Warm Spring Days Boosted by Climate Change in U.S. Cities

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KEY CONCEPTS

• Spring warming leads to longer allergy seasons, supplies warm rising air during the most active tornado months, and causes earlier snowmelt tied to rising flood risk and reduced water supply. 

• Heat-trapping carbon pollution drove unusual spring warmth in the U.S. and around the globe this year, according to Climate Central’s Climate Shift Index. 

• New analysis shows which U.S. states and cities felt the strongest influence of human-caused climate change on daily average temperatures this spring (March to May 2024).

• In the U.S., 300 million people — about 89% of the population — experienced at least one unusually warm spring day made at least two times more likely by climate change. 

• Nearly half of the 240 U.S. cities analyzed had five or more unusually warm spring days made at least two times more likely by climate change.

• The U.S. cities with the most unusually warm spring days boosted by climate change were: San Juan, Puerto Rico (91 days); Sarasota, Fla. (39 days); West Palm Beach, Fla. (35 days); Honolulu, Hawaii (32 days); and Miami, Fla. (31 days).

• Read the report: People Exposed to Climate Change: March-May 2024

Global influence of climate change during the last three months

Carbon pollution, primarily from burning fossil fuels, drove abnormal warmth across the globe from March to May 2024, according to new analysis using Climate Central’s Climate Shift Index (CSI).

• About one in every four people on the planet experienced a very strong influence of climate change (CSI level 3 or higher) on temperatures each day from March to May 2024. Global exposure peaked on April 6, 2024, when 2.7 billion people (one in every three people worldwide) experienced unusual heat at CSI level 3 or higher.

• The average person in 58 different countries experienced a very strong influence of climate change on at least two out of every three days during the last three months. These countries were primarily located in: Central, Eastern, and Western Africa; Southeast Asia; Central America and the Caribbean; South America; and small island developing states within Oceania. 

• Notably, about 44% of all people in Africa and one in every three people in South America experienced the most extreme level of attributable heat (at least 60 total days at the maximum CSI level 5) during the last three months.

Read the report: People Exposed to Climate Change: March-May 2024

The CSI, grounded in peer-reviewed attribution science, is based on the relative frequency of daily temperatures in the current climate and in a world without human-caused climate change. Positive CSI levels 1 through 5 mean that climate change made those temperatures more likely. For example:

• A CSI level 2 (3, 4, etc.) indicates local temperatures that were made at least two (three, four, etc.) times more likely due to climate change. A CSI level 2 indicates a strong influence of human-caused climate change. 

Learn more about the CSI: explore the map tool, watch a video explainer, or read the FAQs.

Influence of climate change in the U.S. this spring 

This new global analysis also used the CSI to quantify the influence of human-caused climate change on daily average temperatures in 240 U.S. cities during meteorological spring (March 1 to May 31, 2024). See Methodology below for details.

• Across the U.S., 300 million people — 89% of the population — experienced at least one spring day with a CSI level 2 or higher — indicating warm average temperatures made at least two times more likely by climate change.

• Nearly half of the 240 U.S. cities analyzed  (47% or 112) experienced five or more spring days with a CSI level 2 or higher — indicating warm average temperatures made at least two times more likely by climate change. 

States and cities with the strongest influence of climate change

There was a strong regional contrast across the U.S. between areas with the strongest CSI signals and areas with the largest spring temperature anomalies. 

This is because CSI levels depend on how the daily temperature compares to both: 1) the strength of the local warming trend and 2) the spread, or variance, of possible local temperatures. As a result, locations with relatively large temperature anomalies don’t necessarily experience the strongest CSI signals. 

The U.S. cities and states with the strongest influence of climate change during spring 2024 were primarily located on islands and along coasts. Seasonal temperature variance in these areas tends to be relatively low compared with climates in other (more temperate) parts of the country. 

Among U.S. states, the strongest CSI signals were observed on islands (Puerto Rico and Hawaii) and in coastal states (Florida and Alaska). 

• The average person in Puerto Rico experienced a strong influence of climate change (CSI level 2 or higher) on 90 spring days (98% of the entire season). 

• The average person in Hawaii and in Florida experienced 40 and 27 days, respectively, at these levels. 

These states all had the highest state-wide CSI signals, despite having had relatively low seasonal temperature anomalies this spring. 

San Juan, Puerto Rico, and numerous cities across Florida dominated the list of highest-ranking U.S. cities in terms of CSI. The following cities all experienced at least three weeks worth of days with a strong climate change influence. In San Juan, Puerto Rico, 99% of all spring days had a CSI level 2 or higher. 

States and cities with the most unusual spring warmth

The U.S. states with the highest spring temperature anomalies (Michigan, Ohio, and West Virginia) all had fewer than four spring days at CSI level 2 or higher. 

These high-anomaly states were all located in the Upper Midwest and Ohio Valley — areas that have relatively high seasonal temperature variance and therefore require even larger anomalies to register a strong (CSI level 2 or higher) influence of climate change during the season. 

Additionally, parts of the Upper Midwest (especially northern Michigan) have experienced more modest long-term spring warming trends than the rest of the country, which also raises the threshold for detectable CSI signals. 

Cities in Ohio dominated the U.S. spring temperature anomaly rankings, all with relatively large warm departures from the 30-year spring normal. 

Despite having notably warm spring temperatures, these cities all experienced relatively few days (five or less) with a strong (CSI level 2 or higher) influence of climate change on average temperature. 

This is mainly due to relatively high historical variance in spring temperatures and relatively more modest long-term spring warming trends in parts of the Midwest (especially the Upper Midwest).

Spring warming driven by climate change affects:

Seasonal allergies: Warmer, shorter winters mean earlier spring thaw and later fall freeze. The freeze-free growing season lengthened in 83% (164) of 197 U.S. cities analyzed since 1970 — giving plants more time to grow and release allergy-inducing pollen earlier in spring and later into fall.

Disease-carrying pests: Cold winters and springs can keep the populations of disease-carrying pests like mosquitoes in check. But warmer winters and ‘shoulder seasons’ can extend mosquito seasons and worsen pest-related health risks. 

Growing seasons and agriculture: Spring is not only warming but also shifting. As warm weather cuts into colder months, growing seasons last longer. A longer growing season can bring opportunities and challenges for agriculture, depending on the location, ecosystem, and other factors such as water supply. 

Planting zones that guide farmers and gardeners: As the U.S. warms, plant hardiness zones are shifting north, changing which plants can grow and thrive in different parts of the country.

Western water supplies: Warming winters can reduce mountain snowpack. Warming springs can also contribute to earlier snowmelt, which stresses the region’s limited water resources. Water deficits from earlier snowmelt have consequences for hydropower, agriculture, and drinking water supplies.

LOCAL STORY ANGLES

Is climate change influencing daily heat extremes in your local area?

Climate Central’s Climate Shift Index (CSI) system provides tools, data, custom maps, and local alerts to answer this question in real-time. Here are three ways to use the CSI this summer:

1. 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 in the recent past.

2. Access KML to create custom CSI maps. The CSI is now available in KML format. Fill out this form to join our pilot project, receive the KML links, and create custom maps. 

3. Sign up for alerts. Sign up here to receive custom email alerts when strong CSI levels are detected in your local area.

CONTACT EXPERTS

Andrew Pershing, PhD (he/him/his)
VP for Science
Director of Attribution Science and Climate Fingerprints
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

All Climate Shift Index (CSI) levels reported in this brief are based on daily average temperatures and ECMWF ERA5 (3/1/2024 - 5/28/2024) and NOAA GFS (5/29 - 5/31/2024) for 240 U.S. cities.

Please refer to the Methods section of the full report for detailed methodology behind the global analysis: People Exposed to Climate Change: March-May 2024

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). 

For local graphics included in this brief: Daily average temperature anomalies (relative to 1991-2020 normals) were obtained from the Applied Climate Information System (ACIS) for 240 stations from March 1, 2024 to May 31, 2024. While Climate Matters local analyses typically include data from 247 stations, seven were excluded from this analysis due to insufficient data: Bend, Ore.; Glendive, Mont.; Hattiesburg, Miss.; Idaho Falls, Idaho; Larego, Texas; Myrtle Beach, S.C.; and Panama City, Fla.