SAN FRANCISCO, CALIFORNIA--A treasure trove of old photographs document how much Alaska's vast and frigid landscape has thawed in the past century. Nearly all of the state's glaciers have retreated, leaving lakes or rocky debris in valleys once filled with jagged ice. Thick shrubs cover slopes of formerly bare tundra. Geologists attribute both trends to a pronounced warming of Alaska's Arctic climate.
In recent decades, satellites and aerial surveys have shown that Alaska's glaciers are waning. But it's the thousands of black-and-white photos taken by naturalists, the U.S. National Park Service, and other agencies, dating back to 1883,that have allowed scientists to establish an earlier baseline. Researchers scouring these archives have returned to the exact sites of hundreds of photos to take new images.
The comparisons are striking, geologists reported here 16 December at a meeting of the American Geophysical Union. Of a thousand measured glaciers, only 15 are still growing, says geologist Bruce Molnia of the U.S. Geological Survey in Reston, Virginia. The rest are stagnant or melting dramatically, thinned by hundreds of meters and retreating kilometers upward along mountain slopes. Former fields of ice are now filled by ground-up rock, called moraine, or meltwater lakes. At many sites, meadows and thickets of trees have taken over. "As the glaciers have disappeared, we're getting an amazing vegetative succession," Molnia says.
The clear culprit is a general warming of at least 1.5°C in the 20th century, says glaciologist Matt Nolan of the University of Alaska, Fairbanks (UAF). "These are really simple glaciers. There's no other way to explain the retreats except for changing climate," Nolan says. His team is studying whether snowfall—-which feeds the high-altitude sources of glaciers—-has declined as well.
The treeless tundra of the state's North Slope also has changed markedly, notes UAF geologist Kenneth Tape. Tape and coworkers replicated hundreds of detailed helicopter photos of the tundra, taken in the 1940s for oil exploration, and the new images show an "explosive increase" in shrubs of alder, willow, and birch at 93% of the sites. The dense ground cover constricts stream channels and alters the basic nature of the world's major high-latitude ecosystem, Tape says.