Glaciers around the world store precious freshwater resources, but that is changing. These images of Glacier National Park, Montana, illustrate the effects of global warming over nearly a century. In the top photo, circa 1910, Boulder Glacier is substantial and solid, but the second image, from 2007, shows the glacier nearly gone.

The park once boasted about 150 glaciers but, according to U.S. Geological Survey scientist Daniel Fagre, as of spring 2008 there are fewer than 27 left, with some close to disappearing. “All the glaciers are getting smaller each year,” he said. (1910 Elrond/Glacier National Park Archives; 2007 Fagre&Pederson/USGS)