Average annual temperature across the United States has risen by about 0.7°C (1.2°F) over the past century, with more than 97% of the land-surface having increases. The largest changes have been in the western United States, where average temperature increased by more than 0.8°C (1.50°F) in Alaska, the Northwest, the Southwest, and also in the Northern Great Plains. The Southeast experienced the least warming, driven by a combination of natural variations and human influences. From a seasonal perspective, warming was greatest and most widespread in winter, with increases of over 0.8°C (1.5°F) in most areas. In summer, warming was less extensive (mainly along the East Coast and in the western third of the Nation), while cooling was evident in parts of the Southeast, Midwest, and Great Plains.
There has been a rapid increase in the average temperature of the contiguous United States over the past several decades. There is general consistency on this point between the surface thermometer record from NOAA and the middle tropospheric satellite records from Remote Sensing Systems (RSS), NOAA’s Center for Satellite Applications and Research (STAR), and the University of Alabama in Huntsville (UAH). In particular, for the period 1979–2015, the rate of warming in the surface record was 0.256°C (0.460°F) per decade, versus trends of 0.223°C (0.401°F), 0.210°C (0.378°F), and 0.130 °C (0.234°F) per decade for RSS version 4, STAR version 3, and UAH version 6, respectively (after accounting for stratospheric influences). All trends are statistically significant.
Shifts in temperature extremes have also been noteworthy. For example, the coldest daily temperature of the year increased at most locations in the contiguous United States through 2015, and the frequency of cold spells has steadily fallen throughout the century. The warmest daily temperature of the year generally increased throughout the West, whereas there were decreases in most locations east of the Rocky Mountains owing to the hot summers of the 1930s Dust Bowl era. Similarly, warm spells increased in frequency until the mid-1930s, became somewhat less common through the mid-1960s, and increased in frequency again thereafter.