Spring snowmelt in Alaska's Arctic is occurring progressively earlier, accelerating the region's climate change and helping produce its warmest summers in at least 400 years, according to a new study.

The earlier snowmelt, itself a product of a warming climate, is one of the "positive feedback" factors that accelerate warming in the far north, said Terry Chapin, a professor of ecology at the Institute of Arctic Biology at the University of Alaska Fairbanks.

"Each of these changes seems to trigger other changes that mean more changes will occur," said Chapin, the study's lead author.

The National Science Foundation-funded study published in the online journal Science Express found spring snowmelt had been occurring about 21/2 days earlier per decade, exposing dark ground to solar heat earlier in the season.

Heat absorbed by the ground releases energy into the local atmosphere, about three watts per cubic meter each decade, a change that is heating the local atmosphere and adding incrementally to global warming, Chapin said.

"This heat is added to the atmosphere, so the atmosphere in the north becomes warmer and is mixed with the global atmosphere," Chapin said.

Summer warming will be amplified two to seven times if trees and bushes continue their northern migration into Alaska's Arctic, the study also said.

For now, the tundra is still relatively free of large vegetation, although such plants are starting to colonize more northern areas, said Matthew Sturm of the Army's Cold Regions Research and Engineering Laboratory in Fairbanks, one of the study's co-authors.

"When we look hard, we find that most of the warming that's already taken place can be explained by the reduction in winters," Sturm said. "But we know that the change in vegetation is already underway. That has the potential to become even more of a feedback."

Chapin said the earlier snowmelts could have profound impacts in the Arctic.

"Any kind of ecological or human activity that requires snow or snow-free conditions are bound to be affected by that," he said.

Caribou time their spring migrations so they can arrive at the Arctic coast just as the snow is disappearing and the most tender plants are emerging from the tundra, he said.

If the snow melts too early, the plants could be mature and tough, and snow and ice bridges used to cross rivers could be gone when caribou arrive at their coastal calving grounds, he said.