The average temperature on Earth has increased in the past 50 years and the hottest day of the year now comes almost two days earlier than it did prior to 1954, according to a study by scientists from the University of California, Berkeley, and Harvard University.

The study is based 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.

Researchers found that the difference between summer and winter land temperatures has decreased over the past half century, with winter temperatures warming more than those in summer. In non-tropical regions, winter temperatures over land warmed by 1.8 C and summer temperatures increased by 1 C. Ocean warming has been somewhat less.

The study also 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 coming around July 21, temperatures in the period 1954-2007 peaked 1.7 days earlier.

The study limited its focus to non-tropical regions because the seasons are more pronounced outside the tropics.

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 lead author of the study.

Human influence suspected

"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 it 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.

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 research team is now looking for other mechanisms to explain the observed shift in the timing of the seasons. These include a hypothesized drying of the global soils, which would cause the land surface to respond more quickly to the sun, and changes in the amount of solar energy absorbed by the atmosphere due to industrial pollution.

The study was carried out by Stine, Peter Huybers, assistant professor of earth and planetary sciences at Harvard University, and Inez Fung, UC Berkeley professor of earth and planetary science and of environmental science, policy and management, and co-director of the Berkeley Institute of the Environment.

Details are published in the Jan. 22 issue of the journal Nature.