Quotes of the Day

As I got off the plane in the Vermont town of Burlington on Sunday, I felt something new: cold. It wasn't that cold — high temperatures in Burlington were hovering around the freezing mark, a little warmer than average for this city of eager ski bums. But after more than a month of unusually mild weather in New York City — where Januarys can sometimes be nothing short of brutal — it was almost a treat to feel a hazy hint of winter.

That's because 2012 is shaping up to be the year that winter forgot in the U.S. December and the first week of January have seen atypically mild temperatures throughout much of the country — especially in the usually harsh states of the far north and parts of the plains. Fargo, N.D. — which probably exists in most Americans' minds as a big white blur of snow — saw temperatures of 55°F on Jan. 5, breaking a more than century-old record for the warmest day in January. High temperatures in Nebraska at the end of last week were more than 30°F above normal, and in December at least half the U.S. had temperatures at least 5°F above normal.

Nor is the unseasonable warmth confined to the U.S.; Europe has had mild temperatures so far as well. When cold goes missing, snow does too and it's been an unusually green (or brown) winter. At the end of 2011, less than 20% of the continental U.S. was covered with snow, compared with more than 50% at the end of 2010. Ski resorts from California to Vermont are panicked about the possibility of a dry, warm winter leaving slopes bare and skiers looking into beach vacations.

The unseasonable weather is doing weird things to nature too. As Juliet Eilperin and Darryl Fears reported in a piece for the Washington Post at the end of December, early spring flowers are responding to the warmth and blooming months early in the National Arboretum. New England lost most of its fall foliage, as heavier than usual rain and unusually warm nights kept trees green until the leaves suddenly fell. "It's a weird kind of fall blending right into spring," Scott Aker, the head of horticulture at the National Arboretum, told Eilperin and Fears.

This, of course, brings us to the point in any column of this kind at which I remind you — just in case you thought otherwise — that one season does not in itself make a trend, and the warm temperatures of the past month and a half aren't solely driven by climate change. The winter of 2012 may see precious little snow, but the winters of 2011 and '10 saw unusually heavy snowfall — record-breaking in some parts of the U.S. Britain experienced some of the coldest temperatures in its history last winter — and just last fall, parts of the U.S. were hit by the celebrated October Snowmageddon, leading people to predict a ferocious winter was coming.

The fact that things have, so far, been so mild is due in part to some extenuating circumstances. As the meteorologist Jeff Masters wrote last week on the blog Weather Underground, the jet-stream pattern in December was the most extreme on record, which kept cold Arctic air from pushing into the U.S. Those kinds of factors can — and do — change fast, so say it with me now: weather is what happens day to day, but climate is what happens over years and even decades.

Still, even such a big-picture perspective does indicate that truly cold temperatures are becoming less and less common in the U.S. To take one example, since 1996, there have been 48 high-temperatures records set in New York City's Central Park — and one just one record low. Since 1980, nearly every year in the U.S. has seen annual average temperatures higher than the long-term average. Confusion and uncertainty still exists over the exact impact of climate change on extreme-weather events like hurricanes or tornadoes, but there's one thing we can be pretty sure of: it will be less cold.

To many people that's probably not a bad thing. Extreme cold isn't just uncomfortable and inconvenient — it's also dangerous, particularly for older or poorer people who can't protect themselves from the elements as well as others. But warmer winters can change nature in dangerous ways as well. Western bark beetles — which have ravaged the pine trees of the west — are thriving because they're no longer being knocked out by very cold winters. Dry warm weather can worsen the risk of forest fires, and short winters can end up intensifying the spring-allergy season. A decline in mountain snowpack in the west can mean less water for dry states that are accustomed to meltwater runoff in the spring.

And then there's the less quantifiable, more lyrical value of winter — a cold, frozen, crystalline season that's beautiful and punishing all at once. As the British poet Anne Bradstreet said, "If we had no winter, the spring would not be so pleasant." Climate change disrupts the rhythm of the seasons, that regular passage of time and temperature we assumed was fixed. It turns out we may be wrong, and winter as we know it could one day be a season of the past. As we keep altering the climate, who can tell what else might follow it into unplanned obsolescence.

• Bryan Walsh
• What unusually mild temperatures do -- and don't -- tell us about climate change