Monthly Attribution Overview - July 2024

An analysis of how climate change boosted United States temperatures in July 2024

Using Climate Central’s Climate Shift Index (CSI) tool to measure the impact of climate change on daily temperatures across the United States, as well as NOAA’s Applied Climate Information System (ACIS) to find daily temperature information, we have compiled a high-level overview of how climate change has affected temperature trends in July in cities across the United States. This will be a monthly release, put out early in the days following the end of a month. 

1. High level findings (download local data)

• July temperatures in the U.S. were elevated in the eastern and western thirds of the United States. Meanwhile the middle of the country, from Illinois and Iowa down to Texas, were unusually cool (Figure 1).

• This was the hottest July on record for 15 analyzed cities, and was in the top 5 for 44 cities.

• The West Coast showed particularly elevated July temperatures. Average temperatures across the region were 3.3 °F higher than normal, and 5 out of 10 analyzed cities in California had average monthly temperatures that were at least 5 °F hotter than normal.

• Climate Shift Index (CSI) signal followed a similar trend (Figure 2), with the eastern and western thirds of the country experiencing the highest number of days at or above CSI 3. (Temperatures made at least 3x more likely because of climate change.)

• On average, cities analyzed in the Southwest experienced at least 14 days with temperatures resulting in CSI values greater than or equal to 3.

Figure 1. Threaded ACIS temperature anomalies for July 2024  relative to the 1991-2020 standard normal period. Analysis based on ACIS data.

Figure 2. Days with a CSI of 2 or higher for July 2024 for ACIS threaded stations. Analysis based on ERA5 data (July 1-30) and GFS data (July 31).

2. Temperature anomaly analysis (download local data)

• 134 out of 191 ACIS stations analyzed had positive temperature anomalies, indicating that these cities experienced a warmer July than normal.

• This was the hottest July on record for 15 analyzed cities, and was in the top 5 for 44 cities.

• July temperatures in the U.S. were elevated in the eastern and western thirds of the United States. Meanwhile the middle of the country, from Illinois and Iowa down to Texas, was unusually cool (Figure 1).

• The West showed particularly elevated July temperatures. Average temperatures across the region were 3.3°F higher than normal. 

• The most unusually hot city in July  was Spokane, Washington, where it was 7.24°F hotter than normal. Julys in Spokane have warmed 5.9°F on average since 1970.

• 10 cities had July temperature anomalies greater than 5°F. Half of these were in California.

• Of the cities examined, five out of the ten most anomalously warm were in California (Fresno, San Jose, Bakersfield, Sacramento, and Stockton).

• 182 out of 191 ACIS stations analyzed had positive temperature trends for July, indicating that these cities have been warming on average since 1970.

• Reno, NV had a warm July (with an average daily temperature anomaly of 4.96°F), and it is the fastest warming ACIS station for July, warming 12.7°F since 1970. The second fastest warming ACIS station was Boise, ID. Average July temperatures in Boise have warmed by 8.5 degrees since 1970, and this July, it was 5.20°F hotter than normal.

3. Climate Shift Index analysis (download local data)

• 40 out of 191 ACIS stations analyzed had at least one week with daily CSI values greater than or equal to 3, indicating that temperatures on those days were made at least three times as likely due to climate change in those cities.

• San Juan, Puerto Rico had 29 days at CSI 5, indicating that temperatures on these days were made at least 5 times more likely to occur as a result of climate change.

• 5 out of the top 10 cities with the strongest climate signal were in Florida (Miami, West Palm Beach, Fort Myers, Tampa, and Sarasota). Each of these had more than two weeks of temperatures with CSI values of 5.

• The Southwest showed the strongest climate signal in the country, with the 8 cities analyzed in the region averaging more than two weeks with CSI values at or above 3.

Methods

Calculating the Climate Shift Index

All Climate Shift Index (CSI) levels reported in this brief are based on daily average temperatures and  ERA5 data from July 1 to July 31, 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).

City Analysis

We analyzed 191 Applied Climate Information System (ACIS) stations associated with U.S. cities. For each city, we found the CSI time series from the nearest 0.25° grid cell. We calculated the number of days at CSI levels 2, 3, 4, and 5. We used ACIS data to find the average monthly temperatures, temperature anomalies, and precipitation information, and to derive average monthly warming trends for each city.

Click here for more information about the Climate Shift Index