Spring Coming Earlier, Study Says

John Roach
for National Geographic News
January 21, 2009
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].)

Earth Physics

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.

In this case, the researchers examined how quickly the Earth responds to the sun.

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.

A partial explanation may be changes in a dominant atmospheric pattern called the northern annular mode, which governs factors such as the direction and intensity of the wind, study author Stine noted.

Other contributing factors could include drier soils, which would increase the planet's absorption of heat from the sun. Wetter soils take longer to heat up than drier soils, he explained.

Research suggests that some regions of the world are drying out due to climate change. If true, Stine said, that means that the 30-day lag between the sun's heat and the Earth's response might be reduced.

"Essentially what you are seeing is the Earth is putting up less resistance to what the sun is trying to make it do," he said.

Increasing soot and other particulate matter may also be heating up the atmosphere more quickly. "That may effectively change the timing of the energy that's coming into the surface and that could give you a shift toward early seasons," Stine said.

According to Pennsylvania State's Mann, another factor may be climate-induced changes to the ocean-atmosphere system that causes El Niño and La Niña. These phenomena are governed by surface temperature fluctuations in the Pacific Ocean and can affect weather worldwide.

Missing Physics

What troubles Stine about the observed trends is their poor representation in state-of-the art climate models, which help predict future trends.

"Something is wrong with the modeled physics," he said. "And we don't know what the implications are of having that physics missing."

For example, determining why the seasons are arriving earlier and integrating that data into existing models could make the models more accurate.

Mann said the lack of the representation does little, if anything, to alter the basic storyline of a warming planet, "but it does potentially help us think about some of the more subtle aspects of climate change."

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