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 storm intensified as it continued northwards towards the island of Taiwan, becoming a category 4-equivalent Typhoon on the 24th, with maximum (10-minute) sustained winds of 185 km/h. It made a prolonged landfall in northeast Taiwan on the 24th, bringing both heavy rain and high winds that killed 10 people and injured more than 900, while the agricultural sector reported damages of roughly US$50 million (FocusTaiwan, 2024a, FocusTaiwan, 2024b). It subsequently made landfall as a weaker, but still destructive tropical storm on mainland China on July 25. Gaemi brought heavy rainfall to coastal and inland regions, particularly the Hunan province, as it weakened to a tropical depression. Cyclone-based rainfall is uncommon so far inland in China and the heavy precipitation led to flooding and a mudslide that killed 15 people, and another 15 people in neighbouring provinces.35 remained missing a week after the disaster, and 290,000 people were evacuated (CNN, 2024).

The influence of climate change on tropical cyclones is complex compared to other types of extreme weather events. However, attribution studies are increasingly focusing on these destructive events. Rapid attribution studies to date have focused primarily on severe rainfall from such storms. Here, we use several different approaches to investigate the influence of climate change on multiple aspects of Typhoon Gaemi. The study focuses on the three geographic regions that experienced severe impacts – northern Philippines, the island of Taiwan and Hunan province, and analyse whether and to what extent human-induced climate change affected wind speeds and rainfall. To study the conditions that formed and fuelled Gaemi, we also analyse the role of climate change in high sea surface temperatures and potential intensity, a metric combining sea surface temperature, air temperature and air humidity data to predict maximum typhoon wind speeds. The study combines the established World Weather Attribution protocol with a new approach using the Imperial College Storm Model (IRIS) to analyse the role of human-induced climate change in tropical cyclones. 

Main findings

• Typhoon Gaemi brought destructive winds and rainfall to large regions of southeast Asia, including the northern Philippines, Taiwan, and Hunan . At least 90 people were killed, thousands were injured and hundreds of thousands had to leave their homes. The extreme rainfall and high winds triggered landslides, widespread power outages and severe damage to infrastructure and agriculture. 
• In today’s climate, that has already been warmed by 1.2C due to the burning of fossil fuels, weather observations indicate that rainfall events as severe as those brought by Typhoon Gaemi now occur about once every 20 (5 – 30) years in the northern Philippines, about once every 5 (1.5 – 20) years in Taiwan, and about once every 100 (90 – 160) years in Hunan province.
• To determine the role of climate change we combine observations with climate models. In Taiwan and Hunan, the rainfall was about 14% and 9% heavier respectively due to climate change, and in both regions, the rainfall total was made about 60% more likely by climate change. If the world continues to burn fossil fuels, causing global warming to reach 2°C above pre industrial levels, devastating Typhoon rainfall events in both regions will become 30-50% more likely.
• In the northern Philippines, the analysis did not identify a significant trend up to today. Observations indicate that 3-day rainfall events have increased by about 12%, however, there is large uncertainty in these data sets. Climate models suggest both increases and decreases in rainfall in the current climate, but an increase in a future climate with 2°C of warming. 
• The IRIS model was used to investigate Gaemi’s strong winds by analysing category 4-equivalent storms in the Western North Pacific basin, a region that includes the South China and Philippine seas.
• By statistically modelling storms in a 1.2°C cooler climate, this model showed that climate change was responsible for an increase of about 30% in the number of such storms (now 6-7 times per year, up from 5 times), and equivalently that the maximum wind speeds of similar storms are now 3.9 m/s (around 7%) more intense.
• The conditions that formed and fueled Typhoon Gaemi were studied for links to climate change, using potential intensity and sea surface temperatures surrounding the storm track in July 2024. These conditions occur about every second year for potential intensity and about once every 15 years for sea surface temperatures.
• The influence of climate change on potential intensity is highly uncertain, as observations show a very large increase with warming (about a factor of 100 and a potential intensity increase of 6 m/s) that climate models do not capture. Sea surface temperatures as hot as those observed in July 2024 were almost impossible without climate change and have become about 1 degree warmer. If global warming reaches 2°C, sea surface temperatures are projected to be another 0.6°C warmer, and the conditions associated with Typhoon Gaemi will continue to increase in likelihood by a further factor of about 10. 
• Together, these findings indicate that climate change is enhancing conditions conducive to Typhoons, and when they occur the resulting rainfall totals and wind speeds are more intense. This is in line with other scientific findings that tropical cyclones are becoming more intense and wetter under climate change.
• Rural communities with climate sensitive livelihoods (e.g. agriculture), the urban poor residing in the lowest lying land, and those living on exposed hillsides susceptible to landslides were the most affected by the multitude of hazards stemming from the typhoon. 
• The regions affected by Typhoon Gaemi have early warning systems and comprehensive emergency response systems in place for tropical cyclones that help manage impacts. Flood risk associated with extreme rainfall is well-assessed in the affected regions, but existing urban plans and flood control infrastructure are not able to withstand the more extreme floods that are driven by climate change. Unplanned urban development, including in Metro Manila where the population has rapidly increased, is increasing the number of people at risk, especially in lower lying informal areas.

Researchers from India, Sweden, the United States, and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of extreme rainfall that led to devastating  landslides and floods.

The soils in Wayanad were highly saturated, which is common in the region during the rainy monsoon season, meaning the meteorological cause of the landslide was the heavy rainfall on the preceding day of the event. Wayanad has been determined to be the most susceptible district to landslides in Kerala  (Sharma, Saharia & Ramana, 2024). 

To characterise the event, we analyse the 1-day maximum rainfall (RX1day) during the monsoon season from June to September, focusing on a region of northern Kerala (red outline, figure1). 

Main findings

• The Wayanad landslides resulted in devastating loss of life and occurred in a mountainous region with loose, erodible soils after 140mm of precipitation fell on saturated soils. 
• In today’s climate, which is 1.3°C warmer than it would have been at the beginning of the industrial period, an event of this magnitude is expected to occur about once every 50 years. The event is the third heaviest 1-day rainfall event on record, with heavier spells in 2019 and in 1924, and surpasses the very heavy rainfall in 2018 that affected large regions of Kerala.
• To assess if human-induced climate change influenced the heavy rainfall, we first determine if there is a trend in the observations. Heavy one-day rainfall events have become about 17% more intense in the last 45 years, over a period when the climate has warmed by 0.85°C. Longer-term trends in the pre-satellite era are not clear, which may relate to lower quality weather data. 
• To quantify the role of human-induced climate change we analyse climate models with high enough resolution to capture precipitation over the relatively small study region. Overall, the available climate models indicate a 10% increase in intensity. Under a future warming scenario where the global temperature is 2°C higher than pre-industrial levels, climate models predict even heavier 1-day rainfall events, with a further expected increase of about 4% in rainfall intensity. 
• Given the small mountainous region with complex rainfall-climate dynamics, there is a high level of uncertainty in the model results. However, the increase in heavy one-day rainfall events is in line with a large and growing body of scientific evidence on extreme rainfall in a warming world, including in India, and the physical understanding that a warmer atmosphere can hold more moisture, leading to heavier downpours. 
• While the extreme rainfall was well forecast by the Indian Meteorological Department (IMD) and warnings were issued, the information was at the state-level, making it difficult to discern which localities would be impacted by landslides (one of the potential impacts of heavy rainfall listed in the warning) and would therefore require evacuation. Slope-specific landslide early warning systems can be extremely costly and difficult to implement, but those would provide the best opportunity for effective early action. Given this, reducing exposure of people and assets to landslide-prone places may be a more effective strategy. 
• While the linkage between land cover and land use changes and landslide risk in Wayanad is mixed in the limited existing studies, factors such as quarrying for building materials, and a 62% reduction in forest cover, may have contributed to the increased susceptibility of the slopes to landslides when the heavy rain fell. 
• The increase in climate change-driven rainfall found in this study is likely to increase the potential number of landslides that could be triggered in the future, raising the need for adaptation actions that may include the reinforcement of susceptible slopes, landslide early warning systems, and construction of retaining structures to protect vulnerable localities. 

Located at the border with Bolivia and Paraguay, the Brazilian Pantanal comprises more than 15  million hectares. The wetland floods seasonally, from November to April, then drains in the dry season from May to October. It holds a huge range of unique species, is home to many indigenous groups, provides important ecosystem services for the surrounding area, supports the livelihoods of tens of thousands of ranchers, farmers and fishers, and is a vast carbon store. 

Indigenous and traditional communities are among the worst affected by the wildfires, as traditional lands are destroyed, cultural practices disrupted and people displaced. Economic activities such as tourism and agriculture are also threatened, with crop losses and livestock deaths. The fires have also killed innumerable wild animals and birds, destroyed vital habitat and made life much more difficult for the animals that were able to escape, as food and water has become increasingly scarce.

Human-induced climate change is increasing wildfires in many regions of the world, as hot, dry and windy weather conditions increase the risk of fires both starting and spreading. Researchers from Brazil, the Netherlands, Sweden and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the weather conditions that fuelled the Pantanal wildfires, and how the conditions will be affected with further warming. Due to the difficulty of accounting for human activity in both starting and suppressing wildfires, we attribute the fire weather conditions, not the burned area itself.

To illustrate the extent and duration of extreme fire weather in the region, we use the cumulative Daily Severity Rating (DSR) for June, averaged over the Brazilian Pantanal (indicated by the solid black outline in Figure 1a). The DSR indicates how difficult it is to control a fire once it starts and it is commonly used to assess fire weather over monthly or longer periods. The DSR is derived from the Fire Weather Index (FWI), which uses meteorological information (temperature, humidity, wind speed and precipitation over the preceding weeks and days) to predict the expected energy release per length of the fire-front if a wildfire occurs. We focus on the Brazilian Pantanal where nearly all active fires in June occurred; however, including the wider region, which extends into Bolivia and Paraguay, would likely yield similar results. 

Main findings

• Fire weather is a critical driver of wildfires, although changes in vegetation (wildfire fuel) and fire management strategies also contribute to future wildfire risk. In the Pantanal, land use and land cover changes, such as clearing natural vegetation for pasture or agriculture, contribute to drier conditions and increase the availability of flammable vegetation. 
• In today’s climate with 1.2°C of global warming, intense fire weather conditions like the ones that drove the wildfires in the Brazilian Pantanal during June 2024 are a relatively rare event, expected to occur once every 35 years. This means there is about a 3% chance similar June fire weather conditions will occur in any given year.
• Observations show that similar June fire weather conditions, as defined by DSR, are about 3 times more impactful than they would have been in a 1.2°C cooler climate. They would have been about a factor 100 rarer had the climate not been warmed by humans. 
• To determine the role of climate change, we combine fire weather observations with climate models. Human-induced warming from burning fossil fuels made the June 2024 DSR about 40% more impactful and 4-5 times more likely. 
• These trends will continue with future warming. If warming reaches 2°C, similar June fire weather conditions will become around twice as likely, expected to occur on average about once every 17 years, and will become 17% more impactful. 
• To understand how the June fire-weather conditions are affected by human-induced climate change, we also investigate the weather variables comprising the DSR: maximum temperature, relative humidity, wind speed and rainfall. Most of these variables broke records in June 2024: it was the driest, hottest, and windiest June since observations began. Only relative humidity was the second lowest on record. 
• Next, we analyse how climate change alters likelihood and intensity of these four main weather variables. In the observations there is a strong drying trend and, as expected, increasingly high temperatures (figure 1b) accompanied by a reduction in relative humidity, while there is no clear trend in wind speeds. Thus, the increase in DSR can be explained by increasing temperatures – driven by climate change – and decreasing rainfall.
• Yearly rainfall in the Pantanal has been decreasing for over forty years. While natural decadal variability and deforestation in large ecosystems are known to affect rainfall patterns across South America, climate change may also be influencing the drying trend.
• The June 2024 fires spawned multi-ministry response actions to try to contain fires and save wildlife and livelihoods, such as the establishment of 13 new bases to accelerate the deployment of firefighters to remote areas. However, while significant steps have been taken to address the Pantanal wildfires, there are still substantial challenges to containment and extinguishment efforts. It is imperative that government agencies at all levels act swiftly and prepare for increasingly critical situations, as projections indicate a rise in such events.  

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. 

Extreme and persistent heat has been overwhelming south western parts of the US, Mexico, and the northern countries of Central America. The area has been lying under a large and lingering region of high pressure, known as a heat dome, whereby hot air is trapped close to the ground and further heated under blue skies and sunshine. Whilst heat domes have a well known mechanism for intensifying heatwaves, these past weeks have seen records broken in both daytime and nighttime temperatures in several countries, including Mexico, Guatemala, Honduras and in the south western US.

Heatwaves are among the deadliest types of extreme events. Even though often the death toll is typically underreported especially during or straight after the event, during this hot season Mexico has already reported 125 deaths (SwissInfo). The coinciding ongoing drought is enhancing impacts of the heat even more.

Scientists from Mexico, Panama, the Netherlands, the United Kingdom, the United States, and Sweden collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme heat in a region that included the US southwest, Mexico, Guatemala, Belize, Honduras and El Salvador. Using peer-reviewed methods, we analyse 5-day maximum daytime and nighttime temperatures in May and June over a large region (see Figure 1) encompassing the region where impacts associated with extreme temperature records were reported.

Main findings 

• The extreme heat in the north and central America has resulted in severe impacts, including more than 125 heat-related deaths in Mexico since March, thousands of cases of heat stroke, and power outages. We likely do not know the full picture of heat-related deaths, since they are usually only confirmed and reported months after the event, if at all. 
• Existing drought conditions have further aggravated the situation by preventing the dispersion of polluting particles, decreasing water availability, and reducing hydropower generation and electricity supply. 
• Observations show that 5-day maximum temperatures in May-June such as recorded this year are expected to occur about every 15 years in today’s climate that has been warmed by 1.2C. However, around the year 2000, when global temperatures were half a degree lower than now, such events were expected to occur only about once every 60 years.
• The night time temperatures over the same 5-day period were also high, but not extreme in today’s climate; there is now a 50% chance per year of similar temperatures occurring. At the turn of the millennium such events would only have been expected to occur with a 13% chance in any given year.
• These return times are estimated for the region as a whole. It is important to highlight that the heat was more rare in the southeastern part of the region, especially for the nighttime temperatures with return periods of up to 1000 years in individual locations. 
• To determine the role of climate change we combine observations with climate models and we conclude that human-induced warming from burning fossil fuels  made the 5-day maximum temperature event about 1.4 degrees hotter and about 35 times more likely. For nighttime temperatures this is about 1.6 degrees hotter and about 200 times more likely.
• These trends will continue with future warming and events like the one observed in 2024 will be very common in a 2C world. 
• Extreme heat warning systems and action plans can help fill important gaps in preparedness across Central America. Heat safety protection laws can be enacted and implemented to protect outdoor workers across all countries. 
• Strengthened grid resilience and water conservation strategies are critical to ensure reliable services during heat events. Improved urban planning, more green spaces, and enhanced infrastructure in informal settlements will also help protect the most vulnerable.

These episodes occurred just outside of the region’s main winter rainfall season, which runs from November to early April. The unusually high rains and subsequent floods in April and May followed a three-month dry period from December to March

Researchers from Pakistan, the Netherlands, Sweden, the United States, Canada, France, Germany, and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the weather conditions at the time of the most impactful floods. 

To analyse the event, we focus on a region centred on Afghanistan, bounded on the west by the Iranian provinces of Razavi Khorasan, Sistan and Baluchestan, Hormozgan, Kerman, and South Khorasan, and on the east by Balochistan and Khyber Pakhtunkhwa provinces of Pakistan. This area covers the flood-impacted regions through April and May 2024. Due to the atypicality of this season, occurring outside of the usual rainfall period and featuring an unusual number of storms that made it wetter than normal, we choose the seasonal accumulated precipitation during April and May for the temporal definition. Fig. 1 shows the total rainfall during April-May 2024 and the anomaly with respect to 1991-2020 average, over the region.

Main findings 

• Afghanistan and Pakistan are highly vulnerable to flooding due to factors such as limited transboundary water management, unplanned urban expansion, and deforestation which are contributing to increased flood risks, in combination with socio-economic conditions and compounding natural hazards, e.g. earthquakes, landslides, and drought. While Iran is less vulnerable than the other countries studied, urban infrastructure-related vulnerabilities in some cities in the northeast contributed to the impacts.
  
• The floods also occurred on top of existing vulnerabilities linked to complex crises. Displaced populations were particularly impacted, especially as limited essential infrastructure was destroyed and already vulnerable populations were exposed to more waterborne diseases.
  
• The event, despite occurring outside the usual rainy season, is not a particularly rare event in today’s climate that has been warmed by 1.2°C with a return time of about ten years under the current El Niño Southern Oscillation (ENSO) conditions. 
• The declining El Niño Southern Oscillation, a naturally occurring climate phenomenon, is  important to explain the variability in the observed rainfall, consistent with previous research. In observations, as compared to a neutral ENSO year, the declining El Niño resulted in a consistent increase across all datasets by a factor of about two  in likelihood and about 8% in intensity.
• To assess the role of human-induced climate change we combine observation-based products and climate models that include the observed ENSO relationship and assess changes in the likelihood and intensity for the heavy rainfall in the study region. While the last 40 years of  observational data show an increase, climate models have a very different signal, depending on the model, with some showing an increase and some a decrease. Consequently, without further analysis into why the models show such different behaviour we can not attribute the observed increase, which is also not consistent across observation-based products, to human-induced climate change. 
• The disagreement between model results and observations prevents us from concluding with certainty that human-induced climate change is the main driver making this event more likely. However, given the observed trend over the last 40 years, the absence of evidence does not mean that human-induced climate change is not a driver of increasingly heavier rainfall in this region and season in a warmer climate.
  
• There are ample opportunities to improve climate adaptation and resilience through, for example, investing in building resilient infrastructure and reinforcing existing structures to withstand extreme events, implementing more comprehensive nature-based solutions, increasing the coverage of early warning systems, and improving flood risk management policy and planning. 

Researchers from Brazil, the United Kingdom, Sweden, the Netherlands, and the US collaborated to answer the question of whether and to what extent human-induced climate change altered the likelihood and intensity of the rainfall that caused the flooding. They also investigated the role of the El Niño Southern Oscillation (ENSO). 

Rainfall in Southern Brazil (comprising the states of Paraná, Santa Catarina, and Rio Grande do Sul) is characterised by a subtropical climate (transition between tropical and temperate climate) with a continuous supply of moisture from the Atlantic Ocean and the Amazon region thus no distinct rainy seasons exist. Rainfall varies from year to year depending on large scale climate phenomena such as ENSO. 

To capture the nature of the extreme rainfall that resulted in extreme flooding across Rio Grande do Sul, two event definitions are analysed in this study: the 4- and 10-day rainfall accumulations, averaged over the state of Rio Grande do Sul. The 4-day window captures the most severe single event in which record rainfall fell across several consecutive days, while the 10-day window (encompassing 26th April – 5th May, inclusive) captures the succession of heavy rainfall events, including the very wet individual days either side of the major 4-day peak (figure 1).

Main findings

• The unprecedented 2024 April-May floods in Rio Grande do Sul have affected over 90% of the state, an area equivalent to the UK, displacing 581,638 people and causing 169 deaths. While Rio Grande do Sul is often perceived as a well-off region, it still has significant pockets of poverty and marginalisation. Low income has been identified as a significant driver of flood impact. Informal settlements, indigenous villages, and predominantly quilombola (descendants of enslaved Africans) communities have been severely impacted. 
• The lack of a significant extreme flood event, until recently, in Porto Alegre led to reduced investment, and maintenance of its flood protection system, with the system reportedly beginning to fail at 4.5m of flooding despite its stated capacity to withstand water of 6m. This, in addition to the extreme nature of this event, contributed to the significant impacts of the flood and points to the need to objectively assess risk and strengthen flood infrastructure to be resilient to this and future, even more extreme, floods. 
• Both rainfall events characterised above, the 10-day and 4-day events, were found to be extremely rare in the current climate, with return periods of 100-250 years. To increase the statistical stability of the analysis given the relatively short data records, we use the 1 in 100 year event for the analysis in this study. This return period is also typically considered a benchmark for risk analysis. 
• The El Niño Southern Oscillation, a naturally occurring climate phenomenon, was found to be important to explain the variability in the observed rainfall, consistent with previous research. Most previous heavy rainfall events in the area occurred during El Niño years. 
• The role of El Nino alone is comparably large. In observations, compared to a neutral ENSO phase, the current (December-February) El Niño resulted in a consistent increase across all datasets and for both events: by a factor of 2-3 in likelihood and 4-8% in intensity for the 10-day event, and a factor of 2-5 in likelihood and 3-10% in intensity for the 4-day event. 
• To assess the role of human-induced climate change we combine observation-based products and climate models that include the observed ENSO relationship and assess changes in the likelihood and intensity for the 10-day and 4-day heavy rainfall over Rio Grande do Sul and find an increase in likelihood for both events of more than a factor of 2 and intensity increase of 6-9% due to the burning of fossil fuels. 
• These findings are corroborated when looking at a climate of 2oC of global warming since pre-industrial times where we find a further increase in likelihood of a factor of 1.3-2.7 and an increase in intensity of about 4% compared to present day. Again results are similar for both event definitions.
• While environmental protection laws exist in Brazil to protect waterways from construction and limit land use changes, they are not consistently applied or enforced, leading to encroachment on flood-prone land and therefore increasing the exposure of people and infrastructure to flood risks.
• Forecasts and warnings of the floods were available nearly a week in advance, but the warning may not have reached all of those at risk, and the public may not have understood the severity of the impacts or known what actions to take in response to the forecasts. It’s imperative to continue to improve the communication of risk that leads to appropriate, life-saving action.

Over the 12-month period, 6.3 billion people (about 78% of the global population) experienced at least 31 days of extreme heat (hotter than 90% of temperatures observed in their local area over the 1991-2020 period) that was made at least two times more likely due to human-caused climate change.

Over the last 12 months, human-caused climate change added an average of 26 days of extreme heat (on average, across all places in the world) than there would have been without a warmed planet.

Hundreds of people lost their lives in the floods and more than 700,000 were affected by the floods across all countries due to infrastructure damages, school closures, lost livestock and thousands of hectares of damaged crops. 

Researchers from Kenya, the Netherlands, Germany, Sweden, Denmark and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the rainfall that led to the severe flooding in the most affected region. 

The impacts were most severe in the region around Lake Tanganyika, Lake Victoria, the central Highlands (including Nairobi), southeast lowlands of  Kenya and coastal Tanzania between the end of March and most of April. To capture this event we looked at the 30-day maximum accumulated rainfall during the long rains (March to May) in the area outlined in red in Figure 1. 

Main findings

• Countries in East Africa have been facing disaster after disaster, including prolonged drought between 2020-23,  and multiple episodes of torrential rainfall leading to severe flooding. These disasters combine to create a complex humanitarian emergency that includes displacement, infrastructure loss, food insecurity, health risks, disrupted livelihoods, and overall weakened resilience. 
• Rapid urbanisation in cities across East Africa is amplifying flood risks, especially in large informal areas that are located on flood-prone land, lack adequate structural protections from the rains, and whose residents lack resources to recover and rebuild. Land-use changes, including deforestation and conversion to agricultural land are also occurring to different degrees in each of the countries studied, adding to flood risk. 
• The East African long rains were observed to show a drying trend towards the end of the 20th century, while climate models projected an increase in heavy rainfall with global warming. While this so-called East Africa Paradox is not as pronounced anymore, with observed precipitation increasing and a new generation of climate models showing weaker or no wettening trend, interpreting observations and climate models is still challenging in this region. 
• The observations, independent of the exact region and data product, do not show a long term trend, but instead a drying trend towards the end of 20th century up until around 2008 and a wettening in the last 15 years. Regardless of whether the recent recovery is being enhanced by human-induced climate change, the increased precipitation does bring an increased risk of flooding to the region.
• To understand if human-induced climate change is indeed playing a role, we also assess whether there are wettening or drying trends in the region for the long rains in climate models. While the trends are not statistically significant, they do show a wettening. On average, an event like this has become about twice as likely and 5% more intense in today’s climate, representing the effect of 1.2C of global warming. 
• Looking at the future, for a climate 2°C warmer than in preindustrial times, models suggest that rainfall intensity and likelihood will increase further.  
• We also examined whether the current phase of the El Nino Southern Oscillation or the Indian Ocean Dipole played a role in the intensity and likelihood of the wet March-May rainy season. Both modes of natural climate variability have been found to exhibit a negligible influence on the 2024 long rains in the study region. 
• Taking these findings and the known physical relationship that heavy rainfall is expected to increase in a warming world, we conclude that the observed increase in rainfall in the region over the last 15 years is in part driven by human-induced climate change. 
• Therefore, investing in flood resilience with future warming is paramount. 
• While early warning systems in each of the countries exist and warn of extreme rainfall, there is room to expand the action taken based on warnings to adequately protect people from the rainfall impacts. Social protection programs can fill gaps in instances where it’s not possible to avoid all impacts, in order to help people recover their assets and livelihoods after the disaster. 
• Disaster preparedness policies, flood preparedness and protection infrastructure, and early warning systems that are in place across Kenya, Tanzania and Burundi are all steps in the right direction, but must be integrated and implemented at scale in order to reduce impacts.