People across the northern hemisphere are battling a cold snap or bracing for record wintry conditions, but this tells us little about climate change. If you want something to blame it on, try the Arctic oscillation

People across the northern hemisphere are facing the fact that a warming planet doesn’t get rid of winter. Britain is currently having snow and chill, and on Monday the heaviest snow on record plastered Seoul. Later this week the central US will experience its most brutal cold wave in 10 to 20 years. And most of western Europe will be encased in a deep freeze by this weekend.

Those happy souls you see dancing through the icicles? They’re the ones who believe that humans are now off the hook for climate change, even as greenhouse gases continue to accumulate in our air. Cowering in the corner is another group: those who fear that Greenland’s melting ice sheets are already starting to pinch off the Atlantic’s warm conveyor belt, a hypothesis explored most luridly in the film The Day After Tomorrow. In real life, any such slowdown would be expected to unfold in decades rather than weeks.

Rather than seeking vindication or catastrophe in this cold snap, now is a good time to remind ourselves that weather, like death and taxes, will always be with us. Spectacular regional swings in temperature and precipitation, sometimes lasting for months, often emerge from the natural jostlings of atmosphere and ocean. By themselves, none of these prove or disprove a human role in climate change.

In any given year, there could be a season as shocking as Britain’s epic winter of 1962—63 — when snowdrifts were measured in metres, and temperatures stayed below freezing for most of January — or the summer of 2003, when tens of thousands died in some of the worst heat ever recorded in Europe.

What’s different now is that climate change is shifting the odds towards record—hot summers and away from record-cold winters. The latter aren’t impossible; they’re just harder to get, like scoring a straight flush on one trip to Vegas and a royal flush the next.

It’s also critical to remember the “global” in global warming. Even if every inch of land in the northern hemisphere were unusually cold, that would only represent 20% of Earth’s surface. There’s plenty of warmth elsewhere. National Oceanic and Atmospheric Administration data through November hints that 2009 may end up ranked as the southern hemisphere’s warmest year on record. For the planet as a whole, last year falls solidly among the 10 warmest years of the past 100. And despite all the talk about Earth having cooled since the late 1990s, this past decade trumps the 1990s as the warmest on record.

If you’re craving a scapegoat for this winter, consider the Arctic oscillation. The AO is a measure of north-south differences in air pressure between the northern midlatitudes and polar regions. When the AO is positive, pressures are unusually high to the south and low to the north. This helps shuttle weather systems quickly across the Atlantic, often bringing warm, wet conditions to Europe. In the past month, however, the AO has dipped to astoundingly low levels -- among the lowest observed in the past 60 years. This has gummed up the hemisphere’s usual west-to-east flow with huge “blocking highs” that route frigid air southward.

Handy as it is, the AO describes more than it explains. Forecasters still don’t know what sends the AO into one mode or the other. What we do know with crystal clarity is that the atmosphere’s load of greenhouse gases is increasing by more than 10 million tonnes every year. The tepid agreements out of Copenhagen are unlikely to change that trend any time soon.

If this winter tells us anything, it’s that we’ll have to remain on guard for familiar weather risks as well as the evolving ones brought by climate change. Juggling all of these will not be easy.