Out to dry

Natural History, Feb, 2007 by Graciela Flores

Lakes in Alaska are vanishing, and the most probable culprit is--you guessed it--global warming. A trio of ecologists led by Brian Riordan at the University of Alaska Fairbanks analyzed aerial images from the past half-century to track changes in the surface areas of the lakes in nine regions throughout the state.

The investigators spatially aligned digitized aerial photographs from the 1950s, infrared aerial photographs taken between 1978 and 1982, and digital satellite images taken between 1999 and 2002, then manually outlined each lake in the images. The result is a meticulous inventory of more than 10,000 lakes. The investigators then estimated the change in the number of lakes and the area of their surface waters. They also compiled meteorological data for each of the nine regions.

Since the 1950s, they discovered, the total surface area of the lake water in eight of the nine study regions shrank by 4 to 31 percent. What's more, the total number of lakes in all nine regions declined by 5 to 54 percent. And mean annual temperatures increased significantly.

The shrinkage could be caused by any of several effects of rising temperatures, the ecologists argue: increased water loss through evaporation, increased transpiration by nearby vegetation during the longer, warmer growing seasons, or increased drainage into the surrounding soil as the permafrost thaws. In any case, the phenomenon may be a first sign of more widespread changes to come. (Journal of Geophysical Research)