A partial explanation may be changes in a dominant atmospheric pattern called the northern annular mode, which governs factors such as the direction and intensity of the wind, study author Stine noted.
Other contributing factors could include drier soils, which would increase the planet's absorption of heat from the sun. Wetter soils take longer to heat up than drier soils, he explained.
Research suggests that some regions of the world are drying out due to climate change. If true, Stine said, that means that the 30-day lag between the sun's heat and the Earth's response might be reduced.
"Essentially what you are seeing is the Earth is putting up less resistance to what the sun is trying to make it do," he said.
Increasing soot and other particulate matter may also be heating up the atmosphere more quickly. "That may effectively change the timing of the energy that's coming into the surface and that could give you a shift toward early seasons," Stine said.
According to Pennsylvania State's Mann, another factor may be climate-induced changes to the ocean-atmosphere system that causes El Niño and La Niña. These phenomena are governed by surface temperature fluctuations in the Pacific Ocean and can affect weather worldwide.
What troubles Stine about the observed trends is their poor representation in state-of-the art climate models, which help predict future trends.
"Something is wrong with the modeled physics," he said. "And we don't know what the implications are of having that physics missing."
For example, determining why the seasons are arriving earlier and integrating that data into existing models could make the models more accurate.
Mann said the lack of the representation does little, if anything, to alter the basic storyline of a warming planet, "but it does potentially help us think about some of the more subtle aspects of climate change."
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