Hotter seasons coming earlier, research finds
Harvard News Office
An analysis of global temperatures between 1850 and 2007 has illuminated some climate change details, showing that winter temperatures have risen more rapidly than summer temperatures and that the seasons are coming nearly two days earlier than they were 50 years ago.
Perhaps most worrisome, however, is that none of the dozens of computerized climate models used by the Intergovernmental Panel on Climate Change – the globe-spanning collaboration of scientists that analyzes climate change scenarios – had predicted the earlier seasons.
“We’re talking about over 60 different models and variants that are sometimes interpreted as showing the range of possibilities. It indicates the models are missing some important process,” said Peter Huybers, assistant professor of earth and planetary sciences at Harvard who worked on the research. “There could still be surprises.”
The research, conducted by Huybers and by colleagues at the University of California, Berkeley, Alexander Stine and Inez Fung, was published today in the journal Nature, providing further confirmation that the globe is warming unevenly not only from place to place, but also from month to month.
The study examined global temperature measurements compiled by the University of East Anglia’s Climate Research Unit for variation in the annual temperature cycle. It excluded tropical measurements, since the seasons are weak there.
Monthly temperatures rise and fall along a regular curve, with peak temperatures typically lagging peak solar radiation by about 30 days over land and 60 days over the ocean, because it takes more energy to heat the ocean than it does the land. The researchers found that the seasons are coming an average of 1.7 days earlier over land than they were 50 years ago.
The records also showed that during the last 50 years winter temperatures have risen nearly twice as fast as summer temperatures: 1.8 degrees Celsius in winter compared with a 1 degree Celsius increase in summer temperatures.
“We see 100 years where there is a seemingly 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,” Stine said.
Biologists have also noticed large changes in the arrival time of many signs of spring over the same decades. Buds have been seen to pop open earlier, birds migrate earlier, snow melts earlier, and sea ice breaks up earlier. These changes, however, have been explained simply by the fact that the Earth is warming — the assumption being that the temperature in any given month has increased. In contrast, however, this new study finds that individual months have been warming at different rates, and that as a result, the peak summer temperature and bottom winter temperature now both come earlier in the calendar year.
Researchers examined possible mechanisms for the shifting seasons and found a relationship with a pattern of atmospheric circulation called the Northern Annular Mode, though this seems to explain only part of the shift. They are now looking for other mechanisms, including 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. Huybers said the oceanic temperature patterns were more complex than those of the land and will also be the subject of future research.
“While the two-day shift toward earlier seasons is small compared to year-to-year changes at any one place, the average taken over many years and locations shows a distinct shift in the seasonal cycle by which we set so much of our lives,” Huybers said. “This looks to be yet one more component of climate change.”