WEDNESDAY, Jan. 21, 2009 (HealthDay News) -- If it seems that flower buds are popping open earlier in the spring, and birds are migrating sooner, new research may explain why.
Not only have average worldwide temperatures been rising for the last 50 years, but the hottest day of the year has shifted almost two days earlier.
That's the conclusion of a new study by scientists at the University of California, Berkeley, and Harvard University that was published in the Jan. 22 issue of the journal Nature.
While greenhouse gases seem to be the cause of global warming, human activity may also be prompting this shift in the seasons, Alexander R. Stine, a graduate student in UC Berkeley's department of earth and planetary science and first author of the report, said in a university news release.
"We see 100 years where there is a very natural pattern of variability, and then we see a large departure from that pattern at the same time as global mean temperatures start increasing, which makes us suspect that there's a human role here," he said.
Although it's not clear what might be behind this seasonal shift -- which has only been seen over land masses, but not the oceans -- the researchers said the trend may partly owe to a shift in a certain pattern of winds that also has been changing over the same period of time. This wind circulation -- called the Northern Annular Mode -- helps determine why one winter in the Northern Hemisphere is different from another. These winds also play a key role in controlling the arrival of the seasons each year, the study authors said.
For their study, the scientists relied on a publicly available database of global surface temperatures over both land and ocean from 1850 to 2007 that was compiled by the University of East Anglia's Climate Research Unit in the United Kingdom. Using non-tropical data only, the researchers found that, while land temperatures in the 100-year period between 1850 and 1950 showed a simple pattern of variability, temperatures in the period 1954-2007 peaked 1.7 days earlier.
Biologists have noticed many changes in the arrival time of spring over the past 50 years, such as buds opening sooner, birds migrating earlier, and sea ice breaking up earlier. While some scientists thought these changes were due to global warming, the new study found that individual months have been warming at different rates than others. That means the peak summer temperature and lowest winter temperature both now arrive earlier each year, the news release said.
"We're saying that, on top of the long-term trend of warmer summer and winter highs, peak warming is coming earlier within the year," Inez Fung, co-director of the Berkeley Institute of the Environment, said in the news release. "It's not just the onset of spring, but the peak."
More information
To learn more about climate change, visit the U.S. Department of Environmental Protection.
