Monthly Attribution Overview December 2024

An analysis of how climate change boosted United States temperatures in December 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 December in cities across the United States.

1. High Level Findings

• December temperature anomalies in the U.S. were elevated across much of the country: 166 out of 191 analyzed cities were warmer than normal—the highest anomalies stretched from Montana south to Texas (Figure 1).

• Montana stood out as the state with the most anomalously warm cities. The three hottest cities analyzed across the country were Helena, Great Falls, and Billings. Missoula, the last Montana city analyzed, ranked 8th overall.

• This was amongst the top five hottest December on record for 32 cities.

• The Southwest, as well as the Northern Rockies and Plains Region showed particularly elevated December temperatures, where cities on average were more than 6°F warmer than average.

• Climate Shift Index (CSI) values meanwhile followed a different trend: overall, there was a low CSI signal across most of the countries. However, a few cities in California, the midwest, and Florida experienced the highest number of days with climate change-influenced temperatures at or above CSI 3 (temperatures made at least 3x more likely because of climate change) (Figure 2).

• In general, cities across the country did not experience many days with CSI values at or above 3. However, a CSI level 3 indicates a very strong climate influence on a day's temperatures. The high variability inherent in winter weather means that CSI levels of 3 or higher are extremely rare during December. The strong temperature anomalies and long-term warming trends during December (see Climate Central’s 2024 Winter Package) indicate that climate change is influencing winter conditions, although the signal is less clear at the daily scale.

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

Figure 2. Days with a CSI of 3 or higher for December 2024 for ACIS threaded stations. Analysis based on ERA5 data (December 1-28) and GFS data (December 29-31).

1. Local Temperature Anomaly Analysis

• The most unusually hot city in December was Helena, Montana where it was 11.77°F hotter than normal.

• 36 cities had December temperature anomalies greater than 5°F. These included 15 in Texas and all four in Montana.

• Montana stood out as the state with the most anomalously warm cities. The three hottest cities analyzed across the country were Helena, Great Falls, and Billings. Missoula, the last Montana city analyzed, landed as the 8th hottest city analyzed.

• The average temperature anomaly across all cities was 2.8°F.

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

• Presque Isle, Maine had an unusually warm December (with an average daily temperature anomaly of 1.87°F), and is the fastest-warming ACIS station for December on average, warming 9.2°F on average since 1970. 

Table 1. Top 10 ACIS stations with the highest December 2024 temperature anomaly.

Table 2. Top 10 ACIS stations with the fastest warming December since 1970.

1. Local Climate Shift Index Analysis

• Four 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 26 days at CSI 5, indicating that temperatures on these days were made at least 5 times more likely to occur because of climate change.

• Honolulu, Anchorage, and Juneau all stood out: these cities experienced 26,16, and 13 days at a CSI of 3 or higher respectively.

• In general, cities across the country did not experience many days with CSI values at or above 3. However, a CSI level 3 indicates a very strong climate influence on a day's temperatures. The high variability inherent in fall weather means that CSI levels of 3 or higher are extremely rare during December. The strong temperature anomalies and long-term warming trends during December (see our 2024 Fall Package) indicate that climate change is influencing fall conditions, although the signal is less clear at the daily scale.

Table 3. Top 10 ACIS stations with the highest number of days at or above a CSI of 3 during December 2024.

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 December 1 to December 28, 2024, and GFS data from December 29 to December 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.