Yesterday afternoon, this tweet by ClimateCentral’s Heidi Cullen grabbed my eye:
@HeidiCullen: On this anniversary of Snowmageddon, a quick comparision: 2/6/11: 48.3% US #snow covered – 2/6/12: 26.2%: bit.ly/83ViYS #climate
22% seems like a big difference, but the data geek in me wanted more information. I went to the National Snow Analyses website and grabbed the data for snow cover on February 6th of the past nine years. Over that time, the portion of the U.S. covered by snow on February 6th (green line/area) has fluctuated around an average of 40.9% (orange line). In this context, neither this year nor last year seems terribly unusual.
But snow cover on one particular date doesn’t tell the whole story. This winter really has been one of the warmest and driest on recordin the U.S. In contrast, 2010 and 2011 both saw prolonged periods of extremely cold, snowy weather.
The question that seems to be on many minds and tongues is whether either (or both) of these conditions are the result of climate change. The link between a warmer-than-usual winter and global warming seems obvious (although it may not be as simple as it appears .. read on). But last winter plenty of ink was dedicated to the less intuitive potential link between rapid warming in the Arctic and cold, snowy winters in North America.
So which is it, you ask? Does climate change make winters colder and snowier or warmer and drier? Or could the answer somehow be both?
As it turns out, that’s a conversation I had with Andrew Freedman a couple of weeks ago. Freedman is an author of the Capital Weather Gang blog and one of Climate Central’s go-to guys when it comes to extreme weather. He explains that the key to understanding the past three winters is the Arctic Oscillation, an index of the barometric pressure in the Arctic relative to that at lower latitudes.
The past three winters have been extreme, although they’ve been extreme in different directions. Consider that we’ve experienced two winters with record to near record negative Arctic Oscillations, and now this winter the Arctic Oscillation Index has flirted with an all-time high so far. And partly as a result of this, we’re see-sawing between all or nothing winters when it comes to snow and cold.
These details are not captured well by long-term climate models. In general, climate studies show that during the next several decades, average winter temperatures should continue to warm across much of the US, and what would be snow events today may increasingly become rain events, particularly at lower elevations. (This represents an existential threat to many ski areas in the Northeast.)
However, heavy snowfalls will continue to take place, and the additional atmospheric water vapor, courtesy of global warming, leads to heavier rain and snow events, which was something that was much discussed last winter.
It’s not yet clear how the loss of Arctic sea ice, which is directly connected to global warming, is influencing wintertime atmospheric circulation in the Northern Hemisphere. Some studies show a connection to the cold and snowy winters of 2009/10 and 2010/2011. There is also some evidence that there is more variability now in East Coast snowfall from year to year, from very snowy winters to snow droughts, and this increased variability may be related to climate change.
So, in sum, yes – it’s quite possible that global warming is contributing to the snow drought of 2011/2012, AND that it contributed to blizzards during the prior winters. But the size of that contribution is uncertain right now.
I also emailed a handful of climate scientists and got similar answers. The consensus seems to be that we can expect a general warming trend, but also greater variation around that trend.
Still, as much as the human mind loves a pattern, it’s a bit too soon to know whether the past few years are harbingers of what’s to come or simply an oddity.