An ice sheet is a large mass of ice that covers land. The two main types of ice sheets are continental and alpine. Continental ice sheets are found on Greenland and Antarctica. They are very thick, sometimes more than 3 kilometers (2 miles) thick, and cover a large area. Alpine glaciers are found in high mountain ranges around the world, such as the Alps, Rocky Mountains, Andes, and Himalayas. Most alpine glaciers are much thinner than continental ice sheets—usually only about 30 to 100 meters (100 to 300 feet) thick—and they cover a smaller area.
Ice sheets form when snow accumulates faster than it can melt or evaporate. Over time, the layers of snow compact into dense glacial ice. Ice sheets grow slowly at first, but their growth rate can increase over time. For example, during periods of global cooling like the last Ice Age (which ended around 11,700 years ago), growth rates were much higher than they are today because there was less melting occurring.
Continental ice sheets can take millions of years to form fully; however, they can also shrink relatively quickly if conditions become warmer and there is more melting than accumulation taking place. For example, during the last century alone Greenland lost about 9 percent of its total ice volume due to increased melting from climate change .
Alpine glaciers respond more quickly to changes in temperature because they’re generally found at lower elevations where it’s warmer overall. As a result, they often advance or retreat depending on whether temperatures rise or fall over time .
While individual glaciers within an ice sheet may advance or retreat depending on changing conditions in their local environment , the overall size of an entire ice sheet will only change significantly if there is a major shift in regional or global climate .
Climate change is causing both polar regions to warm at an accelerated pace , which is resulting in increased melting of both continental and alpine glaciers around the world . If this trend continues unabated , it could have major implications for global sea level rise , freshwater resources , ecosystems , and human populations that live near coasts or depend on glacier-fed rivers .