Seasons arriving two days earlier, study says

ByABC News
January 23, 2009, 3:09 AM

— -- 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 also 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 nontropical 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.