It's cold outside. What happened to global warming?

As top scientists and policymakers continue to wrangle over how to limit greenhouse gas emissions, a growing number of Americans are offering the following response: "Brrrrrr!"

There's no denying it: It's cold. How cold is it? It's so cold that Wall Street bankers have their hands in their own pockets. It's so cold that Richard Simmons was spotted wearing pants.

As the Monitor's Patrik Jonsson noted Tuesday, a blast of Arctic air has swept over the United States, bursting water mains in Georgia, imperiling orange groves in Florida, and driving up energy prices everywhere.

It's not just the US that's having its collective nose hairs frozen. In northern India, a cold snap has killed scores and shut down thousands of schools. Beijing has been hit with the biggest snowfall in half a century. Seoul is reporting the heaviest single-day snowfall in recorded history. And snow and ice has blanketed much of Europe, snarling trains, planes, and buses. Overall, it feels as though the entire Northern Hemisphere has wandered into a giant meat locker.

Those who doubt global warming are having a field day – albeit the indoors kind. "Perishing Cold This Global Warming. Isn’t It?" observes Iain Martin, a political blogger for The Wall Street Journal. "Burn more coal. The Earth could use some heat," writes the Christian Broadcasting Network's Dale Hurd. It seems that cheap cracks about global warming are quickly becoming the new "Cold enough for you?"

Do they have a point? Does the current Hoth-like weather cast the scientific basis of global warming into doubt?

No. And here are three reasons why:

It's not actually that cold

Yes, it takes chutzpah to say this amid reports of seniors in Britain burning books to stay warm, but it's true. It was actually colder in London this week last year. Atlanta Journal Constitution blogger Jay Bookman points out that Atlanta's temperature dropped to as low as 18 degrees F. Tuesday. This sounds rather frigid, he writes, but it's downright balmy compared to the city's record low for that day of 3 degrees F., set in 1884. In Petersburg, Va., the air temperature on Tuesday was about 35 degrees F., or about 10 degrees colder than normal for this time of year, according to the Petersburg Progress-Index, and well above the city's record low of 7 degrees F. set in 1918.

It's the same story all over the Northern Hemisphere. Yes, it's colder than what we're used to in January, but we're not breaking very many new temperature records.

Instead, we're experiencing the kind of weather that was more common a few decades ago, before our atmosphere started becoming choked with man-made greenhouse gases.

Some places are really hot right now

Look around and you'll find plenty of warm spots on the planet, particularly on the bits that are currently angled toward the sun. On Christmas Day, the Australian Weather Bureau reported that Central Pacific Ocean temperatures are now at their warmest in more than a decade. For Australia itself, 2009 was a scorcher, the second hottest year on record after 2005.

Some parts of Northern New Zealand are sweltering in record breaking heat this week. And oddly enough, so are some places in Bulgaria, where a hot spot over the Black Sea has warmed one town to a pleasant 72 degrees. Not bad for a city at the same latitude as Portland, Maine.

Globally, the 2000s were the hottest decade on record, just over a third of a degree F. warmer than the 1990s, which was the second hottest decade. But the heat wasn't felt uniformly. The World Meteorological Organization notes that, in what must seem like a cruel joke on environmentalists, only the United States and Canada experienced conditions that were cooler than average during this period. This striking August 2009 temperature map from the National Oceanographic and Atmospheric Administration shows a big red blob over Australia, which was pummeled by deadly heat waves and wildfires all year, and a big blue blob over North America. The blue blob looks almost as if it's intentionally hovering over the more politically conservative states.

It's easy to see how these anomalously low temperatures have contributed to the steep decline in the percentage of Americans who say they believe in global warming. According an October 2009 poll by the Pew Research Center, only 57 percent of Americans believe that there is solid evidence that the earth is warming, down from 71 percent in April 2008.

Nobody said it would never get cold again

No sane climate scientist would say that global warming means never having another severe cold snap. What it does mean is a gradual shifting of the odds away from record-breaking cold days and toward record-breaking hot ones.

And that's exactly what we're seeing, even though we still get the occasional Arctic blast. The National Center for Atmospheric Research, a nongovernmental research group based in Boulder, Colo., released a report in November showing that, in the continental United States, the ratio of record high temperatures to low temperatures has increased dramatically in recent decades. In the 1950s, the center reports that ratio slightly exceeded 1 to 1, meaning that there were about as many record-breaking cold days as there were hot ones. By the 2000s, there were twice as many record highs as lows. On a warming planet, we'll still get the lows, just not as often.

But what's particularly troubling is that, even though temperatures have been high, the sun has been throwing a little less heat our way lately. According to NASA, we are currently experiencing the lowest solar minimum in nearly a century. Nobody knows when the sun will resume its previous output – in the 17th century, the sun went into a 70-year lull that correlated with cooler temperatures on Earth – and there's a chance that it may never come back.

Yet, the world is still warming. If and when the sun does reverse its downturn, we'll get a more accurate sense of how much of a mess we've made of our planet's climate.

As for the current frigid conditions, you can blame something called the Arctic Oscillation, a halo of counterclockwise winds blowing around the top of the world. When these winds are strong, they lock the cold air in place up there. But when the winds are weaker, the Arctic air slides southward, across the Norhern Hemisphere, and through the gaps in your windows where it eventually settles on your spouse's feet at night.

The New York Times's Andrew Revkin noted Monday that the Arctic Oscillation has entered the deepest cold phase since 1950, although there were times in the 1960s, '70s, and '80s that it came close. Mr. Revkin notes that this phenomenon is one of the least understood atmospheric patterns, but the graph he provides show a clear, if slight, trend: the conditions that brought about this week's freeze are getting less and less likely.

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