Climate Matters•September 13, 2023
Hot Summer Days Linked to Climate Change in U.S. Cities
KEY CONCEPTS
Summer 2023 was the planet’s hottest on record. New Climate Central analysis shows which U.S. cities, states, and regions felt the strongest influence of climate change on summer heat.
In the U.S., 326 million people—97% of the population—experienced at least one summer day with temperatures made at least 2x more likely due to human-caused climate change.
The U.S. South, Southwest, and Southeast experienced the strongest climate fingerprints on summer daily average temperatures.
Of the 244 U.S. cities analyzed, 71% (175) had seven or more days with average temperatures made at least 2x more likely by climate change.
In 45 U.S. cities, at least half of all summer days had heat made at least 2x more likely by climate change. More than half of these cities were in Texas (17) and Florida (11).
U.S. cities with the most hot summer days made at least 2x more likely by climate change were: San Juan, Puerto Rico (90 days); Victoria, Texas (80 days); Lafayette, La. (74 days).
This summer was also marked by relentless heat streaks. Some 15 cities in Louisiana, Texas, New Mexico, Alabama, and Arizona had record streaks of summer days above 100°F.
2023 has had a record 23 billion-dollar weather and climate disasters so far totalling $57.6 billion.
Download data: Summer 2023 Climate Shift Index and record heat streaks in U.S. cities
This brief focuses on the U.S. Check out our previous Climate Matters brief for a global analysis of June-August heat boosted by climate change.
Influence of climate change during Earth’s record-hottest summer
June, July, and August 2023 temperatures shattered global records for all three months.
Summer’s record heat was strongly influenced by human-caused climate change around the globe, according to Climate Central’s Climate Shift Index (CSI).
Nearly half (48%) of the world’s population experienced at least 30 days during June-August with a Climate Shift Index (CSI) level 3 or higher — indicating temperatures made at least 3x more likely due to climate change.
Summer heat boosted by climate change brought extreme heat to many parts of the globe. In the U.S. and North America:
Relentless, record-breaking July heat waves in Arizona, New Mexico, and Texas would have been virtually impossible without human-caused climate change, according to World Weather Attribution analysis.
Climate change also more than doubled the likelihood of extreme fire weather conditions in Eastern Canada in May and June that began a record Canadian fire season and brought unhealthy smoke to many U.S. cities.
Record-breaking summer heat also occurred in the ocean, elevating the risk of rapidly-intensifying hurricanes like Idalia and Lee and coral bleaching events like those impacting Florida and the Caribbean.
The rising global frequency and intensity of extreme heat is consistent with well-established science on the warming effects of carbon pollution — mainly from burning coal, oil, and natural gas.
New Climate Central analysis shows which U.S. cities, states, and regions felt the strongest influence of climate change on heat over the 92 days of this record-breaking summer.
Influence of climate change on hot summer days in the U.S.
This analysis used the Climate Shift Index (CSI) to quantify the influence of human-caused climate change on daily average temperatures experienced in 244 U.S. cities during summer 2023 (June 1, 2023 to August 31, 2023). See Methodology below for details.
In the U.S., 326 million people—97% of the population—experienced at least one summer day with temperatures made at least 2x more likely (CSI level 2 or higher) due to human-caused climate change.
The U.S. South, Southwest, and Southeast experienced the strongest influence of human-caused climate change on daily average temperatures. In the South, the average city had 49 summer days (53% of the season) with heat made at least 2x more likely by climate change.
Hot summer days boosted by climate change in U.S. cities
Of the 244 U.S. cities analyzed:
nearly all (237 cities, or 96%) had one or more summer days with heat made at least 2x more likely due to climate change.
175 cities (71%) had seven or more days with heat made at least 2x more likely due to climate change.
one-quarter (65 cities) had 30 or more days with heat made at least 2x more likely due to climate change.
U.S. cities with relentless heat driven by climate change
Of the 244 U.S. cities analyzed, 45 cities had at least half of all summer days (at least 46 out of 92 total summer days) with heat made at least 2x more likely due to climate change.
More than half of these 45 cities were in Texas (17 cities) and Florida (11 cities), but also included cities in: Puerto Rico, Louisiana, Mississippi, New Mexico, Alabama, Hawai’i, Alaska, and Arizona.
The nine cities that felt the most relentless influence of climate change experienced heat made at least 2x more likely by climate change on 77% to 98% of all summer days.
| City | Summer days at CSI 2 or higher | Percent of summer days at CSI 2 or higher |
|---|---|---|
| San Juan, Puerto Rico | 90 | 98% |
| Victoria, Texas | 80 | 87% |
| Lafayette, La. | 74 | 80% |
| Ft. Myers, Fla. | 73 | 79% |
| West Palm Beach, Fla. | 73 | 79% |
| Houston, Texas | 72 | 78% |
| Sarasota, Fla. | 72 | 78% |
| Tampa Area, Fla. | 72 | 78% |
| Baton Rouge, La. | 71 | 77% |
In San Juan, Puerto Rico, 98% of the summer (all but two days) had heat made at least 2x more likely by climate change. Even at the highest level on the CSI scale (CSI level 5), 92% of all summer days (85 days) in San Juan were made at least 5x more likely due to climate change.
Heat events with a CSI level 5 would be very difficult to encounter in a world without climate change — not necessarily impossible, but very, very unlikely. In other words, nearly every summer day in San Juan had an exceptional influence of climate change on average temperatures. The second-ranked U.S. city (Victoria, Texas) had 68 days (74% of the summer) at CSI level 5.
Record heat streaks in U.S. cities
This summer was also marked by record-breaking heat streaks.
Some 15 cities in Louisiana, Texas, New Mexico, Alabama, and Arizona had record-breaking heat streaks at or above 100°F. Topping the list were: Tucson, Ariz. (53 consecutive days); Bryan, Texas (50 consecutive days); and Austin, Texas (45 consecutive days).
At even higher levels of heat, Phoenix, Ariz. had record-breaking streaks at or above 105°F (56 days) and 110°F (31 days).
Relentless back-to-back days of extreme heat amplify the risks of heat-related illness — especially for vulnerable populations such as the elderly and outdoor workers.
Billion-dollar weather and climate disasters in 2023 so far
According to NOAA, in the first eight months of 2023 (as of September 11) the U.S. has already experienced a record 23 billion-dollar weather and climate disasters — with combined estimated losses of $57.6 billion. The year-to-date count of billion-dollar weather and climate disasters puts 2023 ahead of the previous annual record of 22 billion-dollar disasters set in 2020.
Ten of these events occurred during spring (March-May) and 11 occurred during summer (June-August).
Most of these events (18) were severe storms primarily in the South and Central U.S. The frequency of billion-dollar severe storms has increased sharply since 1980. Learn more: Extreme Weather Toolkit: Severe Weather.
The 23 total year-to-date billion-dollar disasters also include devastating August wildfires in the historic town of Lahaina on Maui, and flooding in California (January-March) and Vermont and New Hampshire (July). NOAA’s latest (September 11) update did not include damage estimates for Hurricane Idalia, which made landfall on August 30 as a Category 3 major hurricane and swept across Florida and Georgia. It is worth noting that, while extreme heat is deadly, heatwaves like what the U.S. experienced this summer in the South and Southwest are not included in these disaster statistics.
LOCAL STORY ANGLES
CSI tools, data, custom maps, and local alerts
Here are four ways to use this attribution analysis from Climate Central:
Use the tools. Climate Central’s Climate Shift Index map tool shows which parts of the world are experiencing high CSI levels, every day. Explore the global CSI map for today, tomorrow, and any day from June through August.
Download the data. Data from this analysis are available to download and explore in more detail the daily global population exposure to high CSI levels, and summary conditions for 202 countries and territories for June through August 2023.
Access KML to create custom CSI maps. The Climate Shift Index is now available in KML format. Fill out this form to join our pilot project, receive the KML links, and create custom CSI maps.
Sign up for alerts. Sign up here to receive custom email alerts when strong CSI levels are detected in your local area.
Quick facts on the links between climate change and extreme heat:
Climate Central’s Extreme Weather Toolkit: Extreme Heat
World Weather Attribution’s Reporting Guide for Journalists
Heat and health
Extreme heat can trigger heat exhaustion and heat stroke, a life-threatening condition. Monitor weather.gov for the latest U.S. heat alerts. Visit the National Integrated Heat Health Information System Planning and Preparing guide for symptoms and safety information.
Everyone can be affected by extreme heat but those most at risk include older adults, young children, pregnant people, individuals with chronic conditions, and outdoor workers.
The Centers for Disease Control and Prevention Heat and Health Tracker maps heat-related illnesses at the census-tract level in real time. Use the National Integrated Heat Health Information System (NIHHIS) Heat Equity Mapper to find census tracts in your area with the highest heat burden.
CONTACT EXPERTS
Andrew Pershing, Ph.D. (he/him/his)
VP for Science
Director of Attribution Science and Climate Fingerprinting
Climate Central
Relevant expertise: Climate change attribution
Contact: pgirard@climatecentral.org (Peter Girard, VP for Communications)
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 average temperature anomalies (relative to 1991-2020 normals) were obtained from the Applied Climate Information System (ACIS) for 244 stations from June 1, 2023 to August 31, 2023. While Climate Matters local analyses typically include data from 247 stations, Bend, Ore., Glendive, Mont., and Idaho Falls, Idaho were excluded from this analysis due to insufficient data.
All Climate Shift Index (CSI) levels reported in this brief are based on daily average temperatures from June 1, 2023 to August 31, 2023. 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).