A new study has brought to the fore another adverse effect of global warming, suggesting that it has led to Earth's seasons to arrive 2 days earlier than before.
The study, by scientists from the University of California, Berkeley, and Harvard University, has determined that not only has the average global temperature increased in the past 50 years, but the hottest day of the year has shifted nearly two days earlier.
Just as human-generated greenhouse gases appear to the be the cause of global warming, human activity may also be the cause of the shift in the cycle of seasons, according to Alexander R. Stine, a graduate student in UC Berkeley's Department of Earth and Planetary Science and first author of the report.
Stine and his team based their study on a publicly available database of global surface temperature measurements 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 team found that, while land temperatures in the 100-year period between 1850 and 1950 showed a simple pattern of variability, with the hottest day of the year in the Northern Hemisphere around July 21, temperatures in the period 1954-2007 peaked 1.7 days earlier.
Although the cause of this seasonal shift, which has occurred over land, but not the ocean, is unclear, the researchers said that the shift appears to be related, in part, to a particular pattern of winds that also has been changing over the same time period.
This pattern of atmospheric circulation, known as the Northern Annular Mode, is the most important wind pattern for controlling why one winter in the Northern Hemisphere is different from another.
The researchers found that the mode also is important in controlling the arrival of the seasons each year.
According to Stine, whatever the cause, current Intergovernmental Panel on Climate Change (IPCC) models do not predict this phase shift in the annual temperature cycle.
Temperatures at any given time of the year can be very different on land than over the ocean, and a change in the strength and direction of the winds can move a lot of heat from the ocean onto land, which may affect the timing of the seasons, he added.
"However, this seems to be only a partial explanation, because the relationship between this pattern of circulation and the shift in the timing of the seasons is not strong enough to explain the magnitude of the seasonal shift," said Stine.