Friday, January 22, 2016

Global warming increases extreme precipitation events

People who live in areas of cold winters have noted that it is more like to snow at temperatures somewhat below freezing than it is when the temperature is way below freezing.

Why Big Blizzards In Winter Don’t Disprove Global Warming

by Joe Romm Jan 22, 2016

Another epic blizzard threatens 50 million people on the East Coast, with a bulls-eye on Washington DC. And leading climatologists again explain how human-induced climate change, especially warming-fueled ocean temperatures, are super-charging the amount of moisture in the atmosphere the storm will dump on us.

First, though, I think the name, Winter Storm Jonas, doesn’t do justice to this blizzard, especially since the Jonas brothers are a pretty harmless pop rock band. I’m suggesting the name, Superstorm (Edward) Snowed-In: Because it will turn DC upside down, bring the government to a standstill, and then flee the country.


Mann, Director of Penn State’s Earth System Science Center, explained: “There is peer-reviewed science that now suggests that climate change will lead to more of these intense, blizzard-producing nor’easters, for precisely the reason we’re seeing this massive storm — unusually warm Atlantic ocean surface temperatures (temperatures are in the 70s off the coast of Virginia).”

When you mix extra moisture with “a cold Arctic outbreak (something we’ll continue to get even as global warming proceeds),” as Mann points out, “you get huge amounts of energy and moisture, and monster snowfalls, like we’re about to see here.”

Mann’s bottom line:

While critics like to claim that these massive winter storms are evidence against climate change, they are actually favored by climate change.

Trenberth, former head of the Climate Analysis Section at the National Center for Atmospheric Research, agrees: “At present sea surface temperatures are more the 3F above normal over huge expanses (1000 miles) off the NE coast and water vapor in the atmosphere is about 10 to 15% higher as a result. Up to half of this can be attributed to climate change.”

So Superstorm Snowed-In is able to sweep in vastly more water vapor thanks to human-caused warming.


Before this latest superstorm, we’ve seen a long-term pattern of more extreme precipitation, most especially in Northeast 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 also 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.


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:


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.”


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 literature review here).


What of the future? Trenberth has explained that, “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.


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