Winter may not officially start until December 21, when Earth's northern hemisphere is tilted farthest away from the Sun. But meteorological winter arrived December 1, so we thought it would be a great time to discuss winter temperature trends.

Over the past 40 years or so, winters have gotten gradually warmer. From 1970-2012, winters in the continental U.S. have warmed by 0.61°F per decade on average.

That’s just an average, though. California and Nevada have only warmed at a rate of 0.17°F per decade over that time (slowest winter warming region in the country), and the Southeast has gone up just 0.29°F per decade. A couple of places — northeastern Nevada and south-central Wyoming — have actually cooled during that time.

However, the winter warming trends for both the Upper Midwest and the Northeast have been higher than the national average. The Upper Midwest is warming the fastest of any region at 1.12°F per decade and the Northeast comes in second at .78°F per decade. The Ohio Valley rounds out the top three, warming at .66°F per decade.

Even in a world that’s generally warming, there will be regional variations and rounds of record cold -- like the one that's currently descending through the Northwest and Rockies into the Plains this week. However, the overall long-term temperature trend is still upward, which is consistent with what you’d expect to happen as heat-trapping greenhouse gases like carbon dioxide continue to build up in the atmosphere.

Article contributed by Climate Central