Alaskan lakes are drying up, signal of warming climate

Between 1950 and 2002, more than 10,000 Alaskan lakes have shrunk in size or completely dried up, according to a newly published study. Over this period, Alaska has experienced a warming climate with longer growing seasons, increased thawing of permafrost, and greater water loss due to evaporation from open water and transpiration from vegetation; yet there has been no substantial change in precipitation.

Three scientists at the University of Alaska Fairbanks' Bonanza Creek Long-Term Ecological Research Program studied 50 years of remotely sensed imagery and conclude that these landscape- level changes in Arctic ponds are associated with recent climate warming in Alaska and may have profound effects on climate and wildlife.

The shrinking of these closed-basin ponds may be indicative of widespread lowering of the water table throughout low-lying landscapes in Interior Alaska, write the authors. A lowered water table negatively affects the ability of wetlands to regulate climate, because it enhances the release of carbon dioxide by exposing carbon in the soil to aerobic decomposition. Their report appeared 10 October in the Journal of Geophysical Research– Biogeosciences.

“No one has done a state water-body inventory of this magnitude, said Brian Riordan, lead author of the study. “It will allow land managers to stop speculating about possible water body loss and begin to address the implications of this loss.”

“Alaska is important in terms of waterfowl production, and if you have a lowering of the water table, that could have a potentially huge impact on waterfowl production,” said David Verbyla, a co- author of the study.

“This is an issue relevant to flyway management, in terms of all the ducks that might use the Yukon Flats National Wildlife Refuge and overwinter elsewhere, and this is something that goes beyond the refuges in Alaska,” said David McGuire, the third member the research team.

National Wildlife Refuges cover more than 31 million hectares [77 million acres] in Alaska comprising 81 percent of the national refuge system. These refuges provide breeding habitat for millions of migratory waterfowl and shorebirds that overwinter in more southerly regions of North America.

Using black and white aerial photographs from the 1950s, color infrared aerial photographs from 1978-1982, and digital Landsat satellite images from 1999-2002, Riordan outlined each pond by hand. “With automated classification, your accuracy goes down,” he said.

Cloud shadows can look like water, and Alaska rarely experiences a cloudless day, said Verbyla. The most difficult part of the four- year project, said Riordan, was “having the patience to circle 10,000 ponds for each time period.”

The main study area was the subarctic boreal region of Interior Alaska, which spans more than five million square kilometers [two million square miles] bounded on the north by the Brooks Range and on the south by the Alaska Range. To contrast the semi-arid, subarctic sites of discontinuous permafrost in Interior Alaska, the authors also selected study areas in the Arctic Coastal Plain, where the temperatures are much colder, the growing season much shorter, and the permafrost is continuous, as well as a more maritime site south of the Alaska Range.

All ponds in the study regions in subarctic Alaska showed a reduction in area of between four and 31 percent, with most of the change occurring since the 1970s. The ponds in the Arctic Coastal Plain showed negligible change.

The research project was funded in part by NASA, the National Science Foundation, and the U.S. Department of Agriculture.