SMALL BUT steady increases in sea level, the gradual march of disease-bearing mosquitoes into areas once too cold for them - most of the effects of global warming are barely visible. But the shrinking of mountain glaciers in places like Alaska and the Alps makes a more arresting picture. "Double Exposure," an exhibit at the Museum of Science through June 22, juxtaposes 15 glacier photos from decades ago with matching shots from 2005 to show the dramatic retreat of the ice because of global warming.
The original aerial photos were the work of the explorer and founding director of the museum, the late Brad Washburn. A few years ago, David Arnold, a former reporter and designer for the Globe, decided to retrace Washburn's work to demonstrate how much ice has already been lost as global temperatures rise.
As closely as he could, Arnold took his shots in the same month of the year and from the same altitude and angle as Washburn. The results make for a stark illustration of what happens as the level of carbon dioxide in the atmosphere rises and traps solar heat close to the Earth's surface.
While Washburn took some of his photos in the 1930s, one of the most telling comparisons is the one between his 1960 shot of the Matterhorn in the Alps and Arnold's 2005 photo.
In the more recent photo, it looks as though a huge windshield scraper has simply stripped the ice off the mountain.
In remarks at the exhibit's opening reception last week, Arnold noted that the Alaskan coastal town of Yakutat now has surfing. One recent summer its water temperature reached 67 degrees, about the same as Chatham in mid-July, according to Arnold.
Alaskan surfing may be a global warming plus. A great minus is the loss of glacial ice in places like Asia and South America, where spring melt from the mountains supplies water for much of the world's people. The exhibit sounds an urgent alarm.
