Despite winters like this year's, which has brought weeks-long periods of frigid air and dumped multiple rounds of snow across the eastern United States, the start of spring is slowly but surely moving earlier on the calendar with each passing year, proof that Earth's warming is leaving its footprint on a wide range of climatic signals.
"It has been a brutal winter" across the eastern half of the U.S., where temperatures have averaged 5 to 7 degrees below normal for the season, said Dr. Michelle Thaller, a scientist with NASA's Goddard Space Flight Center, in an interview Thursday.
"But if you actually look at a global perspective, it's been a warmer-than-average winter," she said. "In the western U.S. and across Europe, it's been quite a warm winter. It's a very good demonstration of the difference between weather locally and long-terms trends in climate."
Since the 1950s, the date of winter's last frost – which usually occurs sometime in mid- to late March – has been creeping farther north, and now occurs about five days earlier than it used to across most of the U.S.
That has left places known for a particular climate feeling a lot more like their counterparts much further south, according to Dr. Compton Tucker, a senior scientist at NASA.
"The analogy I use is ... as if Winnipeg moved to Minneapolis-St. Paul over 50 years," he said, noting a Canadian and American city separated by more than 450 miles. "The onset of spring in Winnipeg now occurs like the onset of springs in Minneapolis-St. Paul in the 1950s."
A pair of recent studies bolsters this claim. Last year, a study in the journal Nature Climate Change found that temperature seasonality – the difference between summer and winter temperatures – is growing narrower over time, especially in areas above the U.S.-Canadian border.
And another study published in 2012 found that the experiments researchers use to mimic the effect of warmer temperatures on plants – which are used to forecast how plants will respond to future climate change – actually under-predict real-world warming by a large margin.
How do we see that unfold on the ground? Plants leaf out and flower earlier than they used to, snowpack melts sooner than it used to, birds migrate at different times, and insects like the bark beetle can invade forests for longer periods, allowing them to do more damage.
"There are parasitic insects, the more frost-free days you have, the more they can take hold," said Dr. Thaller. And because warmer temperatures are melting snowpack sooner in the year, she added, the ground dries out later in the summer – which makes for optimal conditions for wildfires.
"We may be looking at a bad wildfire season in the American West," she said.
“The case of spring coming earlier, this is in fact a long-term trend that we’ve observed," she added. "There are far-ranging consequences, even to a small shift in climate."Follow @terrellwrites
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