An amazing event occurred last night: for the first time this year, the thermostat in my drafty old house got up to 71 degrees and the furnace actually shut off for a few minutes. The outside temperatures were in the 40s, and 40 degrees never felt so warm as it does after two week of 30 and below. But today, temperatures fell right back down again.
Inevitably, I've been hearing more and more from people who say that the insanely cold weather in Atlanta and elsewhere is evidence against global warming. Some even say we're experiencing global cooling. After all, not only is it cold all over the USA, but England's having its coldest winter since 1981.
These people don't seem to understand the difference between climate—the long-term, average conditions in a given location—and weather, which is a series of temporary variations on that long term theme. As Michael Lemonick pointed out over at Climate Central, "those variations can be dramatic—heat waves, cold snaps, droughts, severe storms—but they aren’t representative of the underlying climate (that’s why they’re called variations)."
It’s also critical to remember the “global” in global warming. Large swaths of the planet aren’t unusualy cold; there’s plenty of warmth elsewhere around the world. Parts of the U.S. and Northern Europe are indeed colder than normal, but even if every inch of land in the northern hemisphere were unusually cold, that would only represent 20% of Earth’s surface. We’re now seeing warming where climate science said more than 90% of the warming would end up — the oceans. NOAA 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. Despite all the talk about Earth having cooled since the late 1990s, this past decade trumps the 1990s as the warmest on record.
If a small part of the globe is colder than average and a big part is warmer, that suggests that global warming hasn’t stopped by any means. Record cold snaps will continue to happen, even as the world continues to heat up. A recent study cited plenty of record low daily temperatures over the past decade in the continental US. But it shows that record high temperatures occurred twice as often; in a world without global warming, you’d expect them to be roughly equal
In fact, the entire planet just keeps warming thanks to human emissions. To talk about "global cooling" at the end of the hottest decade the planet has experienced in many thousands of years is indeed ridiculous.
Robert Henson, author of The Rough Guide to Climate Change, recently wrote “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."
Sometimes even a global shift can be temporary. In 1998, global temperatures shot up significantly over the previous year. The reason, it turns out, was due to an especially strong El Nino, a massive warming current that shows up periodically in the Pacific. Did that mean global warming had suddenly accelerated? Not at all. But people who want to sound the alarm on global warming sometimes imply that such events—or the record melting of Arctic sea ice in 2007, or even a hurricane like Katrina—as evidence that global warming has indeed gotten worse.
The truth is that it’s risky to consider any one event, even a worldwide event, as proof of anything, even though we instinctively feel that it might be. It's the longer-term trends that give scientific confidence. The trends over the past century, and the trends most climate scientists see for the coming century, do indeed point to global warming as a real and present danger. But that’s not an obvious point to most people—and it’s one that even people who know better sometimes forget.
The best antidote to misinformation, as Michael Lemonick notes, is information.