POSTED ON JANUARY 26, 2015 AT 3:49 PM UPDATED: JANUARY 26, 2015 AT 4:52 PM
“The Climate Science Behind New England’s Historic Blizzard”
Another epic blizzard is bearing down on New England. There is a “big part” played by “human-induced climate change,” especially warming-fueled ocean temperatures, according to Dr. Kevin Trenberth, former head of the Climate Analysis Section at the National Center for Atmospheric Research.
I asked Dr. Trenberth to comment on the role climate change has on this latest storm, which is forecast to set records. He explained:
The number 1 cause of this is that it is winter. In winter it is cold over the continent. But it is warm over the oceans and the contrast between the cold continent and the warm Gulf Stream and surrounding waters is increasing. At present sea surface temperatures are more the 2F above normal over huge expanses (1000 miles) off the east coast and water vapor in the atmosphere is about 10% higher as a result. About half of this can be attributed to climate change.
Before this latest storm, we’ve seen a long-term pattern of more extreme precipitation, particularly in New England winters. Climate scientists had long predicted this would happen in a warming world. Here’s why.
Like a baseball player on steroids, our climate system is breaking records at an unnatural pace. And like a baseball player on steroids, it’s the wrong question to ask whether a given home run is “caused” by steroids. As Trenberth wrote in his must-read analysis, “How To Relate Climate 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. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”
One of the most robust scientific findings is the direct connection between global warming and more extreme precipitation or deluges. “Basic physics tells us that a warmer atmosphere is able to hold more moisture — at a rate of approximately 7 per cent increase per degree [Celsius] warming,” as the U.K. Met Office explained in its 2014 update on climate science. “This is expected to lead to similar percentage increases in heavy rainfall, which has generally been borne out by models and observed changes in daily rainfall.”
This means that when it is cold enough to snow, snow storms will be fueled by more water vapor and thus be more intense themselves. So we expect fewer snowstorms in regions close to the rain-snow line, such as the central United States, though the snowstorms that do occur in those areas are still likely to be more intense. It also means we expect more intense snowstorms in generally cold regions. This may appear to be counterintuitive — and certainly climate science deniers like to play up big snowstorms for that reason. But the fact is that the warming to date is not close to that needed to end below-freezing temperatures during midwinter over parts of the globe like New England, while it is large enough to put measurably more water vapor into the air.
A 2014 MIT study by Prof. Paul O’Gorman found that “snowfall extremes actually intensify” even many decades from now, in a future with high levels of warming:
O’Gorman found that there’s a narrow daily temperature range, just below the freezing point, in which extreme snow events tend to occur — a sweet spot that does not change with global warming….
“People may know the expression, ‘It’s too cold to snow’ — if it’s very cold, there is too little water vapor in the air to support a very heavy snowfall, and if it’s too warm, most of the precipitation will fall as rain.”
We’ve long known that warmer-than-normal winters favor snow storms. A 2006 study, “Temporal and Spatial Characteristics of Snowstorms in the Contiguous United States” found we are seeing more northern snow storms and that we get more snow storms in warmer years:
The temporal distribution of snowstorms exhibited wide fluctuations during 1901-2000…. Upward trends occurred in the upper Midwest, East, and Northeast, and the national trend for 1901-2000 was upward, corresponding to trends in strong cyclonic activity….
Assessment of the January-February temperature conditions again showed that most of the United States had 71%-80% of their snowstorms in warmer-than-normal years…. a future with wetter and warmer winters, which is one outcome expected, will bring more snowstorms than in 1901-2000. Agee (1991) found that long-term warming trends in the United States were associated with increasing cyclonic activity in North America, further indicating that a warmer future climate will generate more winter storms.
The U.S. Global Change Research Program (USGCRP) U.S. Climate Impacts Report from 2009 reviewed that literature and concluded, “Cold-season storm tracks are shifting northward and the strongest storms are likely to become stronger and more frequent.
So it is no surprise that a 2012 study found extreme snowstorms and deluges are becoming more frequent and more severe. The 2014 National Climate Assessment (NCA), which is the most comprehensive analysis to date of current and future U.S. climate impacts, pointed out, “The mechanism driving these changes is well understood.” The congressionally-mandated report by 300 leading climate scientists and experts, which was reviewed by the National Academy of Sciences, explains: “Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming…. This extra moisture is available to storm systems, resulting in heavier rainfalls. Climate change also alters characteristics of the atmosphere that affect weather patterns and storms.”
That final point is very important. The worst deluges have jumped not merely because warmer air holds more moisture that in turn gets sucked into major storm systems. Increasingly, scientists have explained that climate change is altering the jet stream and weather patterns in ways that can cause storm systems to slow down or get stuck, thereby giving them more time to dump heavy precipitation (see my recent literature review here).
The National Climate Assessment noted that this “remains an active research area” but pointed outthat: “Heavier-than-normal snowfalls recently observed in the Midwest and Northeast U.S. in some years, with little snow in other years, are consistent with indications of increased blocking (a large scale pressure pattern with little or no movement) of the wintertime circulation of the Northern Hemisphere.”
You can see these the remarkable swings in the fraction of annual precipitation coming from extreme deluges in New England from NOAA’s Climate Extremes Index for the past century:
In the case of the blizzard bearing down on New England, we have both the extra moisture off the East Coast and an “odd configuration of the jet stream (once again) is moving the low pressure system through a pattern that will create an epic blizzard.”
What of the future? Trenberth explains, “In mid winter, it is expected with climate change that snowfalls will increase as long as the temperatures are cold enough, because they are warmer than they would have been and the atmosphere can hold 4% more moisture for every 1F increase in temperature. So as long as it does not warm above freezing, the result is a greater dump of snow.” On the other hand, “at the beginning and end of winter, it warms enough that it is more likely for rain to result.” The net result is that average total snowfall may not increase.
The 2014 MIT study provides some more regional specificity about how the future will play out in terms of average snowfall versus extreme snowfall:
The study found that, under high warming scenarios, those low-lying regions with average winter temperatures normally just below freezing would see a 65 percent reduction in average winter snowfall. But in these places, the heaviest snowstorms on average became only 8 percent less intense. In the higher latitudes, extreme snowfall became more intense, with 10 percent more snow, even under scenarios of relatively high average warming.
Of course, climate science deniers and their allies will no doubt continue to misinform the public and policymakers by arguing that monster snowstorms are somehow evidence against the theory of human caused global warming, when they are no such thing. The latest science means the deniers will continue to have increasingly intense extreme “snowmageddons” in colder regions like New England, especially in midwinter, to mislead the public for decades to come.