A US researcher says the frost-free season across North America is now 10 days longer than it was a century ago, mainly because of altered atmospheric circulation patterns and, to a lesser extent, global warming.
“If you ask a U.S. forecaster what determines the first fall frost, they’ll say a cold air mass coming down out of Canada, clearly due to circulation,” said University of Utah atmospheric scientist Court Strong. “There’s a role for warming, but on the other hand, forecasters will tell you there’s clearly a role for circulation as well.”
Of the 10 additional days that North America is now frost-free, Strong says only three can be directly attributed to a warmer climate. But other scientists have said that global warming has actually altered atmospheric circulation patterns.