Welcome to the front lines of global warming in the United States – the Harding Ice Field in Alaska, the biggest icefield in the United States.
   At the Exit Glacier north of Seward – the only glacier in the Kenai Fjords National Park reachable by foot – the giant cerulean blue ice sheet gives every sign of staying put.
   But one only has to glance at the many signs along the roadway and footpath to the glacier’s edge to mark its retreat  – it hit its peak size in 1815 and has been receding ever since. Signs along a footpath leading to the base of the glacier show just how far it has retreated.
   The glacier lost about 10 feet from its front face over the summer of 2008.
   Since the 1980s, land-based glaciers and ice caps like this one in Alaska have contributed the most to sea level rise than any other source within their category, which includes other land-based glaciers like Tanzania’s Mount Kilimanjaro and the Chacaltaya Glacier near La Paz, Bolivia, said Brenda Ekwurzel, a climate expert with the Union of Concerned Scientists.
   Unlike the ice cover around the North Pole or giant floating ice sheets, land-based ice contributes directly to sea-level rises.
   According to a 2007 report by the U.N.’s Intergovernmental Panel on Climate Change, glaciers and ice caps have the potential to raise global sea levels by between .15 meters and .37 meters.
   That pales in comparison to the giant ice sheets in Greenland and Antarctica, which could raise sea levels by 63.9 meters if they fully melted.
   At the Aialik Glacier in the Harding Icefield – reachable by boat or plane, the living nature of the ice was more evident.
   On a visit to the glacier via tourboat on Aug. 15 on a trip hosted by the Knight Center for Environmental Journalism, several chunks of ice broke apart and crashed into Aialik Bay.
   Throughout the visit, the ice cracked and groaned, with a sound like thunder claps that punctuated the still air.