Research Report by Climate Central

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Report Summary

While the U.S. as a whole has seen a warming trend that has raised annual average temperatures by 1.3°F over the past 100 years, warming varies seasonally, and it’s winter that has seen the fastest warming.

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An analysis of data from the U.S. Historical Climatology Network of weather stations shows that the coldest states are warming the fastest, and across the country winter warming since 1970 has been more than four-and-a-half times faster per decade than over the past 100 years. Winter nights across the country have warmed about 30 percent faster than nights over the whole year. Some states cooled or failed to join the warming trend over the past 100 years, but since 1970, every state has shown winter-warming.

To better understand winter warming patterns we analyzed average-daily temperatures and overnight-low temperatures for December through February for the continental 48 states from 1912-2012 and 1970-2012.

We found:

• Since 1970, winters in the top 5 fastest-warming states — Minnesota, North Dakota, Wisconsin, Vermont and South Dakota — heated up four-and-a-half times faster than winters in the 5 slowest-warming states: Nevada, California, Oregon, Colorado, and Washington. The five fastest-warming states have seen at least 4oF warming in winters since 1970.

Rates of warming (°F per decade) for average winter (Dec.-Feb.), 1970-2012.
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• Winter warming accelerated almost everywhere since 1970, and all states have warmed since that time. Nationwide since 1970, winters warmed more than four-and-a-half times faster per decade than over the past 100 years.

• In contrast, over the past century, winters in 13 states — 10 in the South — bucked the warming trend and either cooled significantly or exhibited a non-significant slight cooling trend.

• Winter nights have warmed in all but one of the lower 48 states since 1970. Across the continent, winter nighttime temperatures have warmed about 30 percent faster than nighttime temperatures over the entire year. Since 1970, overnight winter temperatures in Wisconsin, Minnesota and Vermont have warmed faster than 1.29°F per decade, or more than 5°F in just 43 years.

• Since 1912, states with average winter temperatures below 32°F warmed three times faster than states with average temperatures above 32°F. Since 1970, winter warming has accelerated almost everywhere and states that previously cooled began to warm in winter. Yet, the coldest states (below 32°F) have still warmed nearly twice as fast as the rest of the country on average. And during that time, winter nights in the coldest states warmed up to five times faster than those in warm states.

• The pattern of winter warming is different than the pattern of warming throughout the whole year, which was illustrated in Climate Central’s June 2012 analysis of annual temperatures, The Heat is On. Some of the fastest-warming states overall, such as Arizona, New Mexico and Colorado, have had some of the slowest-warming winters, both since 1970 and over the past 100 years.