In this place, nostalgia roams, patient as slow hands on skin, transparent as melt-water. Nights are light and long. Shadows settle on the shoulders of air. Time steps out of line here, stops to thaw the frozen hearts of icebergs.
Sleep isn't always easy in this place where the sun stays up all night and silence has a voice.
—Claire Beynon, "At Home in Antarctica"
Earth surface temperatures are close to the triple point of water, 273.16 K, the temperature at which water vapor, liquid water, and ice coexist in thermo-dynamic equilibrium. Indeed, water is the only substance on Earth that is found naturally in all three of its phases. Approximately 35% of the world experiences temperatures below the triple point at some time in the year, including about half of Earth's land mass, promoting frozen water at Earth's surface. The global cryosphere encompasses all aspects of this frozen realm, including glaciers and ice sheets, sea ice, lake and river ice, permafrost, seasonal snow, and ice crystals in the atmosphere.
Because temperatures oscillate about the freezing point over much of the Earth, the cryosphere is particularly sensitive to changes in global mean temperature. In a tight coupling that represents one of the strongest feedback systems on the planet, global climate is also directly affected by the state of the cryosphere. Earth temperatures are primarily governed by the net radiation that is available from the Sun. Because solar variability is modest on annual to million-year timescales (less than 1% of the solar constant), the single most dynamic control of net radiation is the global albedo—the planetary reflectivity—which is heavily influenced by the areal extent of snow and ice covering the Earth. The simple but illuminating global climate models of Mikhail Budyko and William Sellers explored this feedback in the late 1960s, demonstrating the delicate balance between Earth's climate and cryosphere.
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