The effects of global warming are already visible in many areas of the world. For example, in Montana's Glacier National Park, where about 150 glaciers were once located, only 25 glaciers larger than 25 acres remain, according to the U.S. Geological Survey (USGS). A commonly accepted guideline for identifying a glacier — a body of snow and ice that moves — is that the object must be about 101,000 square meters or about 25 acres in size. Below this size, the ice is generally stagnant and does not move, unless it is on a steep slope.