The number of days that northern New England’s rivers have a substantial amount of ice dropped sharply over the last century, suggesting the region’s winters are growing warmer, according to a new U.S. Geological Survey study.
It found that 12 of 16 rivers in Maine, New Hampshire and Vermont showed a significant fall in the number of days each year when they had enough ice to affect their flow.
The scientists said other evidence -- some compiled by the U.S. Geological Survey and some by other scientists -- shows winters in rural and mountainous northern New England growing warmer but they stopped short of tying this to global warming.
“Questions of the broader impact, the cause of this trend, and whether the warmer climate in New England is linked to global climate change are beyond the scope of these studies,” said Glenn Hodgkins, the study’s lead author.
From 1936 to 2000, the total number of days when ice on the nine longest rivers in northern New England was strong enough to affect the flow of rivers fell by an average of 20 days, the study said.
Robert Dudley, a hydrologist at the U.S. Geological Survey’s Maine Water Science Center, said the biggest change showed up in spring when the last day of significant ice -- known as the “ice-out date” -- was earlier now than in the 1930s.
On average, the ice-out dates were 11 days earlier in 2000 than in 1936, with most of the change occurring since the 1960s, Dudley told Reuters.
He said 12 of the 16 rivers had a significantly earlier spring “ice-out.” In comparison, the first date of substantial ice showed up later at four of the 16 rivers, suggesting autumn was growing warmer though the change was less dramatic than in the spring, the study said.
“Evidence of changes is strong and is consistent with warming temperatures in the late-winter and spring in New England in the last 30 to 40 years,” Hodgkins said.
The study, published in the journal Climatic Change, said near-freezing temperatures in late fall, winter and early spring make New England’s rivers sensitive to small temperature changes and a good barometer for measuring climatic trends.
A 2004 study also by the U.S. Geological Survey showed snowfall in the region had decreased significantly in favor of rain in the last half of the 20th century. A July 2003 study said winter/spring high river flows, which are influenced by snow melt, occurred significantly earlier in the 20th century in northern New England with most of the 1-to-2-week change occurring over the last 30 years.
Other studies have shown earlier last-frost dates and lilac-bloom dates, suggesting milder New England winters.