Report•January 6, 2026
Monthly Attribution Overview – December 2025
An analysis of how climate change boosted United States temperatures in December 2025
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 December in cities across the United States. (Dataset downloadable as Excel workbook here.)
1. High-Level Findings
Overall, the U.S. saw above-average temperatures in December, with an average anomaly — or difference from normal — of 0.4°F across 194 cities.
December temperature anomalies were evenly split across cities: 95 cities were cooler than average, and 95 were warmer than average. The remaining four cities recorded near-normal conditions (0.0°F anomaly).
Twelve cities experienced their hottest December on record.
The CSI signal was minimal across most of the country in December.
Despite the mixed conditions this December, long-term December warming trends show that nearly all cities analyzed have warmed since 1970, consistent with findings from Climate Central’s 2025 Winter Package.
Figure 1. Threaded ACIS temperature anomalies (differences from normal) for December 2025 relative to the 1991-2020 normal. Analysis based on ACIS data.
Figure 2. Days with a CSI of 2 or higher for December 2025 for ACIS threaded stations. Analysis based on ERA5 data.
2. Local Temperature Anomaly Analysis
The average temperature anomaly — or difference from normal — across the 194 cities analyzed was 0.4°F.
December temperatures were evenly split: there were 95 cities that experienced warmer-than-average temperatures, while another 95 experienced cooler-than-average temperatures. The remaining four cities had average December temperature anomalies of 0.0°F.
Temperatures were elevated across many areas in the southern and western U.S. The most unusually hot region was the Southwest, where average temperature anomalies exceeded 9.4°F.
Twelve cities experienced their hottest December on record.
Cities in the Southeast, Ohio Valley, Upper Midwest, and Northeast experienced cooler-than-average temperatures. The three cities analyzed in Alaska also experienced a cooler-than-normal December: Anchorage (-5.9°F), Fairbanks (-18.5°F), and Juneau (-12.4°F).
The most unusually warm city in December was Casper, Wyoming, which was 12.1°F warmer than usual.
Of the 194 ACIS stations analyzed, nearly all (191) showed warming trends for December, indicating that these cities have been warming on average since 1970.Although Wheeling, West Virginia, had a cooler-than-average December, with a temperature anomaly of -2.9°F, it has seen the greatest December warming since 1970. The average December in Wheeling is now 10.7°F warmer than it was in 1970.
Table 1. Top 10 ACIS stations with the highest December 2025 temperature anomaly.
City | State | Temperature anomaly or difference from normal (°F) | Average temperature (°F) | Warming since 1970 (°F) |
|---|---|---|---|---|
Casper | WY | 12.1 | 36.9 | 3.4 |
Salt Lake City | UT | 11.5 | 43.7 | 4.1 |
Denver | CO | 11.1 | 42.3 | 2.1 |
Helena | MT | 10.6 | 34.0 | 6.7 |
Cheyenne | WY | 10.3 | 39.0 | 3.4 |
Albuquerque | NM | 10.1 | 47.1 | 4.8 |
Grand Junction | MT | 9.9 | 38.3 | 1.9 |
Missoula | CO | 9.6 | 34.0 | 4.7 |
Boise | ID | 9.5 | 41.6 | 4.0 |
Colorado Springs | CO | 9.3 | 41.0 | 4.5 |
Table 2. Top ACIS stations with the fastest warming December since 1970.
City | State | Warming since 1970 (°F) | Temperature anomaly or difference from normal (°F) | Average temperature (°F) |
|---|---|---|---|---|
Wheeling | WV | 10.7 | -2.9 | 31.4 |
Presque Isle | ME | 8.5 | -5.6 | 14.3 |
Milwaukee | WI | 7.7 | -5.0 | 24.6 |
Fargo | ND | 7.7 | -3.1 | 12.7 |
Burlington | VT | 7.7 | -5.2 | 22.9 |
Waterloo | IA | 7.5 | -4.5 | 20.9 |
Duluth | MN | 7.4 | -5.3 | 11.9 |
Green Bay | WI | 7.4 | -5.6 | 19.0 |
Sioux Falls | SD | 7.4 | 0.6 | 23.0 |
Minneapolis | MN | 7.3 | -3.1 | 18.9 |
3. Local Climate Shift Index Analysis
San Juan, Puerto Rico, had 10 days at CSI 5 in December, indicating that temperatures on those days were made at least five times more likely because of climate change.
On average, there was minimal CSI signal across the country. Only four cities experience at least one week’s worth of days at or above a CSI 2.
Table 3. Top 10 ACIS stations with the highest number of days at or above a CSI 2 during December 2025.
City | State | Days at CSI 2 or higher | Days at CSI 5 | Average temperature (°F) | Temperature anomaly or difference from normal (°F) |
|---|---|---|---|---|---|
San Juan | PR | 12 | 10 | 79.2 | 0.5 |
Seattle | WA | 8 | 0 | 45.0 | 2.9 |
Yakima | WA | 7 | 1 | 39.0 | 8.3 |
Honolulu | HI | 7 | 3 | 76.2 | 0.7 |
Juneau | AK | 6 | 3 | 17.9 | -12.4 |
West Palm Beach | FL | 5 | 2 | 72.1 | 3.1 |
Lewiston | ID | 5 | 3 | 42.6 | 7.5 |
Los Angeles | CA | 4 | 0 | 61.5 | 3.7 |
Salt Lake City | UT | 4 | 0 | 43.7 | 11.5 |
Spokane | WA | 4 | 0 | 37.5 | 8.5 |
Anchorage | AK | 4 | 1 | 13.5 | -5.9 |
METHODOLOGY
Calculating the Climate Shift Index
All Climate Shift Index (CSI) levels reported in this brief are based on daily average temperatures and ERA5 data. 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 194 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 (compared to 1991-2020 normals), and precipitation information, and to derive average monthly warming trends for each city.
Regions
Regions are defined by the National Centers for Environmental Information climate regions.