Cryosphere Elements

Snow  forms when the moisture in clouds that have air temperatures below 0˚C (32˚F) freeze into ice crystals that stick together. When these get heavy enough, the snow falls as precipitation. Snow has many different forms, including snowflakes, graupel and sleet.

Once fallen, snow can be classified by type and formation. Firn, slush, cornice and sastrugi are a few common examples. Firn, sometimes referred to as névé, is snow that has a high density and has existed for more than a year. Slush refers to fallen snow that has mixed with water. Sastrugi and cornices are are common where there are strong winds. A cornice is an overhanging accumulation of wind-blown snow and ice (usually found on a ridge or cliff face). Sastrugi are sharp, irregularly formed ridges on a snow surface, parallel with the wind direction, that can grow many metres long and as much as a metre high.

Terrestrial snow has the largest geographic extent of the cryosphere components. It covers nearly 50 million km² of the Northern Hemisphere in winter, affecting heavily populated mid-latitude regions as well as higher latitudes. The high albedo of snow reduces solar energy absorption and promotes lower surface temperatures. Snow insulates the land surface from large energy losses in winter and reduces the severity of soil frost. Snow smooths the land surface, reducing wind resistance and modifying energy exchanges with the atmosphere. These interactions strongly influence the land surface energy budget, with local and regional effects on atmospheric circulation.

The high sensitivity of snow to changes in temperature and precipitation makes it a primary indicator of climate change and implicates it in climate change hypotheses concerning the redistribution and acceleration of the water cycle.

Solid precipitation originates in clouds where air temperatures are below 0˚C (32˚F). Solid precipitation has a variety of forms including snow, snow grains, snow pellets, diamond dust, hail and ice pellets. Snow is an aggregate of ice crystals. Ice Crystal Precipitation is made up of small ice crystals that float with the wind and fall very slowly. It is also called "diamond dust". Snow grains: Precipitation of very small opaque white particles of ice which fall from a cloud and which are fairly flat or elongated. Snow pellets are white and opaque ice particles that fall from a cloud. These particles are generally conical or rounded. The term "graupel" is also used. Hail is precipitation of either transparent or partly or completely opaque particles of ice (hailstones), which can be spheroidal, conical or irregular in form.

Sea ice is formed at the sea surface by the freezing of seawater, which occurs at a lower temperature than pure water due to its salinity (~1.8˚C vs 0˚C, respectively). In contrast to icebergs, glaciers, ice sheets and ice shelves, sea ice is formed and melts entirely in the ocean. Sea ice is relatively thin, ranging from a few centimetres to a few metres in thickness.

Sea ice is categorized by age, thickness and form. In terms of age, sea ice is most commonly divided into first-year and multiyear ice. Multiyear ice has survived one or more melt seasons. Other age and thickness categories include new ice (less than 5cm), nilas (5~10cm), grey (10~15cm), grey-white (15~30cm), thin first-year (30~70cm), medium first-year (70~120cm) and thick first-year (120~180cm).

Frazil ice is the first stage of sea ice growth, where crystals or plates of ice are suspended in the water. Grease ice is frazil ice that has coagulated to form a "greasy" or soupy layer on the surface without distinct ice floes. Other sea ice forms include pack ice (relatively uniform ice cover over a large area), floes (an individual piece of ice at least 20 m in diameter), pancake ice (pieces less than 3 m in diameter), drift ice (pack ice in concentrations less than six-tenths) and fast ice (attached to the shore, also called landfast ice). Melt ponds are a common response to summer melting of the sea-ice surface. Warm temperatures also contribute to the formation of polynyi (singular; polynya), which are nonlinear-shaped openings in sea ice that may be open water or covered with new, thin ice. Leads are a feature that are similar to polynya but are formed by dynamics, not temperature, as sea ice pulls apart and creates long channels. Pressure also contributes to the formation of ridges and hummocks. Ridges form when sea ice is haphazardly piled one piece over another, and hummocks are ridges that have been smoothed by melting and other forms of erosion.

Sea ice limits exchanges of heat and moisture between the ocean and the atmosphere, acting as a “thermal blanket”. Because of the large difference in reflectivity between ice (bright) and ocean (dark), a reduction in the extent of sea ice will result in more heat being absorbed by the ocean instead of reflected back into the atmosphere. This is likely to amplify the effect of warming in high latitudes, making the extent of sea ice a potential early and sensitive indicator of climate change. Sea ice also redistributes salt and freshwater, as it rejects brine when it freezes. Brine rejection in turn produces saline, cold, dense water in convective regions and polynyas.

The seasonal sea-ice zone is highly productive biologically, which makes sea ice a key component in the carbon cycle. Fast ice provides important habitat for wildlife, and the shear zone between fast and drifting ice frequently results in open water in these regions, to the benefit of the biota.

Lake and river ice forms on the surface of freshwater bodies. Lake and river ice play a key role in the physical, biological and chemical processes of cold region freshwater. The presence of freshwater ice also has economic ramifications as it impacts, for example on transportation (ice-road duration, open-water shipping season) and the occurrence and severity of ice-jam flooding that often causes serious damage to infrastructure and property.

In the arctic and sub-arctic regions of the Northern Hemisphere, lakes are a major component of the terrestrial landscape. Estimates of their areal coverage range from 15% to 40% depending on location. River ice is also one of the major components of the cryosphere. It affects an extensive portion of the global hydrologic system, particularly in the Northern Hemisphere where ice cover develops on 29% of total river length and seasonal ice affects 58%. River-ice duration and break-up exerts significant control on the timing and magnitude of extreme hydrologic events such as low flows and floods.

Seasonal ice cover grows and decays in response to heat transfer through the ice surface layer that is affected by net radiation, surface albedo, on-ice snow depth and density, air temperature, wind speed and water heat flux. Although freshwater ice formation and decay processes are influenced by numerous physical and climatological factors, it has been determined that the timing of break-up and freeze-up correlates best with air temperature during the preceding weeks to months of the event.

Permafrost, defined as sub-surface earth materials that remain at or below 0˚C continuously for two or more years, is widespread in the Arctic, sub-Arctic and high-mountain regions as well as in the ice-free areas of the Antarctic and sub-Antarctic. An area can have continuous, discontinuous, sporadic or isolated permafrost, but the only metric used to truly identify an area of permafrost is temperature. In the Northern Hemisphere permafrost regions cover approximately 23 million km², an area nearly 2.5 times that of the continental United States. Actual land area underlain by permafrost occupies between 12 and 17 million km². The uppermost layer of seasonal thawing is termed the “active layer”. Seasonally frozen ground combined with intermittently frozen ground occupies approximately 54.4106 km², or 57% of the exposed land areas of the Northern Hemisphere and includes the active layer over permafrost and soils outside the permafrost regions.

Permafrost and its processes have a significant impact on the land it covers. In turn, the land covered by permafrost has a significant impact on the thickness and extent of permafrost. Soil type, plants and other vegetation, snow, slope of the ground, lakes and rivers, and an area’s climate all influence permafrost conditions. Pingos and ice wedge polygons are two notable formations that occur in the presence of permafrost. A pingo is earth-covered ice in the shape of a mound and is usually about the size of a small hill; most small pingos have rounded tops, while larger pingos may have breaks in their tops, or even small freshwater lakes if a crater was formed. Ice wedge polygons begin as a small crack in the soil of permafrost. As overlying snow melts, water fills in the crack, and expands as it freezes. The cracks grow in size until they begin to push surrounding soil up, creating polygons. Eventually, these ice wedges can create pingos, after very wet soil buried below the active layer is compressed to the point at which it pushes upwards.

Some regions that were covered with permafrost are warming in the Arctic, allowing soil to thaw and causing damage to infrastructure that was built entirely on frozen ground. It is not uncommon for buildings or houses to be moved or rebuilt because of changes in the ground beneath them, particularly those located along coastlines. Where sea ice and permafrost used to work to protect the shore, warming has reduced both fast ice (ice connected to the shore) and permafrost extent, sometimes resulting in chunks of land being washed away to sea. Many communities that live in regions affected by seasonally frozen ground are aware of the stress placed on roadways and other infrastructure by the freezing and thawing of soil. Frost heaves are a good example. When water trapped underground freezes, it expands, pushing surrounding material upwards. This force is powerful enough to create hummocks, or raised areas of ground, beneath roads and other structures. Once the soil thaws, the underlying material is disrupted, often times causing potholes and large cracks in pavement. Man-made objects are not the only features affected by thawing ground. Drunken forests, or forests where trees lean at odd angles, are common in areas where permafrost has recently transitioned into only seasonally frozen ground.

Recent studies have begun to look at the tons of carbon dioxide, methane and other greenhouse gasses that are buried within permafrost. The current estimate on the amount of carbon stored in permafrost is almost twice what is currently in the Earth’s atmosphere. Yedoma, a type of permafrost that has existed since the Pleistocene period and is found mostly around Russia and Siberia, contains huge amounts of carbon that could be released into the atmosphere if it were to thaw.

Glaciers are formed over long periods of time by snow that is compressed into ice, gaining mass through snowfall and sediment deposition. Glaciers need perennial snow cover and below-freezing temperatures to retain mass, and therefore are found predominately at higher latitudes but also in high mountain ranges at lower latitudes, such as the Himalayas and Andes. When mass gain (snow accumulation) in the glacier’s accumulation area outpaces mass loss (ablation) in the glacier’s ablation area, the glacier is “surging” forward; vice versa, and the glacier is “retreating”.

The two most well-recognized types of glaciers differ greatly in appearance: piedmont glaciers  form on steep valleys that flows into a flat plain, producing a bulb-like lobe, and tidewater glaciers terminate in the sea. However, other glacier types, such as mountain glaciers and hanging glaciers, can be more difficult to differentiate. Several prominent features are common on all glaciers. Crevasses, or giant cracks that form as the glacier flows, can be found on all types of glaciers. Moraines are long, coloured stripes of rock and debris that are left behind by glaciers as they retreat.

Despite their massive size, glaciers are mobile; their sheer mass, coupled with ablation or accumulation, translates into motion that is usually described in metres per year. Some glaciers may hardly move over the course of a decade, while others, such as the Kutiah Glacier in Pakistan, have been recorded to surge as much as 110 metres (360 feet) per day.

Glaciers occupy about 10% of the Earth’s surface and can be found in a range of sizes, from as small as 100 metres to as large as hundreds of kilometres (km) in length. Ice caps, found on many of the Canadian and Eurasian Arctic Islands, are dome-shaped ice masses with radial flow. Ice fields, found in Alaska and the southern Andes, are characterized by ice thickness that is insufficient to obscure the subsurface topography. Average glacier thickness can be estimated at about half of the surface width of the glacier.

In many mountain ranges, glaciers provide a significant portion of runoff during the dry season, and so are vital sources for drinking water, irrigation and industry. China, India and other Asian countries, as well as the South American Andes, are facing critical glacier loss with hundreds of millions of people affected across those regions.

Glaciers and glacial environments are sensitive indicators of climate change, and, in several mountain regions, important components of the hydrological cycle. Though glaciers and ice caps account for only 0.5% of the total land ice, their contribution to sea level rise during the last century exceeded that of the ice sheets. For the last three decades, the vast majority of glaciers worldwide have been retreating.

An ice sheet is a continental-scale body of ice that flows under its own weight towards the ocean. Ice sheets form over thousands of years as snow accumulates from year to year, gaining mass and compressing older layers into ice. Two major ice sheets remaining from the last ice age blanket most of Greenland and Antarctica. They contain enough ice to raise sea level by 7.2 and 58 m, respectively, if melted (Aschwanden et al., 2019, Morlinghem et al., 2020). The amount of freshwater stored in both the Greenland and Antarctic ice sheets totals over 68% of all freshwater on Earth and over 99% of freshwater ice.

The inland ice rests on bedrock. In Antarctica, it is estimated that the sheer weight of the ice sheet has sunk the underlying ground to approximately 2.4 km below sea level. Ice is carried from the inland ice sheet to the connected ice shelves via fast flowing ice streams and outlet glaciers that breach the mountainous barriers surrounding Greenland and Antarctica. Ice sheets are thickest near the ice divides, where the thickness exceeds 4 km in East Antarctica and 3 km in Greenland, and thinnest at the ice fronts where they can be as thin as 200 m.

The shape, extent, and volume of the ice sheets are controlled largely by the balance between the amount of snow added to the surface, meltwater runoff at the margin, the rate of ice flow in the ice streams, and the amount of ice lost from the ice shelves through melting at their bases and iceberg calving from their fronts. In Antarctica, iceberg calving is the largest source of mass loss. A small amount of mass is also lost through surface melting, and some subglacial water is known to reach the ocean though the exact amount is not known. While large iceberg calving events are necessary for maintaining the mass balance of the ice sheet, the concern is that these events might become more frequent in response to atmospheric and oceanic warming, thus tipping the system out of mass balance. In Greenland, the contribution from surface melting and runoff is comparable to calving and basal melting at the fronts of marine terminating outlet glaciers.

Ice sheets are important archives of past climates on Earth. Ice cores collected from the ice sheets provide detailed information about past climate and environmental conditions on timescales from seasons to decades to hundreds of millennia, depending on the rate of accumulation. They are the only means of studying how closely climate and greenhouse gas concentrations were linked in the past and of demonstrating that very abrupt climate changes can occur.

References

Aschwanden, A., Fahnestock, M.A., Truffer, M., Brinkerhoff, D.J., Hock, R., Khroulev, C., Mottram, R. and Khan, S.A., 2019. Contribution of the Greenland Ice Sheet to sea level over the next millennium. Science advances, 5(6), p.eaav9396.

Morlighem, M., Rignot, E., Binder, T., Blankenship, D., Drews, R., Eagles, G., Eisen, O., Ferraccioli, F., Forsberg, R., Fretwell, P. and Goel, V., 2020. Deep glacial troughs and stabilizing ridges unveiled beneath the margins of the Antarctic ice sheet. Nature Geoscience, 13(2), pp.132-137. 

An ice shelf is a permanent piece of floating ice connected to a landmass. The world’s two remaining ice sheets, in Antarctica and Greenland, both have ice shelves. A few well-known shelves dot the northern hemisphere along Canada’s coast, but the majority are found along the coast of Antarctica. Most ice shelves form along coastlines that protect them from warm seawaters and destructive winds. Because of this, ice shelves can survive for thousands of years and grow into massive structures.

Ice shelves maintain their size and stability by gaining and losing mass periodically. When an iceberg falls off an ice shelf, mass is lost, and the process is called “calving”. Calving is a type of ablation. Ice shelves grow when they gain ice from land, usually from glaciers or ice streams.

Rising global temperatures have already had a noticeable impact on the health of ice shelves and the glaciers that feed them. Ice shelves do not contribute directly to sea level rise because they already float on the ocean surface. However, warmer temperatures take their toll on the waters surrounding shelves, leading to less protection from high winds and warm water. Once a shelf breaks up or breaks off of the coast, the dynamics that controlled the source (like a glacier) are disrupted. Without the backpressure of the ice shelf to slow movement, the glaciers feeding the shelves accelerate more rapidly, leading to more frequent and larger calving events, and in turn raising the global sea level.

Icebergs form from calving glaciers and ice sheets. Icebergs are found all around Antarctica and can drift considerably farther north than sea ice. In the Arctic, large numbers are found around Greenland, throughout Baffin Bay and southward along the east coast of Canada as far as the Gulf Stream; they are found less frequently elsewhere in the Arctic. The size of icebergs range from small “bergy bits” the size of a piano to massive table bergs hundreds of square kilometres in area and tens of metres high above the waterline. Large icebergs like these can significantly affect oceanographic conditions through fresh water inputs from melting. Smaller icebergs pose significant dangers to navigation.