Unlike Rocky Mountain National Park, which is located in a region of high human population growth, Glacier National Park is situated in a large and virtually pristine area of wilderness. Here, at the headwaters of three major continental river systems, grizzly populations are more dense than anywhere else in the conterminous United States, and air and water quality are among the best in the nation. The park is known for its wildflower meadows and abundant wildlife. An extensive network of hiking trails combined with spectacular scenery attracts large numbers of vistors in the summer. Because of its largely pristine condition, the area serves as an important yardstick of climate change in a landscape that is still largely untouched by human influences. Research indicates that the park has not escaped the change brought by global warming and that even more extensive change can be expected in the near future. As befits its name, Glacier is home to approximately 50 glaciers, which because of their small size are highly sensitive to even slight climate changes. During the past century, systematic measurement of some of the larger glaciers has shown decreases in area by more than half. The park's glaciers, including Grinnell, Harrison, and Sperry, started to retreat around 1850, and the rate of recession increased dramatically during a period of dry years with warm summers in the 1920s and 1930s. The period after 1850 saw sustained warming as much of the world came out of the Little Ice Age it had been experiencing. Separating human-induced warming from the natural rebound after the Little Ice Age has been a particularly knotty problem for climatologists. The number of glaciers in the park has dropped from an estimated 150 in 1850 to approximately 50 today (Fagre pers. com.). Since 1968, as the warming trend has worsened, and the human influence on it been more sharply defined, many of the smaller glaciers have disappeared entirely. Park scientists predict that all the glaciers of Glacier National Park will be eliminated by 2030 at current rates of warming (Fagre in press). The loss of glaciers and reduction of summer snowpacks will increase stream temperatures and reduce water volumes in park watersheds. these changes will adversely affect the current aquatic biota, including valued trout species.

Detailed projections of further climate change impacts have focused on the 46,200-ha Lake McDonald watershed, a part of the Flathead River catchment (White and Running 1994). Observations show that water temperatures influence the distribution of stream invertebrates in the McDonald basin. A variety of species of the cadddisfly family, for example, dominate or are replaced by others at different elevations along a stream's length. Warming temperatures would likely alter the invertebrate fauna of these streams, thereby affecting the food web that supports amphibians, fish and waterfowl. Some high alpine invertebrate species could be lost completely if water temperatures rise too high, or if reduced water availability results in intermittent flow of streams (Fagre et al. in press). Coupled forest succession and hydrological models indicate the possibility of widespread changes in vegetation communities, including eventual shifts from forests dominated by lodgepole pine and western red cedar to those dominated by western hemlock and spruce (Fagre in press). Additional changes include invasion of alpine meadows by subalpine fir, nutrient stresses at the tree line, and greater potential for intense wildfire in the valley forests. Recent research shows that forest of the lower slopes and bottomland on the eastern side of the Continental Divide may be particularly sensitive to warming temperatures (White et al. in press). These forests consist largely of deciduous species such as paper birch, black cottonwood, and aspen, and vegetation and fire modelling for Glacier National Park show that smoke levels could double in the park under certain future climate scenarios, with consequences for air quality and visibility (Keane et al. in press).