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, according to a new study by scientists from the University of California, Berkeley, and Harvard University.
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.
“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 the cause of this seasonal shift – which has occurred over land, but not the ocean – is unclear, the researchers say 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.
Whatever the cause, Stine said, current Intergovernmental Panel on Climate Change (IPCC) models do not predict this phase shift in the annual temperature cycle.
Details are published in the Jan. 22 issue of the journal Nature.
Temperatures at any given time of the year can be very different on land than over the ocean, Stine said, 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. However, this seems to be only a partial explanation, he said, 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.
The researchers also found that the difference between summer and winter land temperatures has decreased over the same 50-year period, with winter temperatures warming more than those in summer. They found that in non-tropical regions, winter temperatures over land warmed by 1.8 degrees Celsius and summer temperatures increased by 1 degree. Ocean warming has been somewhat less.