Monthly Attribution Overview - August 2024

Analysis: How climate change boosted United States temperatures in August 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 August in cities across the United States.

1. High level findings (download local data)

• August temperature anomalies in the U.S. were elevated across much of the country. The highest anomalies were in Texas (Figure 1).

• This was the hottest August on record in San Juan and in 20 other cities, was in the top 5 hottest August.

• The Southwest and the South showed particularly elevated August temperatures. Average temperatures across the Southwest were 2.64 °F higher than normal and 2.38 °F across the south.

• Texas had particularly elevated August temperatures. Average temperatures across the state were 3.1°F higher than normal. 6 out of the top 10 unusually warmest cities were in Texas. 

• Climate Shift Index (CSI) signal followed a similar trend (Figure 2)—the South and the Southwest (from southern Nevada down through Florida) experienced 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 15 days with temperatures resulting in CSI values greater than or equal to 3, in the South, this value was 9.5.

Figure 1. Threaded ACIS temperature anomalies for August 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 August 2024 for ACIS threaded stations. Analysis based on ERA5 data (August 1-29) and GFS data (August 30-31).

2. Temperature anomaly analysis (download local data)

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

• This was the hottest August on record in San Juan, Puerto Rico, and was in the top 5 hottest August for 20 cities.

• August temperatures in the U.S. were elevated in the South and the Southwest. Meanwhile, much of the East Coast—from South Carolina to Maine—was unusually cool (Figure 1).

• Texas showed particularly elevated August temperatures. Average temperatures across the state were 3.11°F higher than normal and 6 out of the top 10 unusually warmest cities across the country were in Texas. 

• The most unusually hot city in August was Amarillo, Texas, where it was 6.27°F hotter than normal. Augusts in Amarillo have warmed 4.4°F on average since 1970.

• Four cities had August temperature anomalies greater than 5°F. All of these were in Texas.

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

• Reno, NV had an unusually cool August (with an average daily temperature anomaly of -0.62°F). Nonetheless, Reno is still the fastest-warming ACIS station for August on average, warming 11.2 °F on average since 1970. 

• The second fastest-warming ACIS station is Boise, ID. Average August temperatures in Boise have warmed by 6.3 degrees since 1970. This August, it was 1.77°F hotter than normal.

3. Climate Shift Index analysis (download local data)

• 45 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 because of climate change.

• 6 out of the top 10 cities with the strongest climate signal were in Texas (El Paso, San Angelo, Lubbock, Beaumont, Victoria, and Odessa). Additionally, each of these cities had at least two weeks of temperatures with CSI values of 5.

• The Southwest showed the strongest climate signal in the country. The 8 cities analyzed in the region averaged 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 August 1 to August 29, 2024, and GFS data from August 30 to August 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