Deborah Zabarenko, Environment Correspondent
(Reuters) - This winter's heavy snowfalls and other extreme storms could well be related to increased moisture in the air due to global climate change, a panel of scientists said on Tuesday.
This extra moisture is likely to bring on extraordinary flooding with the onset of spring in the Northern Hemisphere, as deep snowpack melts and expected heavy rains add to seasonal run-off, the scientists said in a telephone briefing.
As the planet warms up, more water from the oceans is evaporated into the atmosphere, said Todd Sanford, a climate scientist at the Union of Concerned Scientists. At the same time, because the atmosphere is warmer, it can hold onto more of the moisture that it takes in.
Intense storms are often the result when the atmosphere reaches its saturation point, Sanford said.
This year, a series of heavy storms over the U.S. Midwest to the Northeast have dropped up to 400 percent of average snows in some locations, said Jeff Masters, director of meteorology at Weather Underground.
The amount of water in that snowpack is among the highest on record, Masters said.
"If you were to take all that water and melt it, it would come out to more than 6 inches over large swaths of the area," Masters said. "If all that water gets unleashed in a hurry, in a sudden warming, and some heavy rains in the area, we could be looking at record flooding along the Upper Mississippi River and the Red River in North Dakota."
That tallies with projections by the U.S. National Weather Service, which last month said a large stretch of the north central United States is at risk of moderate to major flooding this spring.
Spring floods could be exacerbated by spring creep, a phenomenon where spring begins earlier than previously.
"We've documented in the mountains of the U.S. West that the spring runoff pulse now comes between one and three weeks earlier than it used to 60 years ago," Masters said. "And that's because of warmer temperatures tending to melt that snowpack earlier and earlier."
In the last century, global average temperatures have risen by 1.4 degrees Fahrenheit (.8 Celsius). Last year tied for the warmest in the modern record. One place this warmth showed up was in the Arctic, which is a major weather-maker for the Northern Hemisphere, according to Mark Serreze, director of the U.S. National Snow and Ice Data Center.
One driver of this winter's "crazy weather," Serreze said, is an atmospheric pattern known as the Arctic Oscillation, which has moved into what climate scientists call a negative phase.
This phase means there is high pressure over the Arctic and low pressure at mid-latitudes, which makes the Arctic zone relatively warm, but spills cold Arctic air southward to places like the U.S. Midwest and Northeast.
This negative Arctic Oscillation has been evident for two years in a row, the same two winters that have had extreme storms and heavy snowfalls.
The only underlying explanation for these events is climate warming due to heightened greenhouse gas levels, Serreze said.
(Editing by Mohammad Zargham)