Science Wiki

The cryosphere is the collective term for the portions of the Earth system where water is in solid form. It includes ice sheets, glaciers, ice caps, seasonal snow cover, sea ice, lake and river ice, and frozen ground, including permafrost. The cryosphere is studied within Earth system science and is one of the five components of the climate system, alongside the atmosphere, hydrosphere, lithosphere, and biosphere.

Although frozen water covers a limited fraction of the planet, the cryosphere strongly influences climate, sea level, and ocean circulation. Ice and snow reflect sunlight, store large amounts of freshwater, insulate the underlying ground and ocean, and respond sensitively to changes in temperature and precipitation. Changes in the extent and mass of ice are therefore widely used as indicators of climate change.

The cryosphere is divided into components that differ in physical properties, residence times, and dynamical behavior.

• Ice sheets and glaciers: Ice sheets are continental-scale masses of land ice, currently found only in Antarctica and Greenland. Together they hold about 68 percent of the world's freshwater and more than 99 percent of the freshwater that is not stored as ice or in groundwater is a small remainder, so the ice sheets are by far the largest reservoir of freshwater ice. The Antarctic ice sheet contains enough ice to raise global mean sea level by roughly 58 meters if it melted completely, and the Greenland ice sheet by roughly 7 meters. Ice sheets form where snowfall exceeds melt over long periods, so that snow compacts into firn and then into glacial ice, which flows under its own weight toward the margins. Mountain glaciers are much smaller in total volume but are distributed across most mountain ranges and many high-latitude regions, and they supply seasonal meltwater to many river basins.
• Sea ice: Frozen seawater that floats on the surface of the polar oceans, including the Arctic Ocean and the Southern Ocean. Sea ice forms from saline water, retains small pockets of concentrated brine, and typically has a salinity of about 5 to 10 practical salinity units when first-year, decreasing as brine drains out over time. It grows in winter and melts in summer, and it is classified as first-year or multiyear ice depending on whether it has survived a melt season. Because it is already floating, the melting of sea ice has a negligible direct effect on sea level, in accordance with Archimedes' principle, with only a very small contribution arising from the density difference between fresh meltwater and seawater. Sea ice nevertheless strongly affects polar albedo, heat exchange between ocean and atmosphere, and the density structure of the upper ocean.
• Permafrost: Ground, including soil, sediment, and rock, that remains at or below 0 °C for at least two consecutive years. Permafrost underlies roughly a quarter of the exposed land area of the Northern Hemisphere, with extensive coverage in Siberia, Canada, Alaska, and the Tibetan Plateau. The seasonally thawed layer above it is called the active layer. Permafrost stores a large quantity of organic carbon, estimated at roughly 1,400 to 1,600 petagrams (Pg) of carbon in soils, which is about twice the amount currently in the atmosphere. Much of this organic matter was frozen before it could fully decompose.
• Snow cover: A seasonal layer of frozen precipitation that covers large areas of land in winter. Snow cover affects surface temperature through its high albedo and low thermal conductivity, and its melt supplies spring runoff to rivers in the hydrosphere. Snow cover is the most extensive component of the cryosphere in the Northern Hemisphere in winter, reaching roughly 45 to 50 million square kilometers at its maximum.
• Lake and river ice: Freshwater ice that forms seasonally in cold regions. The timing of freeze-up and break-up is a sensitive indicator of regional temperature trends.

Mass balance

The health of a glacier or ice sheet is described by its mass balance, which is the net result of mass gained and lost over a period of time. Mass is gained by accumulation, mainly snowfall, and lost by ablation, which includes surface melt, sublimation, and calving of icebergs into the ocean. For a glacier or ice sheet, the net balance is:

where ΔM is the change in mass, A is accumulation, M is the mass lost by melt and sublimation, and C is the mass lost by calving and ice discharge. A positive value indicates growth, and a negative value indicates net loss. The region where accumulation exceeds ablation is the accumulation zone, the region where ablation exceeds accumulation is the ablation zone, and the two are separated by the equilibrium line. For ice sheets, mass change is routinely measured by satellite gravimetry (for example, with the GRACE and GRACE-FO missions), satellite altimetry, and the input-output method, which compares snowfall and discharge across the grounding line.

A primary way in which the cryosphere influences climate is through its high albedo. Albedo is the fraction of incoming shortwave radiation that a surface reflects:

Fresh snow can have an albedo of about 0.8 to 0.9, and bare sea ice about 0.5 to 0.7, whereas open ocean water has an albedo of only about 0.06. Snow that is aged, wet, or contaminated with black carbon or dust has a lower albedo and absorbs more radiation.

This property underlies the ice-albedo feedback, a positive (amplifying) feedback in the climate system:

1. An initial warming, driven for example by rising concentrations of greenhouse gases such as carbon dioxide and methane, causes snow and ice to melt.
2. Melting exposes darker land or ocean surfaces with a much lower albedo, which absorb a larger share of incoming solar radiation.
3. The increased absorption warms the surface further, promoting additional melt and amplifying the initial warming.

Additional processes strengthen this feedback. Meltwater ponds on sea ice and glacier surfaces lower the albedo, and the growth of dark microbial communities and the deposition of light-absorbing particles on snow and ice can accelerate melt. Snow and ice also insulate the underlying surface, so a reduction in sea ice increases the transfer of heat from the ocean to the atmosphere in autumn and winter.

The cryosphere is closely coupled with the global water cycle and with ocean circulation.

• Sea level: The loss of mass from land ice, which includes glaciers and the Greenland and Antarctic ice sheets, transfers water from the land to the ocean and raises global mean sea level. Ice loss from glaciers and ice sheets, together with the thermal expansion of seawater, accounts for most of the observed sea level rise since the late twentieth century. Contributions from the two ice sheets have increased over recent decades. Melting of floating ice shelves does not directly raise sea level, but it reduces the buttressing that ice shelves provide to inland ice and can speed up the flow of grounded ice into the ocean.
• Ocean circulation and salinity: When sea ice forms, it rejects most of its salt into the underlying water in a process called brine rejection. This produces cold, dense, saline water that can sink and contribute to the formation of Antarctic bottom water and, in the North Atlantic, to dense water formation that drives the thermohaline circulation, including the Atlantic meridional overturning circulation (AMOC). A large increase in freshwater input from melting ice sheets and glaciers can reduce surface water density and has the potential to weaken this overturning circulation. Observational and modeling studies indicate that the AMOC has been relatively stable or has weakened modestly over recent decades, and the degree and timing of any future decline remain active areas of research.
• Freshwater supply: Mountain glaciers and seasonal snowpack act as natural reservoirs that store winter precipitation and release it as meltwater in spring and summer. Regions that depend heavily on this storage include the Indus, Ganges, and Brahmaputra basins fed by the Hindu Kush Himalaya, and the Andes. As glaciers shrink, river discharge initially increases, a stage sometimes called "peak water", and later declines once the ice mass is depleted.

Under the effects of human-caused global warming, much of the cryosphere is in decline. The Arctic has warmed at a rate roughly two to four times the global average over recent decades, a phenomenon known as Arctic amplification. Its causes include the ice-albedo feedback, changes in atmospheric and oceanic heat transport, and the lapse rate feedback. Arctic summer sea ice extent has declined by roughly 12 percent per decade since satellite records began in 1979, and the oldest and thickest multiyear ice has been reduced substantially. Antarctic sea ice has shown more variable behavior, with record low extents in recent years.

Glaciers worldwide have lost mass in nearly every region, and the rate of loss has accelerated since the 1990s. Glacier retreat affects water supply, hydropower, agriculture, and the risk of hazards such as glacial lake outburst floods for communities downstream. The Greenland ice sheet and the West Antarctic ice sheet are losing mass, and some marine-based sectors of Antarctica, such as the Thwaites Glacier, are subject to concerns about marine ice sheet instability, in which retreat of the grounding line on a bed that deepens inland can lead to self-sustaining retreat.

Permafrost is warming and thawing across much of the Arctic. Thaw allows microbes to decompose previously frozen organic matter and release carbon dioxide and methane, a process called the permafrost carbon feedback. Thaw can also cause ground subsidence and the formation of thermokarst landforms, with damage to buildings and infrastructure. The magnitude of the future carbon release is uncertain, and published estimates vary with the emission scenario and with how abrupt thaw processes are represented. Most assessments indicate that permafrost emissions will add to anthropogenic warming but are unlikely to dominate it on a century timescale.

The cryosphere includes elements that are candidates for tipping points, such as the loss of the Greenland ice sheet and the West Antarctic ice sheet, abrupt Arctic permafrost thaw, and changes to the AMOC. For some of these elements, the temperature thresholds and the timescales on which they would take effect are uncertain.