Climate Fact: United States Temperature Trends
In Brief: The United States has experienced a 1.2 degree Fahrenheit warming trend since 1895, with the winter months and February in particular showing the largest positive temperature trends.
The United States Historical Climatology Network (USHCN) is a dataset based on 1,218 quality controlled temperature and precipitation reporting stations, most of which are from NOAA’s Cooperative Observers Program (COOP). These stations were selected out of the larger network of COOP stations because of the completeness of their records and relative lack of potential biases caused by changes in instrumentation or conditions in the surrounding landscape. The best analysis of the data from these stations suggests a warming trend of about 1.2 degrees Fahrenheit for the contiguous United States since 1895, with a period of cooling from the 1940s until about 1970. About 0.2 degrees of this warming can be attributed to the urban warming effect. The urban warming effect results from the urban heat island effect, the tendency for cities to be warmer than surrounding rural areas because of more concrete and other dark surfaces that easily absorb and retain heat, coupled with the fact that the United States has significantly urbanized over the past century. The surface temperature increase can primarily be attributed to warming during the winter months, with February showing the largest warming trend. September showed the smallest trend, although every month showed a warming trend. The three warmest years on record are 1998, 2006 and 1934, while the coolest years on record are 1917, 1895 and 1912. With a duration of 21 months, the 1916-1918 La Niña event, the strongest on record from 1895 to 1998, helped make 1917 the coldest year on record.
Seasons: Winter, Spring, Summer, Fall
Source: Shen, SSP et al. “Uncertainties, Trends, and Hottest and Coldest Years of U.S. Surface Air Temperature since 1895: An Update Based on the USHCN V2 TOB Data.” Journal of Climate 25 (2012): 4185-4203.