seasonal changes in snow and ice cover on the Earth's surface result largely from the Earth's position relative to the sun. However, because ice systems also respond to climate on longer timescales, they are considered good indicators of the Earth's climate as a whole. In addition to being good climatic indicators, glaciers and ice sheets influence the Earth's climate in two important ways. The first is their potential impact on sea-level rise and global water resources under declining ice regimes. Glaciers and ice sheets account for two-thirds of the Earth's freshwater and would contribute approximately 229 ft. (70 m.) to sea-level rise if they were to melt completely.
The majority of this ice is located in the Earth's polar regions, largely in the Greenland and Antarctic ice sheets, which combined account for approximately 97.5 percent of potential sea-level rise. Alternatively, glaciers located outside of the polar regions, commonly referred to as alpine glaciers, play a relatively small role in their overall contribution to global ice volume and, thus, sea-level rise. However, they are perhaps the most critical ice volumes when considering their locations with respect to populated areas. Alpine glaciers are found at the headwaters of river systems throughout the globe and, thus, are an important water source for many regions. These glaciers are also more susceptible to melt owing to their smaller size; as a result these systems have experienced the most visible decreases in ice volume.
Second, decreases in global ice and snow cover are considered particularly important to the Earth's climate system because such changes are considered to be part of a positive feedback loop, commonly referred to as the snow-ice albedo feedback. Under this scenario, as temperatures increase, the extent of snow and ice decreases, and the highly reflective snow or ice surface is replaced with the darker (more absorptive) ocean or land surface underlying it. The increased absorption of energy completes the feedback loop, as more solar radiation is absorbed by the Earth's surface, leading to an overall increase in air and sea surface temperatures, and further decreases in ice and snow volume.
Global air temperatures have increased in the past century. Climate observations indicate that increases in temperature over this timeframe have accelerated in recent decades, and are unlikely to slow under current climatic conditions. One key example of the positive feedback associated with the loss of ice mass is the increased rate of warming observed in the Arctic, which has warmed at nearly twice the global average. Global temperature increases are strongly correlated with decreases in global ice mass, which have been in a relatively constant state of decline throughout the last century. As global temperature rates increase, global ice volumes have experienced the largest rates of decline in recent decades.
Observations of global ice masses are a critical component of climate change research. They give scientists insight into how ice masses are changing over time, how quickly changes are taking place, and allow climate scientists to make informed predictions of how these ice masses are likely to change in the future. Improvements in technology have changed the way in which climate scientists make observations of glaciers, ice sheets, and sea ice. New techniques now include the use of satellite and remote-sensing data as a means of monitoring these systems.
Prior to the 19th century, records of weather and climate were rare. As a result, climate scientists use proxy records (such as ice cores, lake and ocean sediments, and ocean coral) to make inferences about how the Earth's climate and ice masses have changed in the past. The current understanding of changes to the Earth's cryosphere (snow and ice) is comprised, therefore, of a combination of data derived from proxy records (indirect observations) and from direct ice observations. Combined, direct and indirect observations provide insight into how global ice masses are changing over time and provide a context in which to view current changes in global ice masses.
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