Some Antarctic ice has stayed frozen for close to a million years
A snowflake landing today may melt within months, but water locked in glaciers and ice sheets can stay frozen for tens of thousands of years, and deep ice in parts of East Antarctica approaches a million years old. Together this frozen realm, the cryosphere, covers roughly a tenth of Earth's surface and is shrinking fast.
The cryosphere takes in every place where water is solid: snow cover, lake and river ice, sea ice, glaciers, ice caps, ice sheets and frozen ground including permafrost. The name joins Greek kryos, cold or frost, with sphaira, a ball. It counts as one of five parts of the climate system, alongside the atmosphere, hydrosphere, lithosphere and biosphere, and its study draws on geology, hydrology, meteorology and climatology, much like glaciology.
Ice matters to climate because of how it handles energy. Three properties dominate: albedo, the share of sunlight reflected; thermal diffusivity, how readily heat moves through it; and latent heat, the energy absorbed or released when it changes state. Fresh snow that is not melting reflects roughly 80 to 90 percent of incoming sunlight, except where forests poke through. That creates sharp swings in reflectivity every spring and autumn at high latitudes, although clouds blunt the effect; the Arctic Ocean is usually cloudy in summer and autumn, so the feedback from shrinking sea ice is much reduced. Snow has its strongest effect on the planet's energy balance in April and May, when sunshine on snowy ground peaks.
Timescales differ enormously. Snow and freshwater ice are seasonal, and most sea ice lasts a few years at most, outside the central Arctic. Glacier, ice sheet and ground ice can persist from ten years to a hundred thousand or longer. Most of the world's ice by volume sits in Antarctica, mainly the East Antarctic Ice Sheet, but by area the Northern Hemisphere's winter snow and ice win, covering about 23 percent of that hemisphere in January.
Observing all this from space depends on further properties such as surface roughness, which largely controls how strongly radar bounces back. The shrinkage now under way, attributed to climate change, shows up as melting ice sheets, retreating glaciers, declining sea ice, thawing permafrost and thinner snow cover. Scientists call the retreat of these features deglaciation.
Source: Cryosphere
