In the depths of winter, it may provide some comfort to think that summer will be here earlier than usual. But so will next winter.
In fact, the arrivals of all seasons have been sped up by nearly two days, according to new research, part of a worldwide trend that scientists say is tied to climate change.
Not only are temperatures rising, but the hottest and coldest days of the year are falling ever earlier in the calendar, a trend that accelerates from the late 1970s onward.
The research, conducted by scientists from the University of California at Berkeley and Harvard University, is published in the latest edition of the scientific journal Nature.
"We see a shift in the timing of the seasons, a shift which is different from the one you would expect from warming alone," said the study's lead author, Alexander Stine. "In addition to everything warming, there's also a shift to things happening earlier. So the highest temperature is occurring earlier in the year, the lowest temperature is occurring earlier in the year, and both of those are occurring at higher temperatures."
The scientists studied more than 100 years of climate data, and found no similar trend in preceding eras.
"It suggests that something important has changed about the world," Mr. Stine said.
The causes are difficult to identify, but Mr. Stine said human activity is likely to blame.
"We're not just increasing the amount of carbon dioxide in the atmosphere, we're also putting aerosols, soot from factories in the air, and these change where energy is absorbed in the atmosphere, versus the Earth's surface, and that can affect the timing of the seasons."
More significant, he argues, is that none of the 97 climate models produced by the Intergovernmental Panel on Climate Change predict this trend.
"That concerns us. These are the models we're using to make our projections on how the Earth will warm."
The research indicates that the hottest day of the year typically fell 30 days after the summer solstice, around July 21, and that the coldest day of the year typically fell 30 days after the winter solstice.
But, according to data from regions outside the tropics, the hottest and coldest days of the year are now on average 1.7 days earlier than they were in the period up to 1954.
It's possible that a drying climate with less moisture in the soil may require less time to heat up, but it's a hypothesis that needs further study, Mr. Stine said.
Also among the scientists' findings was that temperatures are warming faster in winter than in summer.
David Thomson, a mathematician who studies climate at Queen's University and wrote a companion piece for Nature, said the advance of the seasons has been noted by bird watchers everywhere.
"If you look at when birds or butterflies show up in the spring, you see big shifts. Amateur bird watchers, they all get excited whenever they see their first favourite bird, and a lot of those have shifted a lot all over the world," Prof. Thomson said.
"It's basically more evidence that we're breaking things we don't understand very well."
