Global Warming: It's Melting Away the Winter Season

Close up of melting snow on tree branch over Crater Lake, Oregon, United States
Eric Raptosh Photography / Getty Images

2016 was recently named the warmest year on record for the globe since record-keeping began in the 1880s. But, did you know that the December 2015 to February 2016 period, which makes up the meteorological winter season, was likewise the hottest ever felt for the globe and the Northern Hemisphere?

In fact, nine of the last ten years contain the hottest-ranking Northern Hemisphere winters.

Global Avg Temp (Land & Ocean ) Hottest Year Rank (since 1880) N. Hemisphere Winter Avg Temp (Land & Ocean) Hottest N. Hemi Winter Rank (since 1880)
2016 58.69°F (14.84°C) 1 49.1°F (9.49°C) 1
2015 58.62°F (14.8°C) 2 48.45°F (9.13°C) 2
2014 58.24°F (14.59°C) 3 47.72°F (8.72°C) 4 (ties 2005)
2013 58.12°F (14.52°C) 5 47.5°F (8.6°C) 8
2012 58.03°F (14.47°C) 9 47.39°F (8.54°C) 9
2011 57.92°F (14.41°C) 11 47.32°F (8.5°C) 10
2010 58.12°F (14.52°C) 4 47.63°F (8.67°C) 6
2009 58.01°F (14.46°C) 7 47.61°F (8.66°C) 7
2008 57.88°F (14.39°C) 12 47.25°F (8.46°C) 11
2007 57.99°F (14.45°C) 10 48.24°F (9.01°C) 3
Ranking the Record Heat 2007-1016

Is this a coincidence? Or is it evidence that Earth's rising trend in global temperatures are also warming winters?

Evidence of Winter's Disappearing Act

NOAA scientists would say "yes" to the latter.

There are several reasons why they stand by this belief, one of which is a dwindling air-freezing index (AFI). The AFI—a metric that measures how often and by how much air temperatures remain below the 32°F (0°C) freezing mark during the winter season—has decreased significantly for the majority of the U.S. "[Seasonal] AFI values are typically 14%-18% lower across the [contiguous] United States during 1981-2010 versus 1951-1980," federal climate experts wrote in a 2014. The findings indicate a net reduction in winter severity that's consistent with observed climate change.   

Scientists also look to frost and freeze dates as evidence that the winter season is shortening. What they're seeing is that first frosts (the first occurrence of 32°F in the fall) are happening later and later, while the last frost is happening earlier in the year. Today, the average frost-free season (number of days without frost) is about 2 weeks longer across the U.S. than it was in the early 20th century, and nearly two thirds of that lengthening has happened since the 1990s.      

Mild winters aren't just being felt across the lower 48 states. According to David Philips, Senior Climatologist with Environment Canada, winters in Canada (Earth's second-coldest country) have warmed by an average of (3.3°C) over the past 70 years—twice as much warming as experienced in Canada's springs, summers, or autumns.

Philips has also noted a dramatic drop in the probability of white Christmases across the southern part of the country, the region where most people live.

Even Santa himself has witnessed North America's waning winters. In the Arctic, average temperature has increased at twice the rate as the rest of the globe, and winter temperature more than summer temperature. This has led sea ice—a semi-permanent layer of ice that grows over sea water in winter, and retreats in summer—to shrink by around 3% each February since the late 1970s. At this rate, the Arctic is expected to be ice-free by the year 2030.   

Global Warming's Power

Large-scale warming of air temperatures has helped usher in these environmental changes, but not single-handedly. Climate patterns, including El Niño and the Arctic Oscillation (AO), are equally to blame.

Initial studies suggest that "super" (strong) El Niños are likely to occur twice as often in a warming world. El Niño—abnormally warm waters in the Pacific Ocean (the world's largest ocean) near the equator—is one of the climate patterns that impacts Northern Hemisphere winters the most. The naturally occurring event, which is strongest in winter, generally causes global temperatures to rise, thanks to the release of heat (from warmer ocean waters) into the atmosphere. So, stronger El Niño events would only exacerbate its reputation for causing warmer and drier-than-normal winters.

Scientists are also researching the effects of global warming on the Arctic Oscillation. Over the past century, the AO has alternated between its positive and negative phases, however, since the 1970s, it has tended to stay in the positive phase. During the AO's positive phase, a belt of strong winds around the North Pole confine cold arctic air masses to the polar region, essentially locking frigid winter air out of the middle regions of North America. As a result of this, not only the coldest air, but winter storms, too, are driven farther north.

The Three Seasons

Does all of this mean a three-season year is inevitable in the not-too-distant future?

Scientists can't say for sure since much about our climate future is uncharted territory. More than likely, winters will be re-defined from the cold, snowy season that we know them to be, to a season of spring-like weather sprinkled with week-long stretches of cold snaps. A few isolated locations may actually see more winter snowfall, thanks to the added heat in the atmosphere that will "up" humidity and trigger heavier precipitation. 

One thing is for sure: warmer-than-average winters are the new norm.