for National Geographic News
Tired of winter? Good news: Spring arrives an average of 1.7 days earlier now than it did in the first half of the 20th century, according to a new study.
Summer, fall, and winter are also starting 1.7 days earlier. And there is less of a temperature difference between winter and summer.
The shifts, which are occurring over land (seasonal shifts are different over oceans), appear to stem from as-yet-to-be determined changes in the physics of the Earth's climate system, said Alexander Stine, a graduate student of Earth and planetary sciences at the University of California, Berkeley, and lead author of the study.
The shifts occur at the same time humans are known to be influencing the climate. (Read up on global warming.)
"The Earth is certainly warming," said Stine, whose findings will be published in tomorrow's issue of the journal Nature. "It would seem reasonable that there's some relationship."
(Related: "Warming Sign? Another Early Spring for Rocky Mountains" [April 9, 2007].)
Other research has documented the earlier arrival of spring based on biological factors, such as when plants flower and animals begin to migrate, but the new results are based on physics.
The planet's tilt toward the sun defines its seasons. On land, there's about a 30-day lag between the sun at its maximum intensity and when the Earth is warmest. That's because it takes the Earth those extra 30 days to soak up all the sun's energy.
The study found that the Earth is responding faster, shifting all of the seasons.
Michael Mann directs the Earth System Science Center at Pennsylvania State University, and was not involved in the study. He said the findings are consistent with similar observations he reported more than a decade ago, though what's causing the earlier seasons remains poorly understood.
SOURCES AND RELATED WEB SITES