Climate Matters•June 4, 2025
Climate Change Driving 55-Year Rise in Extremely Hot Summer Days
KEY FACTS
As heat-trapping pollution continues to warm the planet, summers are getting hotter and heat extremes are becoming more frequent — putting health at risk.
New attribution analysis shows that climate change is having a growing influence on the frequency of extremely hot summer days in 88% of 247 major U.S. cities analyzed since 1970.
On average, these cities currently experience about six extremely hot summer days with a strong climate change fingerprint each year, compared to zero such days during the 1970s.
Seven of the top 10 cities with the largest increases in climate change-fueled extremely hot summer days are located in the South, mainly in Texas.
Heat is the leading cause of weather-related deaths in the U.S. The rising frequency of extreme heat due to climate change puts public health at risk from heat-related illness.
Download local data
Extreme heat influenced by planet-warming pollution
Levels of planet-warming CO2 in the atmosphere surged in 2024, growing at the fastest annual rate on record.
The buildup of CO2 and other greenhouse gases is mainly due to pollution from burning fossil fuels for electricity, heating and cooling, transportation, and more.
As this heat-trapping pollution continues to warm the planet, summers are getting hotter and heat extremes are becoming more frequent — putting health at risk.
New analysis used Climate Central’s Climate Shift Index (CSI) to directly assess the influence of human-caused climate change on extreme summer heat across the U.S. every year since 1970.
This analysis calculated the average annual number of extremely hot summer days that were strongly influenced by climate change (CSI level 2 or higher) each decade since 1970 in 247 major U.S. cities (see Methodology).
A CSI level 2 indicates local temperatures that were made at least two times more likely due to climate change.
Here, an extremely hot summer day is defined as having a maximum temperature hotter than 95% of all summer days from 1970-2024. In other words, these are the hottest 5% of summer days recorded in each city since 1970.
Climate change fueling the rise in extreme summer heat
Over the last 55 years, 88% (217) of the cities analyzed have seen an increase in the frequency of extremely hot summer days with a strong climate change signal.
On average, these 217 cities currently experience six extremely hot summer days at CSI level 2 or higher each year (2000-2024). The extreme heat on these days was made at least two times more likely by climate change.
On average, these same cities used to experience virtually none of this climate change-fueled extreme summer heat (an annual average of zero such days during the 1970s).
Some 74 cities now experience at least seven more extremely hot summer days at CSI level 2 or higher each year (2020-2024) than during the 1970s. More than half of these 74 cities are located in the South and Southwest.
Seven of the top 10 cities with the largest increases in climate change-fueled extremely hot summer days (16 to 22 more additional days per year than during the 1970s) are located in the South, specifically across Texas (Abilene, Waco, San Angelo , Austin, and Odessa) and Louisiana (Lafayette).
Extreme heat: a growing health risk in a warming climate
Climate change is increasing exposure to dangerous extreme heat for billions across the globe.
Heat is the leading cause of weather-related deaths in the U.S. In 2023, a record 2,325 people died from heat in the U.S. alone, though such figures are widely thought to be an undercount.
Exposure to extreme heat makes it difficult for our bodies to cool off, resulting in heat-related illnesses including heat cramps, heat exhaustion, and even fatal medical emergencies such as heat stroke.
Extreme Weather Toolkit: Extreme Heat explains the science and collects Climate Central’s recent resources on extreme heat trends and local impacts in the U.S. and across the globe.
Summer heat can also worsen air quality by trapping harmful pollutants close to the Earth’s surface and fueling the formation of ground-level ozone. These pollutants can exacerbate respiratory health issues for people with asthma and other lung diseases.
Heat risks unequally shared
Extreme heat can affect everyone, but some face greater risks of heat-related illness and mortality than others.
Children, adults over 65, pregnant people, and people living with illness are among those most at risk to heat-related illness.
Extreme Heat Risks for Children reviews how climate change is driving more frequent and intense extreme heat that is especially dangerous for children.
Climate Change Nearly Doubled Pregnancy Heat Risk Days in U.S. shows that human-caused climate change has at least doubled the average annual number of pregnancy heat-risk days in 222 countries including the U.S. in recent years.
Humid heat is especially dangerous for weather-exposed workers — presenting health and safety hazards, increasing the risks of heat-related illness, injury, or death, and impacting livelihoods through lost labor hours and wages.
Rising Heat Risks for Workers shows that risky humid heat is becoming more frequent in cities across the U.S.
Structural inequities can also lead to higher extreme heat exposure in some communities. According to a 2021 study, people of color and people living below the poverty line are disproportionately exposed to more intense heat within the urban heat islands of 169 of the largest U.S. cities.
Urban heat burdens are also linked to a history of racially biased housing policy. Recent analysis from Columbia University’s Grades of Heat project shows that historically redlined areas currently experience hotter summers than non-redlined areas in 150 (84%) of 179 major U.S. cities.
Urban Heat Hot Spots in 65 Cities analyzes how and where the built environment boosts temperatures within major U.S. cities that are home to 50 million people, or 15% of the total U.S. population.
LOCAL STORY ANGLES
Is climate change influencing daily heat extremes in your local area?
Climate Central’s Climate Shift Index system provides tools, data, custom maps, and local alerts to answer this question in real-time. Here are three ways to use the Climate Shift Index this summer:
Use the tools. Climate Central’s Climate Shift Index map tool shows which parts of the world are experiencing high Climate Shift Index levels, every day.
Access KML to create custom Climate Shift Index maps. Fill out this form to receive CSI maps in KML format, and create custom Climate Shift Index maps.
Sign up for alerts. Sign up here to receive custom email alerts when strong Climate Shift Index levels are detected in your local area.
How is extreme heat affecting public health in your local area?
The National Integrated Heat Health Information System HEAT.gov is a hub for information to support heat resilience in the U.S. Check out HeatRisk, an interactive map tool from the National Weather Service now available for the contiguous U.S. This color-numeric index shows current and forecast risk of daily local heat-related impacts. The CDC’s Heat and Health Tracker maps daily rates of heat-related illnesses across the U.S.
What local measures are in place to protect people from heat?
The EPA maintains a Heat Island Community Actions Database of measures that communities are taking to mitigate local heat island effects. Reports from NOAA’s urban heat island mapping campaigns cover local risk reduction and adaptation strategies in 60+ U.S. cities. Track local climate-related hazards in real-time in the Climate Mapping for Resilience and Adaptation Portal, which also includes federal resources for long-term resilience planning. The NIHHIS provides heat preparedness and planning resources, including many resources in Spanish.
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Daily summer (June 1-August 31) maximum temperatures and Climate Shift Index (CSI) levels were obtained for 247 cities from 1970-2024. An extremely hot summer day that was strongly influenced by climate change is defined as any day that had both:
A maximum temperature at or above the local 95th percentile, and
A CSI level of 2 or higher, which means that the maximum temperature that day was made at least 2x more likely due to climate change
The total number of extremely hot summer days with a strong climate change influence was calculated for each year. Annual totals for each city were averaged over each decade and rounded to the nearest whole number. Note the 2020s include only the five years from 2020-2024.
Local graphics were not produced for 55 cities whose total of average annual extremely hot summer days at CSI level 2 or higher, summed across all decades analyzed, was less than two.
The analysis is based on temperature data from ERA5 and Climate Central’s Climate Shift Index. See the frequently asked questions for details on computing the Climate Shift Index, including a summary of the multi-model approach described in Gilford et al. (2022)