Climate Change and the Escalationof Global Extreme Heat: Assessing
and Addressing the Risks
A look at global extreme heat over the past 12 months, how climate change
has influenced this heat, and strategies to prevent increasingly frequent andintense heat from claiming lives worldwide.
May 28, 2024

Climate Change and the Escalation of Global
Extreme Heat: Assessing and Addressing the Risks
A look at global extreme heat over the past 12 months, how climate change
has influenced this heat, and strategies to prevent increasingly frequent andintense heat from claiming lives worldwide.

What’s inside the report:
Ahead of Heat Action Day, this new report from scientists at World Weather Attribution, the Red
Cross Red Crescent Climate Centre, and Climate Central assesses the influence ofhuman-caused climate change on dangerous heat waves over the past 12 months (May 15,2023 to May 15, 2024).
The period of analysis spans Earth’s hottest year on record (2023) and 11 consecutive monthsof record-breaking global temperatures (June 2023-April 2024). The report found thathuman-caused climate change is boosting dangerous extreme heat for billions, and making heat
events longer and more likely.
Key findings from the report include:●

Using World Weather Attribution criteria, the study identified 76 extreme heat waves thatspan 90 different countries. These events put billions of people at risk, including indensely populated areas of South and East Asia, the Sahel, and South America.

●

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 observedin 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 ofextreme heat (on average, across all places in the world) than there would have been
without a warmed planet.

This report also demonstrates the crucial role of tracking and reporting on impacts in extreme
heat assessment, and offers actionable solutions to heat risk.
➤ Download data: for May 15, 2023-May 15, 2024 for 168 countries and all U.S. states

PAGE 2

Written in collaboration by:
Climate Central
Red Cross Red Crescent Climate Centre
World Weather Attribution

Authors & Reviewers
Julie Arrighi, Red Cross Red Crescent Climate Centre; Global Disaster Preparedness Center of the American
Red Cross; University of Twente; World Weather Attribution
Friederike E. L Otto, World Weather Attribution; Grantham Institute - Climate Change and the Environment,
Imperial College London
Carolina Pereira Marghidan, Red Cross Red Crescent Climate Centre; Royal Netherlands Meteorological
Institute (KNMI); University of Twente
Sjoukje Philip, Royal Netherlands Meteorological Institute (KNMI); World Weather Attribution
Roop Singh, Red Cross Red Crescent Climate Centre; World Weather Attribution
Maja Vahlberg, Red Cross Red Crescent Climate Centre; World Weather Attribution
Joseph Giguere, Climate Central
Andrew J. Pershing, Climate Central
Arielle Tannenbaum, Climate Central
Abbie Veitch, Climate Central
Major funding provided by the Bezos Earth Fund and The Schmidt Family Foundation.

1. Introduction
The year 2023 was the hottest year on record. July 2023 was the hottest month ever recorded
and July 6, 2023 was the hottest day ever. Since then, each one of the last 12 months havebroken their previous monthly record for highest average temperature.
This is not a surprise or an accident — the causes are well known and the impacts devastating.
The continuous burning of coal, oil and gas has released enough greenhouse gasses to warm
the planet by 1.2ºC since pre-industrial times. Year after year, human-induced climate change
manifests through more intense and frequent extreme weather events, with heat waves being
the most dramatically affected.

PAGE 3In the last decades, climate science has come a long way in understanding the role of climatechange in fueling extreme temperatures and heat waves. Thanks to developments in attribution
science and climate models, we can now quantify how much heat climate change has added toan extreme temperature event, how much more frequent and intense heat waves will become if
we don’t stop emitting greenhouse gasses, and how many people have died because of theextreme heat brought about by human-induced climate change.
One of the most consistent findings is that every heat wave happening today has been mademore likely, more intense, and longer-lasting due to humans burning fossil fuels (Clarke et al.,2022). In the last few years, we have also identified many deadly extreme heat events that would
have been virtually impossible without human-induced climate change. Although floods andcyclones make more headlines, heat is arguably the deadliest extreme event, with thousands ofextreme heat-related deaths reported each year and many more that go unreported.
World Weather Attribution (WWA), an international group of scientists that analyzes the role ofclimate change in extreme weather, monitors heat waves and other events across the world.
Using a set of fixed criteria, they identify the most damaging extreme events and decide whether
to analyze them based on their impacts and the team’s capacity to undertake the study.
Unlike what happens with floods or droughts, where the human impacts are rapidly felt andacknowledged, the consequences of extreme heat often go unnoticed. Countries seldom
declare emergency states or appeal for humanitarian support because of a heat wave. That is
why the criteria WWA uses to detect the most potentially impactful heat waves has to bedifferent than for other extreme events. Currently, the team will consider conducting studies onheat waves if they meet one of these two criteria:
1. Record-breaking or unseasonably high temperatures over a large geographic area, for a
prolonged period (3+ days), are either forecasted or observed in global data products.
2. Media reports of ≥10 heat-related deaths or major disruptions to critical sectors (notablytransportation, energy, and manufacturing) are found.
And provided they also meet at least one of these:
3. The heat identified is occurring during the first 3-6 weeks of the hot season (due toheightened vulnerability of early season extreme temperatures).
4. The heat is occurring in a densely populated area (≥200 people/km^2).
5. The heat is occuring in a highly vulnerable area (using ≥4.8 on the INFORM index as anindicator) and/or high lack of coping capacity (≥6.0 INFORM).
Using these criteria, 76 extreme heat events were triggered between May 15, 2023 and May 15,2024 in 90 different countries across all continents, excluding Antarctica. These extreme heat

PAGE 4

events had the potential to affect billions of people in densely populated areas such as Southand East Asia, the Sahel, and the megacities in South America. For most of the triggered events
that fell outside of Europe and North America, we do not have any information on how many
people were actually affected and how. What is increasingly clear, however, is that when we dohave information on impacts, they are dire and are mostly affecting the most vulnerable people
in every country.
Out of the 76 triggered extreme heat events, the WWA team studied seven in depth:
●
●
●
●
●
●
●

The extreme humid heat in South and Southeast Asia in May 2023
The extreme heat in Southern Europe, North America and China in July 2023
The early and prolonged heat that affected millions of people across Argentina, Brazil,
Paraguay, and Bolivia in September 2023
The hottest-ever October (2023) in Madagascar
Dangerous humid heat in the coastal regions of West Africa in February 2024
The deadly heat in the Sahel region during Ramadan 2024
The most recent extreme heat that affected all of Asia, from Israel, Palestine, Lebanon
and Syria in the west, to Myanmar, Thailand, Vietnam and the Philippines in the east
during April 2024.

In all these studies, human-induced climate change was found to be a key driver of the extreme
heat. In many cases, including in Southern Europe, the southern United States and Mexico, theSahel region, West Africa, Thailand, Lao PDR, and the Philippines, the high temperatures
observed would never have occurred without human-induced climate change.
While these studies only give a snapshot of all the extreme heat waves occurring, they all tell thesame story: climate change is an absolute game-changer when it comes to extreme heat, andthreatens, in combination with the urban heat island effect, rapidly growing urban settlements
worldwide (Tuholske et al., 2021). Below, we assess the role of climate change in heat extremes
over the last year across the world, including the distinct 76 heatwaves identified using WWA
criteria. We combine this with an in-depth assessment of the different ways these heat
extremes have impacted society. We conclude with concrete actions that can be undertaken at
different scales to prevent heat-related impacts, most acutely fatalities.
World Weather Attribution is a collective of scientists who use weather observations and climate models to understandhow climate change influences the intensity and likelihood of extreme weather events. Their studies also assess therole of vulnerability and exposure in the extent of the impacts.

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2. Attribution analysis of heat waves
To build off of WWA’s work, Climate Central scientists conducted an analysis of attributable heatover the past year (attributable to human-induced climate change), including all 76 extreme heat
events triggered by WWA's criteria. We compared these events with locations that experienced
climate-altered temperatures, using our Climate Shift Index (CSI) system.
Below, we describe conditions across the planet between May 15, 2023 and May 15, 2024. We
also provide detailed data on 168 countries.
The CSI uses peer-reviewed methodology (Gilford et al., 2022) and quantifies the influence ofclimate change on daily temperatures. We typically express this influence as a change in thelikelihood of the observed temperature due to climate change (for an example, see the CSI
global map). The system also estimates the temperature that would have occurred in a world
without human-caused climate change. We refer to these as the counterfactual temperatures.
For this study, we focused on days with temperatures that people would consider hot based on
their local experience. For each location in our dataset, we calculate the temperature that iswarmer than 90% of temperatures observed at that site over the 1991-2020 period (also referred
to as temperatures above the 90th percentile). We then counted the number of days withtemperatures above this threshold and where the CSI said climate change made thetemperatures at least twice as likely (Figure 1).

Figure 1. Number of days above the 90th percentile with a Climate Shift Index (CSI, see section 2) level 2 or higher.
The yellow outlined regions represent countries with heat wave events identified by World Weather Attribution that
occurred over May 15, 2023 to May 15, 2024. Analysis based on ECMWF ERA5 data. Produced May 22, 2024.

PAGE 6We also used the counterfactual temperatures to make a more direct estimate of how climatechange influenced the experience of people with extreme temperatures during the last 12
months. Specifically, we count the number of days above the 90th percentile level and thensubtract the number of times the counterfactual temperatures exceeded this level. This showsthat human-induced climate change added an average of 26 days of unusually warm
temperatures over the last 12 months (Figure 2).
Key findings:● More than six billion people (6.3 billion) experienced at least 31 days with heat above the90th percentile and at a CSI level of 2 or higher.
● The five countries where the average person experienced the most days with extreme
heat1 above their local heat level were Suriname with 182 days, Ecuador with 180 days,
Guyana with 174 days, El Salvador with 163 days, and Panama with 149 days.
● Without human-induced climate change, the average person in Suriname would haveexperienced 24 such days. That number was 10 days for Ecuador, 33 days for Guyana,
15 days for El Salvador, and 12 days for Panama.
Next, we specifically estimated the influence of climate change on each of the 76 extreme heat
events triggered by WWA criteria. To do so, we calculated the Probability Ratio (PR) of each
event using the Climate Shift Index system (see Methods for an explanation of the calculation).
The PR indicates the increase in likelihood of an event occurring as a result of climate change (aPR of 5 means that human-caused climate change made the event 5 times more likely). Each
event was defined as the five days following the identified onset of the heat wave in the WWA
criteria. For each identified event, the entirety of affected countries were treated as part of theheat wave. It is important to highlight that because we use a consistent definition of these heatwaves for each country (a length of six days), rather than characterizing the heat waves basedon the impacts as done in WWA studies (van Oldenborgh et al., 2021), the changes in likelihood
differ quantitatively from studies undertaken by WWA. However, they all show qualitatively thesame thing: human-induced climate change strongly increased the likelihood of these heatwaves.
Key findings:● The event with the highest PR was experienced in Oceania: the Marshall Islands andMicronesia experienced an event from March 7-12, 2024 with a PR of 35. Two heatwaves covering both Indonesia and the Philippines had the second and third highest
PRs. The first of these events (October 26-31, 2023) had a PR of 25. The second one(April 2-7, 2024) had a PR of 29.
● In Africa, the event with the highest PR occurred in Tanzania from February 21-26, 2024with a PR of 11.

1

Defined as days above the 90th percentile and at a CSI level of 2 or higher

PAGE 7●●●

In North/Central America, the event with the highest PR covered the Dominican Republic,
Costa Rica, Guatemala, Panama, and Honduras from March 22-27, 2024 with a PR of 13.
In Europe, an event covering Spain and Portugal from August 7-12, 2023 had a PR of 8.
In South America, the event with the highest PR covered Brazil, Argentina, Paraguay, andBolivia from November 8-13, 2023, with a PR of 7.

Figure 2. Additional days with temperatures above the 90th percentile in the current climate over May 15, 2023 to May
15, 2024, added by the burning of fossil fuels. Produced May 21, 2024.

Climate Central is an independent group of scientists and communicators who research and report the facts about ourchanging climate and how it affects people’s lives. Climate Central is a policy-neutral 501(c)(3) nonprofit.

3. The crucial role of impacts in extreme heat assessment
Globally, extreme heat is among the deadliest natural hazards, with thousands of people dying
from heat-related causes each year (The Lancet, 2021). For example, in the summer of 2022, anestimated 61,672 heat-related fatalities occurred across Europe (Ballester et al., 2023).
While Europe is a fast-warming continent with infrastructure that is not built to withstand risingtemperatures, plus an aging population highly vulnerable to extreme heat, it is not the only
region vulnerable to extreme heat impacts. However, with some notable exceptions that also
include North America and India among others, impacts of extreme heat are only sporadicallyreported in the media across all regions. The full impact of extreme heat is often not known until

PAGE 8weeks or months afterward, once death certificates are collected or scientists can analyze
excess deaths. In many cases, the full extent of its impact on human health is never truly known
because heat-related deaths are often attributed to other causes like kidney failure or cardiac
arrest, even though those deaths may not have occurred had the high temperatures notexacerbated someone’s pre-existing condition. Often, they are not counted at all. While excess
deaths should show this, many countries are lacking full death records. Limited reporting onheat-related impacts greatly hampers our ability to understand the true scale and full spectrumof impacts associated with extreme heat.
Extreme heat brings multifaceted impacts that compound and cascade across various sectors,
as illustrated in the interconnected pathways in Figure 3. These impacts are most evident for(but not limited to) health, water, agriculture, economy, livability, critical infrastructure, and theenvironment.
In the environment, extreme heat increases the risk of wildfires, air pollution, and the loss of
biodiversity. Extreme heat also strains healthcare services due to heat-related illnesses. Healthimpacts include increased morbidity and mortality, exacerbation of mental health issues, andstrain on emergency medical facilities. These health effects are compounded by criticalinfrastructure failures, such as electricity blackouts and transportation disruptions, which
impede access to essential services.
Water resources are heavily impacted, with reduced water quality and increased demand leading
to potential shortages and a higher risk of waterborne diseases. Extreme temperatures andwater stress adversely affects agriculture, causing crop damage, reduced yields, and heightened
food insecurity. The agricultural impacts then ripple through the economy, reducing foodproduction, affecting livelihoods, and disrupting industrial activities. Water is also a criticalcomponent of energy production; thermoelectric plants use it to cool equipment andhydroelectric plants rely on water flow to spin turbines. A reduction in energy production canhave knock-on effects, increasing power cuts when electricity is most needed.
Livability is compromised as extreme heat intensifies discomfort in indoor and outdoor spaces,
increases aggression, and heightens social conflict. The compounded effects across sectors
underscore the systemic nature of extreme heat impacts, where disruptions in one area cancascade, amplifying challenges in others, ultimately stressing the social, economic, andenvironmental fabric of communities.

PAGE 9Figure 3. Interrelated and compounding impacts resulting from extreme heat.

Despite the occurrence of severe heat-related emergencies across the world in recent years,
reliable data and comprehensive reporting on impacts are often lacking, with the largest gaps
across many developing countries in Africa, Asia, and Latin America (Campbell et al., 2018). For
example, for many of the heat events that have triggered based on the WWA trigger
methodology since last year, we are only aware of extreme temperature records. Information
about impacts to human health, agriculture, or other systems are not available. In fact, few
countries keep comprehensive records of heat waves and the impacts they cause, such as
excess mortality. Notably, this means currently available global mortality figures are likely asevere underestimate (see e.g. Zhu et al., 2024; Zhao et al., 2021).
Conceptualizing, monitoring, detecting, documenting, and communicating heat-related eventsand impacts face numerous challenges and barriers, ultimately impeding effective public healthresponses and a realistic estimate of financial requirements, and thus flows of finance. A
primary challenge is the absence of standardized heat wave definitions and thresholds (Awashti
et al., 2022), complicating systematic monitoring and cross-regional reporting of heat-related
impacts. Additionally, comprehensive and standardized data collection systems to trackheat-related illnesses, deaths, and disruptions across sectors are lacking, leading toPAGE 10underreporting and misdiagnosis of heat-related cases. This issue is exacerbated by limited
data integration and sharing among health, meteorological, and emergency services (PAHO,n.d.), and the absence of robust surveillance systems (IFRC, 2022), particularly in developing
countries, where tracking heat impacts is less feasible due to resource constraints.
Existing monitoring systems often fail to provide nationally representative data, underestimating
the true burden of heat-related impacts. For instance, reliance on emergency department visits
alone skews data, as not all cases are captured (see e.g. Vaidyanathan et al., 2024; Schramm etal., 2021). This is especially prevalent among developing countries with limited health systemcoverage. Further, attributing excess mortality to heat is particularly challenging for people with
underlying health conditions, due to difficulties in isolating heat as the sole cause of death whencomorbidities and older age increase vulnerability to heat-related illness and mortality (Kristie etal., 2021). This also means that heat-related health impacts may be misdiagnosed or attributed
to other causes. Resource and capacity constraints, including limited funding, inadequate
infrastructure for real-time data collection, and a shortage of trained personnel, further hinder
effective monitoring and analysis of heat-related data (Murage et al., 2024; IFRC, 2022).
Coordination and communication barriers present additional obstacles. Responsibilities among
various agencies involved in heat monitoring and response notably are often fragmented,
leading to ineffective communication and dissemination of heat-related information and alerts.
Public awareness of heat-related risks remains low (Eady et al., 2020; WHO, 2018; Howe at al.,2018; Beckmann & Hiete, 2020), impacting the reporting and management of heat-related
incidents. Coordination between meteorological services and health sectors is crucial yet
insufficiently implemented, reducing the potential for meteorological data to inform health andemergency response decision-making.
Vulnerable populations, such as the elderly, low-income, and marginalized communities, often
have less access to healthcare systems. Differences in heat exposure, acclimatization, andadaptation strategies across regions further complicate the attribution of impacts to heat
events, especially when considered alongside other environmental stressors and theoverarching challenge of climate change.
Addressing these multifaceted barriers to fully understand, and in turn prevent, the human toll of
extreme heat requires a concerted effort to improve data and reporting systems, enhancecoordination, and increase public awareness and resources dedicated to heat-relatedmonitoring and response.
The Red Cross Red Crescent Climate Centre is a climate reference center supporting the global Red Cross Red Crescenthumanitarian movement and its partners to reduce the impacts of extreme weather events on the most vulnerable
people.

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4. Actionable solutions to heat risks
Deaths from extreme heat are preventable. There are a myriad of options available to reduce
exposure to extreme heat that range from low or no-cost individual actions, to population-scaleinterventions that decrease the urban heat island effect.
For example, at the individual level, actions can include behavioral adaptations such as
spending less time outdoors during the hottest time of the day, or drinking water at regularintervals. People can use cooling devices to cool the air (fans, A/Cs, evaporative coolers, etc.) or
use low-tech cooling methods that directly reduce body temperature such as self-dousing withwater. People can modify their built environment by increasing shade on exterior surfaces orwindows (e.g. shutters, solar shades, awnings, shade trees), improving ceiling and roof
insulation, or increasing the reflectivity of their roofs by painting them white. All of these are
evidence-based interventions that can reduce the impacts of heat risk, which should becommunicated to people widely. But individual actions are often not enough. Access to water,electricity, and shade when needed is a luxury for the most vulnerable, and the scale of the issue
requires collective action — at the community, city, regional, and country levels.
Cities are hotspots of heat risk, but can also be drivers of action to protect the people living inthem from extreme temperatures through the development of City Heat Action Plans (HAPs).
City Heat Action Plans integrate actions across timescales — from the imminent extreme heatto seasonal preparedness — and long-term urban planning. They provide a plan for different
municipal departments and non-governmental actors to work together, outlining roles andresponsibilities such as communication, emergency response, and health system strengthening.
While many cities in North America, Europe, Australia, and now in India have HAPs, there is alarge gap in Africa, the Middle East, South America, and in Small Island Nations. Rapidly risingtemperatures necessitate that these regions quickly bolster their planning for extreme heat.
Without adaptation measures, the impacts across all sectors, especially human health, arecertain to increase (IPCC, 2023).
Additional measures to reduce extreme heat impacts at the population level include:●

Strengthening crucial service provision systems, such as ensuring emergency healthservices are able to expand and absorb increased demand for treatment during extreme
heat events.

●

Incorporating cooling needs into social protection programs that supplement energy
costs of the most vulnerable.

●

Ensuring water and electricity systems are able to meet increased demands without
being overwhelmed.

●

Bolstering transport systems to ensure they remain operational and accessible during
extreme heat events.

PAGE 12●

Strengthening early warning system coverage and efficacy.

●

Legislating building codes to encourage passive and active cooling measures.

●

Passing and enforcing worker safety laws.

●

Enhancing cold storage to reduce agriculture supply chain losses.

●

Designing towns and cities with cool spaces in close proximity to all residents.

There is evidence that many of the measures mentioned above are effective. A review ofassessments reported that in a majority of places where there was extreme heat, there was a
reduction of adverse effects where preventive measures had been implemented (Boekmann &Rohn, 2014). Despite the availability of actions to reduce heat risks, these are not equallyavailable to everyone everywhere. Many developing countries are playing catch up on adaptation
while also facing the highest rates of increase in temperatures and population-level vulnerability,
putting them at higher risk (IPCC, 2023). It is imperative that in addition to adapting to extreme
heat, we must reduce greenhouse gas emissions to net zero in order to put the brakes on further◾

rising temperatures.

Methods
Calculating the Climate Shift Index (CSI)
Calculating the CSI begins with high-resolution daily temperatures (high, low, and daily average).
For this report, Climate Central scientists (we) use ERA5 data from May 15, 2023 to May 15,2024. We estimate how often the temperature at a particular location is likely to occur in thecurrent climate using both historical observations (ERA5) and 24 climate models. We also
estimate the likelihood in a climate without human-caused climate change. The CSI is built fromthe ratio of these two likelihoods.
Based on the multi-model approach described in Gilford et al. (2022), the CSI combines several
different techniques for estimating the frequency of a given temperature occurring in the current
climate and in a climate without human-caused climate change. Two of the techniques use 70
years of historical temperature reconstructions. The other technique uses 24 state-of-the-art
global climate models run with and without carbon dioxide emitted by human activities over thehistorical period.
Calculating days above the 90th percentile
To calculate the number of days with temperatures above the 90th percentile in the modern
climate with a climate change fingerprint, we compiled ERA5 data from 1991 to 2020. We thencalculated the 90th percentile temperature of all average daily temperatures for each ERA5 grid

PAGE 13cell over this time period. For each average daily temperature from May 15, 2023 to May 15,2024, we found the days where temperatures exceeded their associated 90th percentile
temperature threshold from the 1991-2020 period and experienced a CSI greater than or equal
to 2. The number of days that fulfilled these criteria were added up to compute a final number
for days above the 90th percentile with a climate fingerprint in the modern climate.
To calculate the number of days with temperatures above the 90th percentile in the
counterfactual climate (the climate in a world where climate change hasn’t occurred), we
calculated the likelihood as a percentile of daily average temperatures occurring from May 15,2023 to May 15, 2024 in the modern climate. Next, we applied that percentile to thecounterfactual climate to compute what the equivalent temperature would have been in the
counterfactual climate. We found those days where counterfactual temperatures exceeded theirassociated 90th percentile temperature threshold from the 1991-2020 period, and experienced a
CSI value greater than or equal to 2. The number of days that fulfilled these criteria were added
up to compute a final number for days above the 90th percentile with a climate fingerprint in the
counterfactual climate.
To find the number of days above the 90th percentile added by climate change, the number ofdays above the 90th percentile in the counterfactual climate was subtracted from the number ofdays above the 90th percentile in the modern climate.
To find the average values per country for these values, we averaged final values (number ofdays) across the country by weighting based on population density in each ERA5 grid cell.
Calculating probability ratios for triggered heat events
For each triggered heat wave identified by World Weather Attribution from May 15, 2023 to May
15, 2024, we compiled the onset date of that event as well as the affected countries. We usedan event length of six days. Next, for each event, we found the daily climate factors (a value
derived to generate the Climate Shift Index that is a measurement of the increase in likelihood
due to climate change of a temperature occurring). To find the climate factor of the event as awhole, we found the variance associated with the daily temperatures in each location for days
within the event from 1991-2020. We then calculated the yearly means for those same daily
temperatures from 1991-2020, before calculating the variance of these yearly means. By
multiplying the ratio of the two (the daily temperature variance to the yearly temperature
variance) by the mean climate factor, we calculated variance-scaled climate factors. Thistranslates a sequence of changes in likelihoods to a single comprehensive value. To find themean climate factor of an event, we found the spatial mean of the variance-scaled climate
factors across the affected countries. Finally, we converted climate factors to Probability Ratios.