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.  

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.

It leads to clear skies and high temperatures as well as strong winds due to an inverted pressure gradient between an intense subtropical high extending well into midlatitudes and the coastal low farther north. These conditions are highly conducive to wildfires i.e favour spread, especially during summer when conditions are warmer and drier. On February 2, 2024, wildfires ignited in the mountainous forested areas east of, forested areas east of the city Viña del Mar and around Lake Peñuelas.

The flames rapidly advanced into densely populated city outskirts despite authorities’ efforts to curb their spread. The humanitarian impact continues to worsen, with over 29,000 hectares burnt since February 4, resulting in 132 deaths, 300 missing, 7,200 houses destroyed and 40,000 people affected.

Researchers from Chile, Brazil, Colombia, the Netherlands, Germany, Sweden 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 fires, and how the conditions will be affected with further warming. 

Fire weather is defined by a combination of elevated temperatures, reduced humidity, minimal rainfall, and heightened winds persisting over a period. To capture the characteristics of this event, we use a fire weather index (HDWI) based on high wind speeds, high temperatures and low humidity. While not taking the build-up of fuel into account like other, more complex indices, it is an effective hazard metric for estimating threat to communities and difficulty of containment. We focussed on a coastal region affected by the wildfires that include Valparaíso and Viña del Mar.

Figure 2 shows the index values for the four days of highest fire intensity, when most impacts occurred.

Main findings

• There is significant interaction between extreme climate conditions and land management policies, leading to particularly devastating impacts in informal settlements.
• Fire weather is one important factor that drives wildfires, although changes in vegetation (wildfire fuel), ignition factors, and fire management strategies also contribute to future wildfire risk (‘risk’ refers to the combination of hazard, exposure and vulnerability, following common usage in climate science).
• To combine the different variables leading to high fire danger we compute the hot dry windy index (HDWI) that combines high temperatures, high wind speeds and low humidity. In this index, we find that the hot, dry and windy conditions that drove the wildfires of February 2024 are characterised as a 1 in 30-year event in today’s climate.
• 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-30-year 4-day event over the region of the most devastating fires (fig. 2).
• We find that overall, there is a small increase in the HDWI in the observations and some models, but it is not significant. This is also true for the individual components of the index: maximum temperature, relative humidity and wind speed, none of which show a significant trend.
• These results are not surprising, given the coastal location of the region which has a well established wind-driven sea surface temperature cooling.
• The fires occurred in the coastal range of central Chile, right in the transition of coastal cooling and inland warming, rendering it difficult for most climate models to represent the event well. Only 5 models were able to simulate the event, and only 2 of these perform well in the model evaluation, thus results are very uncertain. Furthermore, the observation-based data are also comparably short and highly uncertain, particularly with respect to the wind component.
• Nevertheless, unless the world rapidly stops burning fossil fuels, fire danger due to high HDWI will increase. Using the same limited models as above, in a world 2°C warmer than preindustrial, the trend towards higher HDWI becomes significant.
• We conclude that despite the trend in HDWI not yet being significant, the risk of an increase in dangerous fire weather conditions attributable to human-induced climate change needs to be taken very seriously.
• We then assess to what extent El Niño is related to dangerous fire weather conditions. We test two different indices to characterise the influence of El Niño and find that El Niño has no significant influence on the HDWI.
• Across the studied area, fire risk is increasing notably due to current land management practices, such as the expansion of Wildland-Urban Interface areas (including the growth of informal settlements in forest zones) and widespread conversion from native to foreign and monoculture plantations.
• The existing investments in fire prevention and adaptation measures, coupled with low-risk perception among residents in fire-prone areas, have shown limited effectiveness in adequately mitigating the fire risk.
• Spared by the deadly flames, the pilot fireproofing program of Villa Botania
  showcases the life-saving potential of preparedness, including measures such as community-led vegetation control, embedding water points across the land, and robust emergency training.
• Measures to address the significant fire risk should encompass improved spatial planning; enhanced coordination, resource allocation, and community engagement in fire prevention and adaptation; and awareness raising campaigns

The large riverine system powers significant portions of the affected countries’ energy through hydropower, with Brazil relying on hydro power for 80% of its electricity, Colombia 79%, Venezuela 68%, Ecuador and Peru 55%, and Bolivia 32% (USaids, 2018). The drought is significantly impacting dam capacities and energy output and led to power cuts in the region as early as June 2023. 

Scientists from Brazil, the Netherlands, the UK and the US used published peer-reviewed methods to assess whether and to what extent the drought has been influenced by climate change as well as the occurrence of El Niño, which is known to be associated with drought in the Amazon. While the drought started earlier in the west of the basin, the whole basin has been in severe or exceptional drought for the second half of the year (Fig. 1 blue outline). 

There are several ways to characterise a drought. Meteorological drought considers only low rainfall, while agricultural drought combines rainfall estimates with evapotranspiration. Increased evapotranspiration due to regional warming can play a major role in exacerbating drought impacts. In this study we assess agricultural drought as well as meteorological drought. The main variable used here to characterise agricultural drought is the Standardised Precipitation Evapotranspiration Index (SPEI) (figure 1) which uses the difference between rainfall and potential evapotranspiration to estimate the available water. The more negative the values are, the more severe the drought is classified. Meteorological drought is analysed here using an index (SPI) based on precipitation alone. 

Main findings

• Highly vulnerable populations were disproportionately affected by the drought. Small-holder farmers, indigenous-, rural- and river communities across the region were among the most vulnerable due to high poverty rates and their high dependency on agricultural food production, availability of freshwater, and import of goods via rivers. 
• Exposure to drought impacts was compounded by historical land, water, and energy management practices including deforestation, destruction of vegetation, fires, biomass burning, corporate farming, cattle ranching and other socio-climate problems which have decreased the water and moisture retention capacity of the land and thus worsened drought conditions.
• In datasets based on weather records the drought is exceptional, even in today’s climate, characterised as a 1 in 100 year event for the meteorological drought (SPI) and approx. a 1 in 50 year event in SPEI. While there is a strong drying trend in the meteorological drought, the trend in agricultural drought is even stronger meaning this agricultural drought would have been extremely rare in a cooler climate. 
• We first assess to what extent El Niño is a driver of this trend. El Niño reduced the amount of precipitation in the region by about the same amount as climate change; however, the strong drying trend was almost entirely due to increased global temperatures, so the severity of the drought currently being experienced is largely driven by climate change.
• In order to assess whether and to what extent human-induced climate change was a driver of this drought we combine observations-based data products and climate models and look at the 6 month meteorological (SPI6) drought as well as agricultural drought (SPEI6). We find that the likelihood of the meteorological drought occurring has increased by a factor of 10, while the agricultural drought has become about 30 times more likely. 
• Using the US drought monitoring classification system, based on agricultural drought, this means that what is now classified an exceptional drought (D4), would have only been a ‘severe drought’ (D2) without the effects of climate change, caused by burning fossil fuels and deforestation. 
• Unless the world rapidly stops burning fossil fuels and deforestation, these events will become even more common in the future. In a world 2°C warmer than preindustrial an event like this would become even more likely by a further factor of 4 for the agricultural drought (every 10-15 years) and a further factor of 3 for the meteorological drought (every ~30 years). 
• While all countries in the affected region have drought management plans, the recent droughts indicate a need to reform policy and better integrate proactive support for forecasts and early warnings, drought contingency plans,  sustainable water management practices and infrastructure investment to cope with future, more intense droughts. 
• These results highlight that despite ‘low confidence’ in IPCC projections for drought in the region, increasing water stress driven by human-induced climate change as well as other systemic factors continues to be a major threat for the population and requires urgent efforts for more effective water management strategies, interdisciplinary humanitarian response, and regional cooperation that includes farmers and other stakeholders in the planning.

Using published peer-reviewed methods, scientists from Brazil, Argentina, the Netherlands, United States of America and the United Kingdom, collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of 10-day maximum temperatures during the months August- September in the region most affected by the extreme heat (fig 1). 

Main findings

• While the full impacts of heatwaves remain unknown until months after the events, 4 casualties and many heat-related illnesses have been reported. Early spring extreme heat events often prove to be particularly impactful as local populations are not yet acclimatised to high temperatures. In addition, high population density, low vegetative cover and water-based spaces, high levels of air pollution, and inequality are additional risk drivers for mortality and morbidity within cities, rendering extreme heat particularly deadly for the urban poor. 
• Using gridded observational products and observations the heat event as defined above is approximately a 1 in 30 year event in today’s climate. 
• While there is a high-level of confidence in the gridded products used to carry out the analysis, these do not capture very local records, many of which were broken during this heat episode. To incorporate these in future studies and inform early warning systems high  quality and readily available weather station data is needed. 
• To estimate the influence of human-caused climate change on this extreme heat we combine climate models with the observations.We find that because of human-induced climate change the event would have been 1.4 to 4.3 °C cooler had humans not warmed the planet by burning fossil fuels. Due to the strong trend the change in likelihood is not well defined: it has increased by at least 100 times. 
• With future global warming, heat events like this will become even more common and hotter. At global mean temperatures of 2°C above pre-industrial levels, a heat event like this would be about another 5 times more likely and 1.1 to 1.6 °C hotter than today. 
• Although ENSO may have influenced the large-scale weather patterns, the direct contribution to the extreme heat is small, compared to the climate change signal. 
• While some losses will inevitably occur due to the extreme heat,  in particular with respect to ecosystems  it is misleading to assume that human impacts are inevitable. Adaptation to extreme heat can be effective at reducing morbidity and mortality. The authors were not able to identify any heat action plans that exist in the affected area, this leaves the potential for a window of opportunity to mitigate heat impacts on vulnerable people. Heat Action Plans that include early warning and early action, awareness raising and behaviour changing messaging, and supportive public services can reduce morbidity and mortality. 

Scientists from Argentina, Colombia, France, the United States of America, the Netherlands and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the low rainfall that led to drought, focussing on the particularly severe three months from October to December 2022.

Using published peer-reviewed methods, the team defined the event by the average precipitation during these three months for the region of largest impacts, outlined in blue in Figure 1, and analysed whether climate change altered the likelihood and intensity of the anomalously low rainfall. Given that the season was also characterised by multiple heatwaves (Rivera et al., 2022), the researchers additionally evaluated the effect of temperature, in particular, whether and to what extent climate change has influenced evapotranspiration, thereby exacerbating the drought.

Main findings

• The ongoing drought has led to severe impacts on agriculture, halving the annual harvests in wheat and soy in Argentina, which in turn is expected to lead to export deficits of 25-50%. The drought impacts hit the population on top of already high inflation and weakening local currency. In Uruguay, more than 75,000 people are suffering from lack of access to potable water; access to water for crops and livestock is also limited.
• Central South America has suffered from drought for the last three years, which saw consecutive La Niña There is a high correlation between the rainfall deficit in the study region during the months of October to December and the Niño 3.4 index. Thus, the rainfall deficit is in part driven by La Niña.
• In order to identify whether human-induced climate change was also a driver of the rainfall deficit, we analysed rainfall over the most impacted region and in 9 representative For the region as a whole, the event has a return period of 20 years, meaning it has a 5% chance of occurrence in any given year. At individual stations, it is a less common event, with return times up to 50 years.
• In the observations over the whole region we observed a trend of reduced rainfall over the last 40 years, although we cannot be confident that this trend is beyond what is expected from natural variability in the region.
• In order to identify whether the reduced rainfall is a real trend beyond natural variability that can be attributed to climate change, we looked at once in 20-year low rainfall events over the same region in climate models and found that the models show that low rainfall events decrease – ie they become wetter; the opposite of the trend observed in most weather records – although this trend is again not significant and is compatible with natural variability. Thus, we cannot attribute the low rainfall to climate change.
• This does not rule out that climate change affected other aspects of the drought. To investigate whether the high temperatures, which are in part attributable to climate change, led to a deficit in water availability, calculated as potential evapotranspiration subtracted from the rainfall, we repeated the analysis for this indicator.
• The results show that, in climate models, the increase in temperature does partly compensate for the increase in rainfall but only to offset the wettening, and does not lead to a significant climate change signal in effective precipitation..
• However, higher temperatures in the region, which have been attributed to climate change, decreased water availability in the models in late 2022, indicating that climate change probably reduced water availability over this period in the observations too, thus increasing agricultural drought, although the study cannot quantify this effect.
• This means even though the reduced rainfall is within the natural variability, consequences of drought are becoming more severe due to the strong increase in extreme heat.
• The high impact of the drought on agriculture and economic activity speaks to the need to reduce vulnerability to drought in this Measures such as improved water efficiency and management, anticipation of drought using seasonal forecasts, and insurance instruments to help farmers weather dry years could improve resilience to these types of events.

The area is also experiencing a prolonged drought that started in 2019 and has become worse since. Drought and heat reinforce each other and exacerbate impacts on agriculture. Harvests are expected to be the worst in seven years, with great economic losses for farmers and the Argentine Treasury, as Argentina is South America’s largest exporter of wheat [Successful Farming, News Dakota, Merco Press]. As a major player in the world’s wheat market, this means further increases in global food prices [National Interest]. Direct impacts of the heatwaves included large scale power cuts and wildfire outbreaks. Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-related causes each year (IFRC, 2020), with early season heatwaves known to be particularly deadly. However, the full impact of a heatwave is often not known until weeks or months afterwards. The heatwave was well forecasted and while Paraguay still has to develop an early warning system for heatwaves, Argentina implemented such a system in 2018 which issued amber and red alerts for most parts of the affected area prior to the heatwave.

Scientists from Argentina, Colombia, France, New Zealand, Denmark, United States of America, the Netherlands and the United Kingdom, collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of these heatwaves.

Using published peer-reviewed methods, we analysed how human-induced climate change altered the likelihood and intensity of the 7-day heatwave event that occurred on 4-10 December 2022, in the most affected region (see Figure 1, black outline).

Main findings

• The 2022 heatwave has led to large-scale power outages, wildfires and, in combination with the ongoing drought, poor harvests. It is estimated to have led to an increase in heat-related deaths, with the impacts unequally distributed across In different cities and municipalities across South America, people living in some areas – often poorer neighbourhoods – experience higher temperatures than others, as they lack green space, adequate thermal insulation from heat, electricity, shade, and water which can be lifelines during heatwaves.
• South America, like the rest of the world, has experienced heatwaves increasingly frequently in recent years. The recent heatwave, averaged over 7 days over the region shown in Figure 1, has an estimated return time of 1 in 20 years in the current climate, meaning it has about a 5% chance of happening each
• We further looked at 7 individual weather stations, to see whether the character of the heatwave differed within the study We found that at most stations the 7-day maximum temperatures observed during this heatwave have return times comparable to the region average – meaning it was equally unusual across much of the region – but the heat was more extreme towards the northwest of the region.
• To estimate how human-caused climate change has influenced the likelihood and intensity of the observed heatwave we combine climate models with the observations-based data.
• We find that human-caused climate change made the event about 60 times more Alternatively, a heatwave with a similar probability would be about 1.4°C less hot in a world that had not been warmed by human activities.
• With future global warming, heatwaves like this will become even more common and hotter. If global mean temperatures rise an additional 8°C, to a total warming of 2°C, a heatwave as hot as this one would be about 4 times more likely than it is now, while a heatwave that happens approximately once in 20 years would be 0.7-1.2°C hotter than this one.
• There is a discrepancy between the modelled and observed change in heatwave intensity in the region with the observations showing a larger While there is no doubt that future heat extremes will become even hotter than they are now, this discrepancy limits confidence in projections of the magnitude of future extremes.
• Heatwaves this early in the season pose a substantial risk to human health and are potentially lethal. This risk is aggravated by climate change, but also by other factors such as an ageing population, urbanisation and the built environment, and individual behavior and susceptibility to the The full impact will only be known after a few weeks when the mortality figures have been analysed. Effective heat emergency plans, together with accurate weather forecasts such as those issued before this heatwave, reduce impacts and are becoming even more important in light of the rising risks.

These events displaced at least 25,000 people and more than 133 people died. While the densely populated Metropolitan Region of Recife (RMR) in the state of Pernambuco was particularly badly hit, with impacts concentrated in low-income neighbourhoods on or near hillslopes, 80 municipalities across Pernambuco and Alagoas declared a state of emergency following the events. Eastern Northeast Brazil is also the region of the country with the highest proportion of the population living in poverty, making it particularly vulnerable to changing intensity and likelihoods of extreme weather events.

To analyse whether and to what extent human-caused climate change altered the likelihood and intensity of this extreme rainfall, scientists from Brazil, the Netherlands, France, the US and the UK used published, peer-reviewed methods to perform an event attribution study, focused on the amount of rain that fell over 7-day and 15-day periods, in an area of coastal tropical climate (10°S-5°S; 36°W-45.5°W) that encompasses the region of the extreme event of May 2022. While the heaviest rain fell over the sea, we limit our analysis to the land, as that is where impacts occur.

Main findings

• The flooding and landslides leading to severe impacts were a direct consequence of extremely heavy rainfall in the week beginning on 23 May and continuing into June. We therefore assess the role of climate change in 7 & 15-day mean rainfall.
• While the full profile of the impacts on human life and livelihoods has yet to be analysed, initial assessments show that the floods and particularly landslides disproportionately affected vulnerable communities, with particular devastation in low-income neighbourhoods . Thus, the magnitude of this disaster on these groups has been exacerbated by pre-existing structural vulnerability in the region.
• Today, there is a dense network of 389 weather stations in the area. However, we select 75 stations that have consistent data since at least the 1970s and are distributed across the study region for our observational analysis. Both the 7- and 15-day events are exceptionally rare events, which have only approximately a 1-in-500 and 1-in-1000 chance respectively of happening in any year in today’s climate, which has been warmed about 1.2°C by human activities.
• Although they are still very unusual, these events are now more likely to happen than they would have been in a climate that had not been warmed by human activities. But, as both events are far outside the previously observed records, it is not possible to quantify how much more likely climate change has made them to happen, based on observations. Warming of the planet has also increased the intensity of the rainfall: rainfall events as rare as these, that occurred in a 1.2°C cooler climate, would have been approximately a fifth less intense.
• To determine the role of climate change in these observed changes we undertake the same assessment using climate models. While many climate models are able to simulate the main precipitation features over the region, we find that for this spatially small event, all models exhibit systematic errors in precipitation magnitudes, partly due to having coarse spatial resolution and misrepresentation of key physical processes (e.g. convection). We can therefore not quantify the role of climate change in the observed increase in likelihood and intensity.
• However, combining observations with our physical understanding of the climate system we conclude that human-caused climate change is, at least in part, responsible for the observed increases in likelihood and intensity of heavy rainfall events as observed in May 2022.
• These findings are consistent with future projections of heavy rainfall in the region and suggest that these trends will continue to increase as long as greenhouse gas concentrations continue to increase. Also, due to climate change, other factors such as rising sea levels and higher tides could increase the vulnerability to heavy rainfall, leading to more urban floods in Recife, for example.
• The extreme nature of the floods made it so that exposure was the main determinant of impact, although long-term impacts and recovery will likely be mediated by socio-economic, demographic and governance factors. An increase in urbanisation, especially unplanned and informal in low-lying flood-prone areas and steep hillsides have increased the community exposure to these hazards. While forecasts and warnings were provided, it is unclear to what extent these informed anticipatory or early action that could have reduced the impacts.
• This indicates the need to review and strengthen the linkage between weather warnings and the process that would lead to anticipatory action based on those warnings. This region also generally has an infrastructure deficit (e.g. housing, roads, water and sanitation etc.). As new infrastructure is built, there is an opportunity to increase resilience by accounting for increasing risks in the design and location, instead of reverting to outdated design standards.

“We found that it is scientifically possible to quantify historical responsibility of individual countries/regions for specific extreme events,” said Dr Friederike Otto, deputy director of the ECI and lead author of the study. “The fact that it is possible to provide such quantification will greatly advance the possibility of an informed discussion,” Otto added. “The aim of the study was to explore what science could contribute to the debate of climate justice.”

We found that it is scientifically possible to quantify historical responsibility of individual countries/regions for specific extreme events.

The team applied two different statistical methodologies to assign contributions of individual countries’ emissions to an extreme weather event, using the example of the Argentinian heatwave of 2013–14. While a previously published attribution study found that anthropogenic climate change overall made the event approximately five times more likely, the new analysis showed that when accounting for all historic emissions from 1850 onwards large emitters like the U.S. and the EU made the event approximately 28% and 37%, respectively, more likely. The differences between the different methodologies were small compared to the overall responsibility assigned to the individual region.

“Overall, we find that choices about how to do the calculations that are not only scientific but also moral and political determine the quantitative results,” said Dr. Jan Fuglestvedt, research director at CICERO.

The manuscript is published in Nature Climate Change.

Introduction

During summer 2014, there was a complete absence of SACZ episodes (Coelho et al. 2015). Previous major droughts occurred in the region in 1953/54, 1962/63, 1970/71, and 2001. While droughts have very complex criteria, these were all characterized by large rainfall deficits while the effect of the SACZ needs further investigation. The 1953/54 rainfall deficit prompted construction of the largest water supply system (Cantareira) used for São Paulo (Porto et al. 2014). The 2014/15 drought had major impacts in São Paulo due partly to a four-fold population increase since 1960 (Figure 1b). Although new water supply systems were constructed after Cantareira, it is still by far the largest in terms of capacity and number of people supplied (until early 2015) and hence is used as an indicator of the impacts of the SEB drought on water supply. In January 2015, Cantareira, which used to supply 8.8 million people in São Paulo, sank to a water volume of just 5% of capacity (Figure 1c), and currently supplies just 5.3 million people. Other systems (Guarapiranga and Alto Tiete) started to supply the excess population, those previously supplied by Cantareira, after the water crisis was established.

In this analysis, we investigate potential changes in the hydrometeorological hazard, defined by accumulated precipitation and the difference between precipitation and evaporation (P − E) in the SEB region. The true impact, however, is due to a combination of a physical event with vulnerability and exposure, in this case on millions of people in the affected area (Field et al. 2012).

The current drought reflects increasing trends in exposure. São Paulo’s population grew by 20% in the past 20 years. Water use has increased at an even faster rate over the same period (Figure 1b). Vulnerability of water supply systems remains high. Recognizing that water governance is key to reducing vulnerability, Brazil has advanced decentralisation of water management (Engle and Lemos 2010).

Other aspects of vulnerability give a more mixed picture. This drought has not resulted in sustained power outages, a common consequence of water shortages. Similarly, no cholera outbreaks have been reported, reflecting major public health investments (Barrato et al. 2011). Dengue, however, has spiked in São Paulo, with a tripling of cases in 2015 compared with 2014, including several deaths.

Data and methods

Drought can be defined in multiple ways and have multiple drivers (Field et al. 2012). Here we employ a multi-method approach to assess whether and to what extent anthropogenic climate change contributed to the 2014/15 drought event over SEB, using both observations and general circulation model (GCM) simulations of 14-month accumulated precipitation and P – E. We chose these measures to robustly assess the combined thermodynamic and dynamic effect of anthropogenic climate change on the drought. Future studies will disentangle these effects and analyze the driving mechanisms (e.g., Coelho et al. 2015). Our methods include: (i) trend and return period estimation for the 2014/15 event based on historical records; (ii) an estimation of the change in return periods of this event by comparing very large ensembles of SST-driven GCM simulations of the current climate with simulations of the climate in a “world that might have been” without anthropogenic greenhouse gas emissions; and (iii) a similar procedure using state-of-the-art coupled climate model simulations (CMIP5; Taylor et al. 2012).

1. The observational analysis is based on the GPCC-V6 analysis up to 2010 (Global Precipitation Climatology Centre; Schneider et al. 2014), GPCC monitoring analysis 2011–14, and GPCC first guess analysis Jan–Feb2015. The monitoring analysis was adjusted to GPCC-V6 using linear regression on the 1986–2010 overlap period. Figure 9.1a shows January 2014–February 2015 precipitation anomalies relative to the 1941–2010 mean. Eastern Brazil, including SEB, shows 25% to 50% deficits. Figure 9.1d shows 14-month precipitation running means averaged over SEB. No evidence of a trend was found in the mean, whereas dry extremes showed a barely significant decrease up to 2013 (Fig. 9.1d, lower panel). The 2014/15 SEB deficit is similar to previous events, with dry episodes around 1963, 1970, and 1954 more severe than the current episode up to February 2015. Figure 9.1e shows a generalised Pareto distribution (GPD) fit to the driest 20% records assuming a stationary distribution. The January 2014–February 2015 deficit (435 mm) return period is about 20 years (95% CI: 10–60 years).
2. We use the distributed computing framework— weather@home—to run the Met Office Hadley Centre atmosphere-only general circulation model HADAM3P (Massey et al. 2014) to simulate precipitation and P − E in two different model ensembles representing: 1) observed climate conditions of 2014/15, and 2) counterfactual conditions under pre-industrial greenhouse gas forcings and 11 different estimates of SSTs without human influence (Schaller et al. 2014). The empirical SEB total precipitation return periods (Figure 2a) show that in this approach dry precipitation extremes have become less likely due to anthropogenic greenhouse gas emissions: what would have been a 1-in-20-year precipitation deficit event like the 14-month 2014/15 event has become approximately a 1-in-30-year event (95% CI: 0 to 35 years). At the same time there is no detectable change in P − E due to human-induced climate change (Figure 2c) because of an increase in evaporation that cancels the increase in precipitation. The decrease in extreme low precipitation seen in SEB however is not uniform (consistent with observations; see Supplementary Figure 2a) across Brazil as a whole (Figure 2e).
3. We use the same approach as described in Lewis and Karoly (2014) and King et al. (2015) to estimate the fraction of attributable risk (FAR; Allen 2003) of precipitation totals below 25%, 20%, 15%, and 10% of the 1961–90 average and P − E below 170 mm (the 10th percentile) in a subset of the CMIP5 ensemble (see supplemental material). In contrast to the weather@home we find an increase in the risk of low precipitation with FARs greater than 0.167 (with 90% confidence) for the observed accumulated precipitation. However, the null result is confirmed with FARs slightly greater than zero for P − E.

Conclusion

While it has been speculated that anthropogenic climate change is a leading driver of the current drought (e.g., Escobar 2015) our multi-method approach finds limited support for this view. Evidence from observations shows large precipitation deficits becoming less common, albeit with large uncertainties. Likewise, large climate model ensembles show a non-significant effect of anthropogenic greenhouse gas emissions on the probability of low water availability (P − E). We therefore conclude the hydrometeorological hazard risk has likely not increased due to human-induced greenhouse gas emissions and the large impact of the 2014/15 event (particularly in the São Paulo region) is more likely driven by water use changes and accelerated population growth.

This does not, however, mean there is no human influence on the hazard itself. We expect (and observe) evaporation to rise due to higher temperatures as a direct consequence of the Clausius–Clapeyron relationship when there is enough water availability. However, this is not the case in droughts where a precipitation deficit over an extended period of time bounds evaporation. Hence the trend in evaporation tends to cancel the trend in dry precipitation extremes, giving a null result in P − E extremes. Overall, our analysis suggests changes in hydrometeorological risk are small while increasing water consumption increases the risk of profound water shortages.

The negative trend in observed dry extremes and large ensemble simulations is in contrast to a positive trend in CMIP5 dry precipitation extremes (Figs. 9.1f, 9.2b). This apparent contradiction could result from the differing physics among the CMIP5 models and weather@home, or from the different underlying assumptions of the different methodologies (e.g., climatological behavior in CMIP5 versus single year simulations using weather@home or SST-forced versus coupled). This highlights the importance of analyzing the same event using multiple methods as a means of better assessing confidence in our results.

Our analysis suggests the specific geographic location of the study area plays an important role in the results as São Paulo sits on the edge of the boundary between decreasing precipitation (to the north) and increasing precipitation (to the south) (Figures 2e,f). Future projections show a continuation of this general pattern, but given the large spread between models, scenarios, and seasons, it is possible the wet–dry boundary will shift leaving São Paulo’s precipitation future uncertain (van Oldenborgh et al. 2013). Hence, while the recent drought impacts were most likely not driven by an increase in hydrometeorological hazard, there is a risk that this may not hold in an even warmer world. Future analyses of the dynamical drivers of the hazard might allow this risk to be quantified.

Acknowledgements

We thank Antonio Divino Moura and David Karoly for their guidance and input on the manuscript and Dina Sperling and Roop Singh for all their help, our colleagues at the Oxford eResearch Centre and the Met Office Hadley Cen tre PRECIS team for their support for the application and development of weather@ home and all participants in climateprediction.net. The work was supported by the EUCLEIA project funded by the European Union’s Seventh Framework Programme (FP7/2007–2013).

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