WASHINGTON Looking forward to spring? The good news is that it is coming two days earlier on average, but so are summer, autumn and winter, researchers said on Wednesday.
They found that on average, the hottest day of the year in temperate regions has moved forward by just under two days, and so has the coldest day of the year.
While the consequences of this shift are not clear, it is worrying, Alexander Stine of the University of California, Berkeley and colleagues said.
"All of the seasons are coming earlier. They are both hotter and they are earlier," Stine said in a telephone interview.
The effect can be seen in both the northern and southern hemispheres, said Stine, whose team studied more than 100 years of temperature data to tease out the pattern.
Writing in the journal Nature, Stine and colleagues said the effect is related to global warming and is very likely to be caused by humans.
The shift in seasons is unsettling because none of the climate change computer models predict this, Stine said.
"There are certain things that we expect from global warming and there are certain things that we don't," he said.
"You expect that, say, the ice is going to melt a little earlier ... and you expect the ice is going to form a little bit later in the year. But what you don't expect to see is for the hottest day of the year to be earlier."
Stine's team found that land temperatures between 1850 and 1950 showed a simple pattern of variability, with the hottest day of the year in the Northern Hemisphere around July 21. Between 1954 and 2007, temperatures peaked 1.7 days earlier.
Peak temperatures come about a month after the solstices -- when the sun's rays hit the Earth more or less indirectly, causing summer and winter. Stine said it takes time for the rays to heat up the land in the summer, and for things to cool off during the period of indirect sunlight.
This pattern suggests the planet has lost something that helps draw out the process.
"The land is putting up less resistance to what the sun is telling it to do," Stine said.
There may be a loss of moisture in the soil, or some other factor like pollution, he said.
"If the way the Earth is responding to the sun is changing, we'd like to know that," Stine said. "There is a concern that we may be missing some important processes."
The finding fits in with other research that shows spring begins earlier in certain areas of Britain, for instance, and that growing seasons have shifted forward.
(Reporting by Maggie Fox; Editing by Julie Steenhuysen)