No single weather event can be attributed directly to climate change. This is common knowledge thanks in part to Kevin Trenberth, a climate scientist who’s played an integral role in explaining how to relate weather extremes to climate change. “The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question,” his most-cited quote goes. “All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”
So if anyone tries to tell you that climate change directly caused Boston’s record-breaking and continuing snowmageddon, that’s not true. What is true, however, is that climate change may have affected the snowstorm — may have made it more likely, may have made it worse than it would have been without so much greenhouse gas in the atmosphere. It bears repeating: “All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”
The question, then, becomes how? How did the warmer and moister environment in which we now live because of human-caused carbon emissions affect Boston’s historic weather event? Wouldn’t a warmer, moister environment mean less snow? How does that even make sense?
Trenberth, who is a senior scientist National Center for Atmospheric Research, offered his explanation via e-mail:
No doubt there is a big element of chance in having the weather pattern of the storm tracks setting up in an optimal fashion to produce big snow storms one after the after. But in mid-winter, there is plenty of cold continental air to ensure that precipitation falls as snow rather than rain.
At the same time, the environment has warmed especially compared to 1978 (when the last set of major snow storms occurred), boosting the odds of huge amounts of snow. Part of the warming is from human activities increasing heat trapping gases (carbon dioxide) which have warmed the global oceans. Part is from climate variability (such as from the quiet hurricane season last summer — when all the activity was shifted to the warm Pacific), and the result is sea surface temperatures off the coast exceeding 7°F above normal in parts and 4°F over huge expanses, thereby resulting in 15 to 20% more moisture in the atmosphere.
That moisture gets caught up in the storms, likely invigorating the storms themselves, and the result is major snow storms.
In other words, Trenberth is saying very generally that climate change has helped boost the warmth of the ocean and atmosphere, which combined (as he has explained before) increase the likelihood of really big snow storms. That’s because a warmer ocean creates more water vapor, and a warmer atmosphere is able to hold more moisture. The atmosphere absorbs the water vapor, and when it can no longer hold the moisture, it forms precipitation. When it’s below freezing — like it is Boston — the precipitation is generally snow.
This explanation would make sense if climate change is helping to intensify the Boston snowstorms, according to Penn State climate researcher Michael Mann, considering how warm sea surface temperatures are off the coast of the northeastern United States right now. For this time of year, Mann said, sea surface temperatures there are warmer than they’ve ever been.
“That means there is both more energy to intensify these storms, and more moisture over the ocean to be turned into snow when the spiraling winds of the storm circulate back over land,” Mann said via e-mail.
So, in sum, Trenberth and Mann aren’t saying the warmer ocean caused the “snowmaggedon” itself (especially considering the storms originated in Canada). They’re saying the warmer ocean likely increased the amount of moisture available within the storm, contributing to record-breaking snowfall. Unfortunately for Bostonians, this is probably going to keep happening — two more major snow storms are expected there this week. And with the city’s “snow farms” nearly full, public schools perpetually facing closure on bad days, and public transportation on the fritz, Trenberth has some advice.
“While a huge inconvenience to some,” he said, “it is best to accept it and enjoy it!”