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Summerlike weather is lasting longer

The lines between seasons are blurring and summer is getting longer in North America, a new study indicates.
/ Source: LiveScience

The lines between seasons are blurring and summer is getting longer in North America, a new study indicates.

Tracing backwards every known rainfall event on the globe, for a 25-year period ending in 2003, scientists wanted to determine where the moisture that supplied each rainfall came from.

While doing that, they found remarkable trends in what they call recycling over that time period, said study co-author, Paul Dirmeyer from the Center for Ocean-Land-Atmosphere Studies in Calverton, Md. The study was published July 29 in Geophysical Research Letters.

Precipitation recycling is the fraction of rain falling over a particular area that originated as evaporation from that same area.

"In other words, the water goes up and then comes back down at that same location as opposed to being blown in from somewhere else," Dirmeyer said.

Typically, in the wintertime, evaporation at high latitudes is minimal because it's cold. In the summer, however, there's a lot more evaporation. And because winds are generally weaker in summer, what does evaporate tends to stay in the vicinity.

In the winter, there's little evaporation and the winds typically are stronger, so what little moisture does evaporate tends to get blown away, Dirmeyer told LiveScience. "Both of those factors tend to amplify the contrast in the recycling between summer and winter."

Dirmeyer found that at higher latitudes, especially in North America, the trend over the years showed water recycling was happening faster during the winter and the spring. The same trend was also observed in the fall season, although not as strong as in spring.

If recycling is increasing in the spring and the fall, it suggests that summer regime is expanding, Dirmeyer said. "In other words, you're getting more summer-like conditions in the spring and in the fall."

This trend observed in high latitudes is consistent with other changes attributed to global warming. Studies have found that the spring season is arriving earlier, and vegetation is lasting later into the fall.

"The curiosity here is if this is in fact a manifestation of a change in the water cycle being driven by global change," Dirmeyer said. "That we have not proven. What we have right now is a correlation; we don't have a cause and effect. But it's very consistent with these other findings."