Spring sprung early in 2012, breaking records across the lower 48 states, a new study finds.
In some places across the central and eastern United States, plants started greening up 20 to 30 days ahead of historical averages that year, the research found. But that early greening wasn't necessarily good: Early spring bloomers were susceptible to late frosts, said study researcher Julio Betancourt, a geoscientist at the U.S. Geological Survey. Michigan alone lost $500 million in fruit from frost-damaged trees.
"It happened all over," Betancourt told LiveScience.
The start of spring
March 2012 was already known to be unusually warm. The month broke or tied more than 7,000 temperature records across the United States. Betancourt and his colleagues examined a different measure: the spring index.
The spring index is a measure of vegetation's spring revival, based on observations stretching back to 1900. Many of these observations are spotty, but since the 1950s, scientists, agriculturalists and citizen observers have collected budding and blooming data on lilacs and honeysuckles.
These plants are handy indicators of spring, for a few reasons. First of all, they are grown in yards and watered by people, so their responses can generally be attributed to temperatures, not to water or lack thereof. Lilacs and honeysuckles also track well with the blooming of other plants, from Anjou pears in Oregon to peaches and dogwoods in South Carolina, Betancourt said. [ 6 Signs That Spring Has Sprung ]
Finally, lilacs and honeysuckles reproduce by cloning, meaning that changing genetics don't influence observations over time.
"You're basically looking at the same individuals at different sites over large areas across the country," Betancourt said.
In areas where there are no lilac or honeysuckle observations, researchers can use computer models to determine how greening up would be going if the plants were nearby, as long as the researchers know daily minimum and maximum temperatures. Combined with satellite data showing the "green wave" of vegetation emerging in the spring, the spring index can measure spring's beginning each year. (By the calendar, spring begins in late March as the spring equinox. But meteorological spring, the quarter of the year between the coldest quarter, winter, and the warmest quarter, summer, varies in onset.)
A record year
Spring 2012 was an early riser, the researchers reported May 14 in the journal Eos. The season came particularly early to the western Great Lakes region and the northeastern United States, where leaves budded more than 20 days earlier than average. (The greenery was cut short starting in June, when a drought set in that turned green into brown throughout the western Great Lakes region and the Corn Belt across the country's midsection.)
"It's been a fairly strange time, and the variance is really high," Betancourt said. "We had this big heat wave and drought in Texas in the summer of 2011, and then we had this unusual record-early spring and record-setting temperatures in March of 2012. Then, that was followed by drought throughout the West and in the Plains. Now, here we are again, and this March was actually an unusually cold spring in the Southeast across to Europe."
The early spring in 2012 was prompted by a dome of high pressure that migrated off the Atlantic onto the eastern United States in late December 2011 and early January 2012, and just stayed put, Betancourt said. This year's cooler spring in the Southeast and Europe is caused by changes in atmospheric patterns all the way up in the Arctic, which result in a "wavy" jet stream injecting cool air southward, he said.
And what is the role of climate change in all this weather weirdness? Betancourt isn't sure.
"It looks to me like there's convincing evidence that events like that can easily be spun out of natural variability," he said. On the other hand, the regional variations seen across the country are probably the result of both natural climate variability and the atmospheric changes caused by climate change, he said.
"Trying to separate whether or not environmental change is caused by natural processes or human impact is one of the most difficult questions — really difficult to disentangle," Betancourt said. "I think we'll continue to struggle with it."