By Chris Mooney
Washington Post news story
The snowfall in Boston lately is simply insane. The local bureau of the National Weather Service has tallied up the data and here’s how it looks — with all time records for snow within a 14-, 20-, and 30-day period: http://www.washingtonpost.com/news/energy-environment/wp/2015/02/10/what-the-massive-snowfall-in-boston-tells-us-about-global-warming.
You could treat this as ordinary weather, or, you could think about it in a climate context. Counter-intuitive though it may sound, the fact remains that — as I have noted previously — some kinds of winter precipitation could indeed be more intense because we’re in a warming world.
Consider, for instance, that sea surface temperatures off the coast of New England are flashing red, showing an extreme warm anomaly. That’s highly relevant — because warmer oceans have atmospheric consequences.
“Sea surface temperatures off the coast of New England right now are at record levels, 11.5C (21F) warmer than normal in some locations,” says Penn State climate researcher Michael Mann. “There is [a] direct relationship between the surface warmth of the ocean and the amount of moisture in the air. What that means is that this storm will be feeding off these very warm seas, producing very large amounts of snow as spiraling winds of the storm squeeze that moisture out of the air, cool, it, and deposit it as snow inland.”
Warmer oceans also increase the temperature contrasts that winter storms encounter when they hit the East Coast, notes Mann — and this ups their strength.
“Heavy snows mean the temperature is just below freezing, any cooler and the amount would be a lot less,” adds Kevin Trenberth, a climate expert at the National Center for Atmospheric Research. “Warmer waters off the coast help elevate winter temperatures and contribute to the greater snow amounts. This is how global warming plays a role.”
Yes, it might sound strange, but it can actually snow more when it’s a bit warmer — not too warm for snow, of course, but not extremely cold, either.
“Increase of extreme precipitation has occurred in all regions of the continental USA and further changes are expected in the coming decades,” adds a recent study.
The mechanisms by which global warming messes with winter certainly do involve some counterbalancing forces. On the one hand, if it’s warmer overall, you’d expect temperatures to reach the threshold required for snow less frequently. You’d also expect snow cover to decline — snow will melt away faster in a warmer world when it does fall.
As Trenberth argues, this means that at the beginning and end of winter, precipitation that might once have fallen as snow would now be more likely to fall as rain.
And yet as long as it’s in the thick of winter and there are cold enough temperatures, global warming can then throw in a snow-enhancing force that involves water vapor — the jet fuel of precipitation. More atmospheric heat means more water vapor contained in the atmosphere — and more rain or snow.
“For most conditions at sea level, there’s a rule of thumb that says the atmosphere can hold 4% more moisture per one degree Fahrenheit increase in temperature,” Trenberth writes.
So in sum: When climate experts see an extremely warm ocean off the east coast and record snowfall in Boston, it fits a big picture for them.
It doesn’t yet for most of us, but sooner or later we’re going to have to get past the idea that global warming and huge amounts of snow are somehow contradictory.