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Global warming to shift timing of North American monsoon

Greenhouse gas-induced heating inhibits early monsoon precipitation

Colorado weather lightning

The North American monsoon usually starts in mid-July in Colorado.

graph showing monsoon changes

The North American monsoon will dry up in June and July and become wetter in September and October. Graphic courtesy NASA and the Coumbia University Earth Observatory.

By Bob Berwyn

SUMMIT COUNTY —Global warming result in a significant shift of the North American monsoon, with less rain during the early part of the season, in June and July, and more rain later in the summer and early autumn. The trend toward a later start to summer precipitation has already started, but will become more pronounced — and easier to distinguish from the background “noise” of natural variability — during the next few decades, according to researchers with NASA and Columbia University’s Lamont-Doherty Earth Observatory.

“We expect that increased greenhouse gases will make the atmosphere more stable and more difficult for precipitation to occur … When the warming is strong enough, it effectively delays the start of the monsoon,” said NASA researcher Benjamin Cook. “One way to overcome that is when the air near the surface is really moist. That’s what happens at the end of the monsoon season. At that point, it leads to an increase in rainfall,” Cook said, explaining that the study points to big change in the total amount of monsoon precipitation, but that the change in timing is still likely to have significant ecological societal impacts. Continue reading

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