decades and events such as this have the power to surprise and be particularly impactful. Direct impacts of the coldwave will have been felt most strongly by vulnerable groups notably people experiencing energy poverty, living in poor housing stock, and/or experiencing homelessness. Elderly people and children are also more vulnerable physiologically to extreme temperatures. The cold also caused traffic disruptions, school closures, power cuts and infrastructure damage.

Scientists from Norway, Sweden, Finland, France, the Netherlands, the UK and the US collaborated to assess whether and to what extent human-induced climate change has modified the likelihood and intensity of this cold wave.

Published peer-reviewed methods were used by the team to analyse the event, on twodifferent scales and locations: Firstly the annual (Jul-Jun) minimum of 5-day averaged minimum temperatures in a region impacted by the onset of the cold wave in Northern Scandinavia, as shown in Figure 1; and secondly the single-day (Jul-Jun) minimum temperature for the station of Oslo, to the south of the study region, impacted slightly later.

Main findings 

● Extreme cold such as experienced during this event can have severe direct impacts on health, and secondary impacts on roads, electricity grids, and services such as schools and public transport.
● Cold waves, like other extreme weather events, put significant pressure on energy, healthcare, and water systems. These systems must be designed and able to absorb additional (or different) needs, and early warnings, preparedness plans, and response capacity are all crucial to mitigate the impacts of these events.
● Homelessness in particular, but also energy poverty and bottlenecks with the associated price hikes and, in some cases, poor housing conditions, exacerbate the impacts of cold waves on particularly vulnerable socio-economic and demographic groups.
● Even though some very low temperatures were recorded, the dataset based on observations characterises the area average 5-day cold spell as a 1-in-15 year event in today’s climate, ranking as 12th coldest since 1950. For Oslo, the one day minimum temperature was rare, approximately a 1-in-200 year event in today’s climate.
● To estimate the influence of human-caused climate change on this extreme cold we use a combination of climate models and the observations. We find that because of human-induced climate change the area-averaged event would have been about 4 degrees colder in a 1.2°C cooler climate. This corresponds to such cold spells having become about 5 times less frequent.
● For the Oslo station the cold single day minimum would also have been about 4 degrees colder without human-induced climate change. This corresponds to such cold days having become about 12 times less frequent.
● At global mean temperatures of 2°C above pre-industrial levels, large-scale cold spells as rare as this one are projected to be about another 2.5 °C less cold for the area average and about another 2 °C less cold for Oslo. They will become even less frequent than today.
● Climate change does not mean that cold waves will no longer happen. In fact, less severe and less frequent cold waves may be more impactful than past ones if risk perception and preparedness decrease due to the less frequent event occurrences.

To assess to what extent human-induced climate change altered the likelihood and intensity of the heavy precipitation and high wind speeds that caused these impacts researchers from the World Weather Attribution initiative undertook an attribution study on the event.

The focus on two indicators allows us to differentiate the role of climate change in two important characteristics of the event: the 3-day mean precipitation (Rx3day), and the 3-day mean of maximum wind speed magnitude (WSx3day), averaged over the study region where the majority of impacts was observed, defined as a box of 40-50N, 25-45E (Figure 1) and considering only land areas. To account for the climate of the region, with relatively dry and warm summers and wet winters, we study the annual maxima based on July to June cycles.

Main findings 

• Storm Bettina hit the Crimean peninsula in the midst of the active Russia-Ukraine war adding to wide-ranging vulnerabilities across the storm affected areas.
• Storms like Bettina are fairly common in the region at this time of year, which is reflected in the return periods of the event which, in the current climate, are 1 in 3 years for the wind speeds and 1 in 20 years for the associated precipitation (which combines snow and rain).
• Because of human-induced warming, an increasingly larger proportion of precipitation associated with storms like this falls as rain instead of snow, leading to larger flood damages.
• We use observations-based data products and climate models to estimate the role of human-induced climate change in storms like this. The results are very different for rainfall compared to wind speeds. 
• For the precipitation as observed during Storm Bettina, we find that the burning of fossil fuels has increased the likelihood of its occurrence by about a factor of 2. The intensity of an event like this has increased by about 5 percent due to human-induced climate change. 
• Looking at the future, for a climate 2°C warmer than in preindustrial times, models suggest that rainfall intensity and likelihood will increase further. 
• For wind speeds as associated with storm Bettina we find that in observation based products there is a decrease in the likelihood and intensity, while climate models show decreases or increase, leading to no change on average. 
• For a climate 2°C warmer than in preindustrial times, climate models show overall a modest further increase in likelihood and intensity of wind speeds as observed in the 2023 Black Sea event.
• These findings suggest that the decrease we see in wind speeds in the observations are not due to climate change but other drivers e.g. natural variability. Given the model results and the scientific literature, the possibility of an increase in strong winds needs to be taken seriously, even if it cannot be attributed to climate change at this time. 
• Particularly vulnerable groups, notably elderly people, children, and people with disabilities, are more likely to be severely impacted by extreme weather shocks. In particular here, the armed conflict will have exacerbated situations of vulnerability and exposure notably by limiting the ability of communities to respond and recover after the storm and compounding situations of displacement. 
• Disaster response for the storm included flood evacuations, first aid provision, and the set-up of warming points by local governments, Red Cross national societies, and other organisations. 

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

Researchers from Greece, the United States of America, the Netherlands, Germany and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the heavy rainfall that led to the flooding.

To capture the different characteristics of the heavy rainfall and subsequent flooding we focus on two regions to assess the role of climate change: one over Greece, Bulgaria and Türkiye encompassing the region impacted by storm “Daniel” characterised as 4 day maximum rainfall averaged over land in the region 36 to 42.5N and 20 to 28.5 E (red box in Figure 1, 4th to 7th of September). Given that the region receives little rain over the summer (JJAS) but much more in winter, we focus for this event on 4-day max rainfall in the summer season. Secondly, we look at 1-day maximum annual rainfall in a smaller region over Libya (32 to 33N and 20 to 23E, red box in Figure 1, 10th September) where most of the heavy rain fell that led to the devastating flooding in Derna and the surrounding area. We do not assess the role of climate change for the event in Spain due to the fact that the rain fell in less than 24 hours.

Key findings

• The severe flooding in Spain, Greece, Türkiye, 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 4 days over Greece and Türkiye.
• For Libya and Spain, we thus evaluate the return period of the annual maximum of 1-day accumulated precipitation; for central Greece, and the larger region defined above the annual maximum of 4-day precipitation. As a summary assessment, we state that the return time for the event in Spain is a 1-in-10 to 1-in-40 year event; for central Greece a 1-in-80 to 1-in-250 year event; for the large GBT region a 1-in-5 to 1-in-10 year event; and over Libya a 1-in-300 to 1-in-600 year event.
• In Libya the event magnitude is far outside that of previously recorded events.
• The uncertainties for the return times are very high and depend on the exact region and dataset chosen. In individual locations they can be very different from the ones shown here.
• To assess the role of climate change we combine observation-based products and climate models and assess changes in the likelihood and intensity of a 1-in-10 year 4-day event over the larger region encompassing Greece, and the parts of Türkiye and Bulgaria that were impacted by flooding, as well as the 1-in-600 year 1-day maximum rainfall event over Libya.
• For the large region including Greece and parts of Bulgaria and Türkiye, we find that 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. 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 to a 1.2C cooler climate.
• The uncertainty in these estimates are high 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: 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. Studies focussing on extreme rainfall with future warming also show an increase in heavy rainfall, rendering it probable that the observed increase in heavy rainfall is indeed a trend due to climate change. For these reasons, we do not give a central estimate of the influence of climate change, as in previous studies, instead giving an upper-bound of the effect.
• In Greece, this has been a summer of extreme heatwaves and fires, including the largest fire ever recorded in the EU, followed by Storm Daniel which devastated the centre of the country. Deforestation and relatively high rates of urbanisation have changed the landscape over time, increasing the number of people and assets exposed to flooding, and reducing stormwater drainage.
• In Libya, the volume of water and overnight timing of the dam failures meant that anyone in the path of the water was at increased risk, not just those who are typically highly vulnerable.
• Ongoing conflict and state fragility in Libya compounded the effects of the flooding, contributing to a lack of maintenance and deterioration of dam infrastructure over time and increasing peoples’ risk and the resulting impacts. The conflict also limits nation-wide adaptation planning and coordination across a range of climate issues facing the country, such as water scarcity, and extreme weather including heat and floods.
• In addition to the lack of maintenance, the Al-Bilad and Abu Mansour dams were built in the 1970s, using relatively short rainfall records, and may not have been designed to withstand a 1-in-300 to 600 year rainfall event. A full after-action review looking at the design criteria of the dams will be required to understand the extent to which the dams’ design, and the lack of subsequent maintenance contributed to the disaster. Even still, catastrophic dam failures and its impacts can be limited through risk reduction protocols that include real-time monitoring of forecasts, water volumes, and warning systems that alert those downstream of possible failures and the need to evacuate.
• While in Libya there was a forecast with a 3-day lead time on the track of Storm Daniel, the impact of that potential rainfall on infrastructure and people was not clearly understood in advance. Further, it is not clear to what extent forecasts and warnings were communicated and received by the general public, or relevant emergency responders. In conjunction with improved emergency management capacity, impact-based forecasts may help to provide a clearer understanding of how the rainfall translates into potential impacts and could lead to improved warnings in the future.
• This disaster also points to the challenge of needing to design and maintain infrastructure for not just the climate of the present or the past, but also the future. In Libya, this means taking into account the long-term decline in average rainfall, and at the same time, the increase in extreme rainfall like this heavy rainfall event; a challenging prospect, especially for a country plagued by crises.

Several heat deaths have been confirmed in the US, including migrants on the US Mexican border. In Mexico alone over 200 people died due to the heat. Spain, Italy, Greece, Cyprus, Algeria, and China also reported heat deaths, as well as a large increase in hospitalisation due to heat related illnesses. Large parts of the population in Italy and Spain and over 100 million people in Southern US are under heat alerts. In all three regions, demand for power spiked and negatively impacted a number of important crops, including olive oil in Spain and cotton in China. 

Scientists from the World Weather Attribution initiative collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme July heat in these three regions.

Using published peer-reviewed methods, we analysed how human-induced climate change altered the likelihood and intensity of 1) 18-day average maximum temperatures over the most affected regions in western US, Texas and northern Mexico (fig 1, top). 2) 7-day average maximum temperatures over land in the rectangular box(5W-25E, 36-45N) covering the most affected region (fig1, middle). 3) 14-day average maximum temperatures over the lowlands of China, again covering the most affected region (fig 1, bottom). 

Main findings

• Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-related causes each year. However, the full impact of a heatwave is rarely known until weeks or months afterwards, once death certificates are collected, or scientists can analyse excess deaths. Many places lack good record-keeping of heat-related deaths, therefore currently available global mortality figures are likely an underestimate.
• In line with what has been expected from past climate projections and IPCC reports these events are not rare anymore today. North America, Europe and China have experienced heatwaves increasingly frequently over the last years as a result of warming caused by human activities, hence the current heat waves are not rare in today’s climate with an event like the currently expected approximately once every 15 years in the US/Mexico region, once every 10 years in Southern Europe, and once in 5 years for China. 
• Without human induced climate change these heat events would however have been extremely rare. In China it would have been about a 1 in 250 year event while maximum heat like in July 2023 would have been virtually impossible to occur in the US/Mexico region and Southern Europe if humans had not warmed the planet by burning fossil fuels. 
• In all the regions a heatwave of the same likelihood as the one observed today would have been significantly cooler in a world without climate change. Similar to previous studies we found that the heatwaves defined above are 2.5°C warmer in Southern Europe, 2°C warmer in North America and about 1°C in China in today’s climate than they would have been if it was not for human-induced climate change. 
• Unless the world rapidly stops burning fossil fuels, these events will become even more common and the world will experience heatwaves that are even hotter and longer-lasting. A heatwave like the recent ones would occur every 2-5 years in a world that is 2°C warmer than the preindustrial climate.
• Heat action plans are increasingly being implemented across all three regions and there is evidence that they lead to reduced heat-related mortality. Furthermore, cities that have urban planning for extreme heat tend to be cooler and reduce the urban heat island effect. There is an urgent need for an accelerated roll-out of heat action plans in light of increasing vulnerability driven by the intersecting trends of climate change, population ageing, and urbanisation. 

To analyse whether and to what extent human-caused climate change altered the likelihood and intensity of this extreme rainfall, scientists from Italy, the Netherlands, France, the US, and the UK used published, peer-reviewed methods to perform an event attribution study. Given that the deluge and the impacts over the Emilia-Romagna region (red outline in fig. 1) were a consequence of a series of short but heavy rainfall spells occurring within a span of three weeks, the team looked at an event defined as the maximum accumulated 21-day rainfall (Rx21d) in April-June (AMJ), averaged over the region.

Main Findings 

• While the full profile of the impacts on human life and livelihoods has yet to be analysed, initial assessments show that the floods and landslides caused 17 fatalities and displaced roughly 50.000 people. The majority of the deceased were elderly and died in their homes, in many cases linked to either reduced mobility or reluctance to evacuate. These deaths highlight how pre-existing vulnerabilities such as disability and limited risk perception exacerbated the impacts in the region. 
• For observational analysis, we looked at spring rainfall in the study region based on a dense network of about 60 weather stations in the area that have consistent data since at least the 1960s. The heavy rainfall over the first 21 days of May 2023 is the wettest event of this type in the record with a return time estimated to be about 200 years. This means that in any given year, the chance of such an event occurring is about 0.5%.
• In the station data as well as other observational data products there is no significant trend in the 21-day spring rainfall: thus the amount of rain that falls in a 200-year event today is the same as in a 200-year event at the beginning of the record.
• To determine whether there is indeed no trend due to human-induced climate change or whether a trend is masked by changes in other drivers of rainfall, such as land-use changes or changes in aerosols, we look at the same 21-day event in climate models with and without the human-induced increase in climate change. Of the 19 models used, none of them show a significant change in the likelihood or intensity of such an event to occur. This suggests that in contrast to most parts of the world, there is indeed no detectable increase in heavy rainfall in the Emilia-Romagna region in spring.
• This finding corroborates earlier research that has found that with human-induced climate change the number of low-pressure systems in the central Mediterranean has decreased. This leads to a reduction in heavy rainfall, offsetting the expected increase in heavy rain from global warming.
• In recent decades, rapid urbanisation and increasingly dense urban fabric has limited space for water drainage and increased risk of flooding, which has exacerbated the impacts of the heavy rainfall. However, this was an extremely rare event, and most infrastructure cannot reasonably be built to withstand such low-frequency events. 
• There are many adaptation options that are robust to multiple types of extremes (e.g. drought, heat, floods) and also have co-benefits for societal well-being and biodiversity, which can increase the resilience of this region to future extremes. Nature-based solutions, social protection, and improved urban planning, are just a few examples. 

These record-shattering temperatures came on top of a historical multi-year drought in those regions, exacerbating the impacts of the heat on agriculture which is already threatened by an increasing water scarcity resulting from the combined effect of climate change and water use.  

While verified mortality data from the current heatwave are not yet available, we do know that in 2022 heatwaves contributed to nearly 4000 deaths in Spain and over 1000 deaths in Portugal (WHO, 2022). Every year, an average of 262, 250, and 116 people die from heat-related illness in Algeria, Morocco, and Tunisia, respectively (Hajat et al., 2023). In Tunis, a review of all-cause mortality between 2005-2007 found that for every degree Celsius over 31.5C, the daily mortality increased by 2% (Bettaieb et al, 2020). Early season heatwaves tend to be particularly deadly because of a lack of acclimatisation of the population, lower preparedness for heat (e.g. people have not yet brought out fans or A/Cs from storage), and harvesting effects (Gasparrini et al., 2016; Lee et al., 2014). 

Scientists from Morocco, France, the Netherlands, the US  and the United Kingdom, collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of this early season heatwave.

Using published peer-reviewed methods, we analysed how human-induced climate change altered the likelihood and intensity of the 3-day heatwave event that occurred on 26-28 of April 2023, in the most affected region (see Figure 1, black outline).

Main findings

• Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-related causes each year. However, the full impact of a heatwave is often not known until weeks or months afterwards, once death certificates are collected, or scientists can analyse excess deaths. Many places lack good record keeping of heat-related deaths, therefore currently available global mortality figures are likely an underestimate.
• Early heatwaves and associated drought conditions also threaten the yield for many crops such as wheat, because it hinders grain filling. This heatwave has come at a critical time for the crop season in the Western Mediterranean countries.
• While Europe and North Africa have experienced heatwaves increasingly frequently over the last years, the recent heat in the Western Mediterranean has been so extreme that it is also a rare event in today’s warmer climate. Our estimate of observed temperatures averaged over 3 days were estimated to have a return period of approximately 400 years (at least 60 years) in the current climate, meaning they have approximately a 0.25% chance of happening in any given year.
• ​​To estimate the influence of human-caused climate change on this extreme heat we combine climate models with the observations. Observations and models both show a strong increase in likelihood and intensity but the change is systematically lower in the models than in the observations. The fact that extreme heat is increasing faster than climate models simulate is a known problem in summer in Western Europe, in all climate models, and is also found here. 
• The combined results, giving an increase in the likelihood of such an event to occur of at least a factor of 100, is therefore likely too conservative. At the same time, a heatwave with a chance of occurrence of 0.25% in any given year (return period of 1-in-400 years) would have been at least 2C cooler in a 1.2°C colder world. 
• This discrepancies between the modelled and observed trends and variability also hinders confidence in projections of the future trends. In a future 0.8°C warmer climate (reaching a global warming of 2C above pre-industrial levels) such a heatwave would be another 1°C hotter, but as above, this is probably a very conservative estimate. 
• Heat-related fatalities have decreased in cities with urban planning for extreme heat. This has proved effective in Spain, and notably in Lisbon, Portugal, where  the urban heat island effect has been reduced through incorporating more green and blue spaces. In addition, early warning systems for heat, simple self-protective behaviours such as drinking enough water, city heat action plans, strong social ties, and improved risk perception have been shown to reduce heat-related health impacts.