ablation zone The zone in which mass losses of glaciers or ice sheets by surface melting, runoff, sublimation, wind scour, and calving exceed accumulation.
accumulation zone The zone in which mass gains of glaciers or ice sheets through precipitation (snowfall) and wind drift exceed losses by ablation.
active layer Near-surface layer above permafrost that freezes in winter and thaws in summer.
albedo The fraction (or percent) of solar radiation (or sun’s energy) striking a surface that is reflected by the surface.
Antarctic Circumpolar Current (ACC) A wind-driven surface current that flows west to east around Antarctica.
Anthropocene The period beginning approximately with the Industrial Revolution when humans have increasingly transformed the Earth’s surface. (Note that varying start periods have been proposed, including 1945—the beginning of the Atomic Era.)
Arctic haze Reduced atmospheric visibility, particularly in late winter and spring, caused by the presence of aerosols (tiny liquid or solid particles suspended in air) of either natural or anthropogenic origin.
Arctic Oscillation (AO) A seesaw pattern of atmospheric pressure between the Arctic and midlatitudes, similar to the NAM (Northern Annular Mode) and to the North Atlantic Oscillation (NAO). During the positive phase, low polar latitude pressure and high midlatitude pressure steer midlatitude storms north, increasing rainfall in Alaska and Northern Europe. During the negative phase, the pressure pattern reverses and cold air outbreaks tend to grip North America and Europe.
asthenosphere A weak zone within the Earth’s upper mantle, ~100 to 200 kilometers (62 to 124 miles) beneath the lithosphere, where rocks deform by plastic flow under pressure. It is the source of lava that erupts at the midocean ridges.
astronomical theory of ice ages The theory that ice ages are caused by periodic variations in the Earth’s orbit. Also referred to as the Milankovitch theory, after one of its first proponents. (See also eccentricity; obliquity; precession.)
Atlantic Meriodional Overturning Circulation (AMOC) North-south deep-ocean circulation driven by density differences (temperature, salinity), winds, and tides.
basal melting Increased melting at the base of an ice shelf or ice tongue, generally due to warmer subsurface ocean water.
bergy bit A small iceberg between 5 and 14 meters (15 and 46 feet) long and 1–5 meters (3–13 feet) high.
bipolar seesaw An out-of-phase relationship in millennial climate variability between the Northern and Southern Hemispheres. The cause has been attributed to changes in relative strengths of hemispheric ocean deepwater formation and circulation following massive iceberg discharges.
boreal forest Northern forest south of the tundra zone, characterized by spruce and larch.
brash ice A mix of ice and icebergs less than 2 meters (6.5 feet) across.
brine rejection The process of expelling salt impurities from sea ice as it recrystallizes and matures.
calving The break-off of ice chunks from the edge of a glacier or a floating ice shelf into the ocean or a lake. The broken ice masses are called icebergs.
Circumpolar Deep Water (CDW) Part of a worldwide system of ocean circulation (or “oceanic conveyor belt”) that flows eastward around Antarctica at depths of between 488 and 914 meters (1,600 and 3,000 feet).
cirque A bowl-shaped, steep-sided hollow high on a mountain, carved out by a glacier.
congelation growth (of sea ice) Growth at the base of a newly formed thin sheet of sea ice in which ice crystals grow vertically with their c-axes oriented parallel to the ocean surface.
continental shelf Shallow seaward extension of a continent, with average depths between 130 and 135 meters (430 and 443 feet).
continental slope The steeper slope between the outer edge of the continental shelf and the continental rise, near the base.
crevasse A deep crack in a glacier caused by extensional stresses as ice moves over an uneven surface.
cryosphere The frozen portions of the Earth’s surface including glaciers, ice sheets and caps, ice shelves, permafrost, snow, and sea ice.
diatoms Unicellular algae living in the ocean, coastal lagoons, or freshwater lakes. Sensitive to various environmental parameters such as temperature, salinity, and water chemistry, they are used to reconstruct past climates and sea levels.
drumlin Smooth, streamlined, oval hill consisting of sediments deposited beneath an ice sheet and shaped by its flow; it is blunt and steeper on the up-glacier side and tapered in the downstream direction.
East Antarctic Ice Sheet (EAIS) Part of the Antarctic ice sheet that lies east of the Transantarctic Mountains.
eccentricity The degree of deviation of the Earth’s elliptical orbit from a circle. It varies over roughly 100,000- and 400,000-year cycles.
El Niño-Southern Oscillation (ENSO) A quasiperiodic oscillation of the ocean-atmosphere system in the tropical Pacific manifesting as changes in air pressure, sea surface temperature, and wind and rainfall patterns, recurring approximately every three to seven years.
equilibrium line altitude (ELA) The altitude at which the annual accumulation of snowfall on a glacier is exactly balanced by ablation (see above).
esker Long, sinuous ridge consisting of sands and gravels deposited by glacial meltwater at the edge of an ice sheet or in an ice-walled tunnel.
eustatic sea level rise A global change in sea level due to both addition of water from ice losses and thermal expansion of seawater. However, the magnitude of the sea level change may vary from place to place (see relative sea level rise).
firn Snow that has survived a summer’s melt season; a transitional stage in the conversion of snow to ice.
first-year ice Sea ice that forms in autumn and winter and melts in summer.
fjord Long, narrow, U-shaped valley bounded by steep cliffs and carved by glacial erosion but subsequently filled with seawater.
foraminifera (forams) Single-celled organisms that live in the ocean, brackish water, and lakes. Marine forams can be either planktonic (free-floating) or benthic (bottom-dwelling). Many species have shells of calcite (calcium carbonate). Variations in oxygen isotope ratios in the shells of marine forams are used to infer past sea levels and ocean temperatures.
frazil ice Spongy, slushy aggregate of small, needle- or plate-shaped ice crystals suspended in turbulent seawater.
frost heave Ground movement due to freezing of water.
gas hydrate (methane hydrate or clathrate) A form of ice in which cage-like spaces can accommodate gas molecules, usually methane, or, less commonly, carbon dioxide, or low-molecular-weight hydrocarbons. It is stable only under high pressure and low temperature, and occurs beneath permafrost and buried marine sediments.
geothermal gradient The average rate of increase in temperature with depth in the Earth’s interior.
glacial erratic A boulder or rock transported by a glacier or ice sheet that has been deposited far from its source.
glacial drift Sediments of all sizes transported and deposited by a glacier.
glacial deposits These include eskers, kettles, kames, drumlins, till, etc.
glacial-interglacial cycle A roughly 100,000-year cycle that includes a glacial period of prolonged cold (glaciation) and advancing ice sheets followed by a warm period (interglacial) during which ice sheets retreat.
glacial isostasy Changes in land elevation resulting from an added or decreased load of ice mass.
glacial isostatic adjustment (GIA) Response of the Earth’s lithosphere and upper mantle to changes in ice mass loading/unloading during glacial-interglacial cycles. (Gravitational, rotational, and isostatic changes due to recent mass redistribution between ice sheets and ocean are often referred to as “fingerprinting.”)
glacial lake outburst flood (GLOF) A flood generated when a dam (either rock, sediment, or ice) containing a glacial lake fails. A jökuhlaup is a similar type of flood caused by a subglacial volcanic eruption (as in Iceland) or, by extension, any subglacial flood.
glacier A large mass of ice on land formed by compaction and recrystallization of snow that slowly slides down the slope of a mountain under the pull of gravity. Glacier types include:
cirque glacier A small glacier that forms within a cirque basin, generally high on the side of a mountain.
hanging glacier A glacier that originates high on the wall of a mountain, but descends only part of the way to the main glacier.
outlet glacier A glacier or ice stream emerging from an ice sheet or ice cap through a valley.
piedmont glacier A fan- or lobe-shaped glacier located at the front of a mountain range. It forms when one or more valley glaciers flow from a confined valley onto a plain, where it expands.
tidewater glacier A glacier that reaches the coast, often with a floating extension (ice tongue or shelf). Also called a marine-terminating glacier.
valley glacier A glacier confined for all or most of its length within the walls of a mountain valley. Also called an Alpine or mountain glacier.
greenhouse effect, anthropogenic Theory that atmospheric greenhouse gases (e.g., carbon dioxide, methane, nitrous oxides) generated by fossil fuel combustion, deforestation, and industrial activity are warming the Earth more than the warming due to naturally occurring greenhouse gases such as water vapor and carbon dioxide. These latter gases heat the Earth by 33°C (59°F) above temperatures that would exist in their absence.
grounding line Boundary between a glacier or ice stream resting on land and an attached ice shelf or tongue floating on water.
growler The smallest-size iceberg—under 5 meters (3 feet) long, ~1 meter (39 inches) high.
hydroisostasy Changes in ocean elevation resulting from the addition or loss of water due to changes in the mass of ice sheets.
hydrological cycle Movement of water between ocean and land reservoirs (lakes, rivers, soil, groundwater, glaciers) by means of evaporation and precipitation as rain, snow, or hail.
ice age Period of ice accumulation near the poles. Ice has covered both poles for at least the last 2.8 million years. The Antarctic ice cores record as many as eight interglacial-glacial cycles within the past million or so years during which the ice sheets retreated and expanded.
iceberg A large chunk of drifting ice, of varying shapes and sizes, that has broken off a glacier or ice sheet.
ice cap A large, dome-shaped ice mass covering underlying topography, generally less than 50,000 square kilometers (19,300 square miles) in area.
ice edge Outer margin of densely packed sea ice.
ice floe Floating slab of ice.
ice front Seaward edge of a tidewater glacier or ice shelf.
ice island Flat iceberg calved off ice shelves (especially around northern Ellesmere Island, Canada).
ice lens Mass of buried ice that grows by drawing in water as the ground freezes; found in permafrost regions.
ice mélange (Fr., a mixture, blend) A densely packed mix of icebergs and sea ice at the terminus of many Greenland glaciers. An ice mélange may impede calving rates.
ice-rafted debris (IRD) Sediment or pebbles transported by icebergs and deposited in the ocean.
ice sheet A large, continental-scale ice mass covering an area of over 50,000 square kilometers (19,300 square miles) that spreads outward from a center of accumulation, usually in all directions. The major ice sheets today are in Greenland and Antarctica.
ice shelf Floating slab of ice attached to the shoreline as a marine extension of an ice sheet or glacier.
ice/snow-albedo feedback Fresh ice and snow are bright and highly reflecting (i.e., have high albedo). A reduced summer sea ice cover (or winter/spring snow cover) exposes more dark surface water (or bare ground) that absorbs more of the sun’s incoming energy and raises surface water (or ground) temperatures. This, in turn, triggers more ice/snow melting (see albedo).
ice stream A faster-flowing portion of an ice sheet. Ice streams behave as glaciers and account for much of the ice shed by ice sheets into the ocean.
ice tongue A narrow, floating extension of a tidewater glacier, usually within a fjord or bay.
Intergovernmental Panel on Climate Change (IPCC) United Nations Environment Programme (UNEP) and World Meteorological Organization (WMO)–sponsored international group of scientists and technical experts convened to assess the current state of climate change and its environmental and socioeconomic impacts. The IPCC has issued reports in 1990, 1995, 2001, 2007, and, most recently, in 2013.
interstadial A warm period within a glacial cycle.
isostasy A state of gravitational equilibrium in which the different elevations of continents and oceans largely reflect differences in thickness and density of crustal blocks. Isostatic changes are further caused by loading or unloading of sediments at river deltas, volcanic lava accumulations, or ice masses (see glacial isostasy).
kame A stratified sand and gravel deposit in a hollow near the edge of a glacier; it forms small hills, mounds, knobs, hummocks, or ridges.
katabatic wind A cold, dense wind blowing off ice sheets and glaciers.
keel Submerged portion of an iceberg. (The exposed portion of the iceberg is called a sail.)
kettle A depression formed by melting of a stagnant block of ice buried in glacial deposits.
landfast ice (fast ice) Floating ice that forms and remains attached to the shore, to grounded icebergs, or between shoals.
Last Glacial Maximum The period near the end of the last ice age when ice sheets and glaciers reached their maximum global extent—roughly between 23,000 and 19,000 years ago.
Last Glacial Termination The period following the last ice age, between around 20,000–19,000 years ago, and 7,000 years ago.
last ice age (or glaciation) The cold period following the end of the Last Interglacial, between ~116,000 and 20,000–19,000 years ago.
Last Interglacial The warm period preceding the last ice age lasting from ~130,000 to 116,000 years ago; also called the Eemian (Europe) or Sangamonian (United States).
lead Linear opening in sea ice.
lithosphere The rigid, strong crust and upper mantle above the asthenosphere. It varies in thickness from several to 100 kilometers (62 miles) beneath the ocean and 100 to 150 kilometers (62 to 93 miles) beneath continents.
Little Ice Age A colder period from 1400 to 1850 CE with temperatures slightly below the millennial average, during which time many glaciers advanced.
loess Wind-deposited dust and silt in unglaciated areas, especially in the central United States, northern Europe, Russia, and China during the last ice age.
marine ice sheet instability (MISI) A potential ice instability that is set up where the grounding line, resting below sea level, begins to retreat into a region where the subglacial topography plunges steeply landward. (See grounding line.)
mass balance (of a glacier or ice sheet) The sum of all processes adding to or removing ice from a glacier or ice sheet, including accumulation (gain), ablation, meltwater runoff, calving and ice discharge (losses). When positive, accumulation exceeds mass losses; when negative, ice losses exceed accumulation. (See also ablation; accumulation; surface mass balance.)
meltwater pulses Periods of very rapid sea level rise during the Last Glacial Termination due to major discharges of glacial meltwater.
Medieval Warm Period (MWP) A period of relative warmth between roughly 950 and 1100 CE, when mean Northern Hemisphere temperatures were several tenths of a degree Celsius above the 2,000-year hemispheric average. The medieval warmth has not been exceeded until recent decades.
methane hydrate A form of ice in which methane or carbon dioxide is locked within cage-like crystal structures under pressure at subfreezing temperatures; a potential source of natural gas found beneath Arctic permafrost and in marine sediments.
moraine A ridge of rocky debris deposited by a glacier at its leading edge or along the sides of the glacier valley.
lateral moraine A sediment ridge on the glacier’s edge formed by the accumulation of rock debris from the valley walls.
medial moraine A sediment ridge located toward the middle of a glacier. It forms when the lateral moraines of two glaciers merge.
recessional moraine A ridge of rock and sediment left by a glacier in successive stages as it retreats up a valley.
terminal moraine The moraine marking the outermost advance of a glacier or ice sheet.
moulin (Fr., mill) A tubular channel through a glacier or ice sheet formed by meltwater.
nilas ice A thin, elastic crust of ice that freezes at the sea surface. Young ice, 10–30 centimeters (3.9–11.8 inches) thick, represents a transitional stage between nilas and first-year ice (typically 0.30–2 meters [11.8 inches to 6.6 feet] thick), which forms during the winter season.
nor’easter An extratropical cyclone, most active between November and March, that affects the Atlantic Coast of North America.
North Atlantic Deep Water (NADW) A branch of the Atlantic Meridional Overturning Circulation (AMOC) where warm, northward-flowing surface currents become cold, dense, saline water that sinks and helps drive ocean circulation (the latter is sometimes called the global ocean conveyor system).
North Atlantic Oscillation (NAO) Climate variability in the North Atlantic region related to the atmospheric pressure difference between Iceland and Lisbon, Portugal (or, alternatively, the Azores). The Arctic Oscillation (AO) and the Northern Annular Mode (NAM) are similar types of climate oscillation.
nunatak Mountain peak or ridge that protrudes above an ice sheet or glacier.
obliquity The angle between the Earth’s rotation axis and the perpendicular to the ecliptic (the plane of the Earth’s orbit around the sun). It is presently 23.4°, but varies between 22° and 25° over ~41,000 years. This angle affects the contrast between seasons.
ogive An arc-shaped, convex, down-glacier-pointing series of waves, or alternating dark and light banding, on the surface of a glacier.
outwash plain A broad, low-sloped plain composed of sediment deposited by glacial meltwater at the margin of a glacier. Typically, the sediment becomes finer grained with increasing distance from the glacier edge.
oxygen isotope variations Changes in the proportions of 18O and 16O in marine carbonate shells due mainly to changes in ice volume and ocean temperatures; also affected by changes in ocean chemistry or by biological activity.
pack ice An area containing high concentrations of sea ice.
pancake ice A type of sea ice that forms in turbulent water as frazil ice fragments repeatedly collide and adhere into pancake-shaped masses with upturned rims. (See frazil ice.)
patterned ground Surface features found in landscapes affected by frost action. These include circles, polygonal networks, and stepped or striped ground on sloped surfaces.
perennial ice Sea ice that survives at least one summer season.
periglacial Relating to landforms and processes associated with present-day cold climates; these landforms are also found at the edges of formerly glaciated terrain.
permafrost Frozen ground where temperatures remain below 0°C (32°F) for two consecutive winters and the intervening summer. Continuous permafrost underlies over 90 percent of an area; discontinuous permafrost underlies between 50 and 90 percent of an area.
pingo Ice-cored, dome-like hill found in permafrost regions.
polynya Area of large and persistent open water within the pack ice.
precession of the equinoxes, axial A wobble in the Earth’s rotational axis, which varies over a ~26,000-year period, caused by the force of the moon and sun on the Earth’s equatorial bulge.
precession, elliptical The change in timing of perihelion with respect to the seasons at the Earth’s closest approach to the sun, which varies over a ~22,000-year cycle. Today, perihelion nearly coincides with the Northern Hemisphere winter solstice; 11,000 years ago it coincided with the Northern Hemisphere summer solstice.
pressure ridge Ridge formed by compression as slabs of sea ice collide.
reference ellipsoid A flattened sphere approximating the Earth’s shape that includes the equatorial bulge and polar flattening caused by rotation. It is used as a reference surface for measuring sea level.
relative (local) sea level rise Geographic differences in the rate of sea level rise caused by ongoing glacial isostatic adjustments, tectonic movements, subsurface fluid extraction, or seasonal to interannual oceanographic processes (e.g., ENSO), in addition to climate-related sea level rise.
roche moutonnée Asymmetric rock outcropping sculpted by glacial erosion. It is smoothly streamlined on the side of the oncoming glacier, shattered and jagged on the lee flank.
sea ice Slabs of ice floating on the surface of the sea.
sea ice area Area occupied by sea ice per cell. For a given sector or an entire hemisphere, it is the sum of the area of each cell times its fractional ice concentration.
sea ice concentration The relative amount of a given area covered by sea ice, expressed as a percentage.
sea level rise equivalent The rise in sea level, in millimeters, due to a given loss in ice mass. Conversion factor: 362.5 GT ice = 1 millimeter of global sea level rise, assumed to be evenly spread over the oceans. Units: one GT (one billion metric tons, or one trillion kilograms) = 1 cubic kilometer of freshwater, or 1.1 cubic kilometer of ice.
steric changes Ocean water density and ocean height changes due to variations in temperature and/or salinity.
storm surge An increase in water level above that of the astronomical tide caused by high winds and low barometric pressure associated with a storm.
sublimation The transformation of ice directly into water vapor without passing through the liquid state.
surface mass balance (of a glacier, ice sheet) (SMB) The net balance between accumulation and surface ice mass loss. When positive, accumulation (snowfall) exceeds mass loss (surface melting plus SUBLIMATION [ice to water vapor] + runoff). When negative, ice loss exceeds accumulation. (See also mass balance.)
taiga Subarctic forest south of the tundra, dominated by conifers (mostly pines, spruces, and larch) and birch. Synonymous with boreal (northern) forest.
talik Unfrozen lens or zone within permafrost.
thermal expansion Increase in ocean water level due to a rise in water temperature.
thermal inertia A measure of the ability of a material to conduct and store heat. It denotes the ability of the material to store heat during the day and reradiate it at night. Materials that have a high heat capacity, such as water, also generally have a high thermal inertia, showing only minor temperature changes during the diurnal cycle.
thermokarst Landforms in permafrost terrain characterized by depressions resulting from thawing of ground ice.
tidewater glacier See under glaciers.
till A heterogeneous mixture of different-sized materials deposited by a glacier.
tundra A treeless, mossy, shrubby plain in Arctic and sub-Arctic regions underlain by permafrost.
utilidor An above-ground, covered utility conduit used in Arctic communities where permafrost prevents burial of water and sewer pipes.
West Antarctic Ice Sheet (WAIS) Part of the Antarctic ice sheet that lies west of the Transantarctic Mountains. It is potentially unstable because much of the ice sits below sea level and slopes down inland. (See marine ice sheet instability [MISI].)
yedoma A type of permafrost that is carbon rich (around 2–5 percent carbon) with an ice content of 50 to 90 percent by volume.