Ice-related items as selected from the Glossary (Annex I of the AR4)

Equilibrium line

The boundary between the region on a glacier where there is a net annual loss of ice mass (ablation area) and that where there is a net annual gain (accumulation area). The altitude of this boundary is referred to as equilibrium line altitude.

Glacier

A mass of land ice that flows downhill under gravity (through internal deformation and/or sliding at the base) and is constrained by internal stress and friction at the base and sides. A glacier is maintained by accumulation of snow at high altitudes, balanced by melting at low altitudes or discharge into the sea. See Equilibrium line; Mass balance.

Ground ice

A general term referring to all types of ice contained in freezing and seasonally frozen ground and permafrost (Van Everdingen, 1998).

Grounding line/zone

The junction between a glacier or ice sheet and ice shelf; the place where ice starts to float.

Ice age

An ice age or glacial period is characterised by a long-term reduction in the temperature of the Earth's climate, resulting in growth of continental ice sheets and mountain glaciers (glaciation).

Ice cap

A dome shaped ice mass, usually covering a highland area, which is considerably smaller in extent than an ice sheet.

Ice core

A cylinder of ice drilled out of a glacier or ice sheet.

Ice sheet

A mass of land ice that is sufficiently deep to cover most of the underlying bedrock topography, so that its shape is mainly determined by its dynamics (the flow of the ice as it deforms internally and/or slides at its base). An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins usually slope more steeply, and most ice is discharged through fast-flowing ice streams or outlet glaciers, in some cases into the sea or into ice shelves floating on the sea. There are only three large ice sheets in the modern world, one on Greenland and two on Antarctica, the East and West Antarctic Ice Sheets, divided by the Transantarctic Mountains. During glacial periods there were others.

Ice shelf

A floating slab of ice of considerable thickness extending from the coast (usually of great horizontal extent with a level or gently sloping surface), often filling embayments in the coastline of the ice sheets. Nearly all ice shelves are in Antarctica, where most of the ice discharged seaward flows into ice shelves.

Ice stream

A stream of ice flowing faster than the surrounding ice sheet. It can be thought of as a glacier flowing between walls of slower-moving ice instead of rock.

Interglacials

The warm periods between ice age glaciations. The previous interglacial, dated approximately from 129 to 116 ka, is referred to as the Last Interglacial (AMS, 2000)

Last Glacial Maximum (LGM)

The Last Glacial Maximum refers to the time of maximum extent of the ice sheets during the last glaciation, approximately 21 ka. This period has been widely studied because the radiative forcings and boundary conditions are relatively well known and because the global cooling during that period is comparable with the projected warming over the 21st century.

Last Interglacial (LIG)

See Interglacial.

Little Ice Age (LIA)

An interval between approximately AD 1400 and 1900 when temperatures in the Northern Hemisphere were generally colder than today's, especially in Europe.

Mass balance (of glaciers, ice caps or ice sheets)

The balance between the mass input to the ice body (accumulation) and the mass loss (ablation, iceberg calving). Mass balance terms include the following:

Specific mass balance:

net mass loss or gain over a hydrological cycle at a point on the surface of a glacier.

Total mass balance (of the glacier):

The specific mass balance spatially integrated over the entire glacier area; the total mass a glacier gains or loses over a hydrological cycle.

Mean specific mass balance:

The total mass balance per unit area of the glacier. If surface is specified (specific surface mass balance, etc.) then ice flow contributions are not considered; otherwise, mass balance includes contributions from ice flow and iceberg calving. The specific surface mass balance is positive in the accumulation area and negative in the ablation area.

Medieval Warm Period (MWP)

An interval between AD 1000 and 1300 in which some Northern Hemisphere regions were warmer than during the Little Ice Age that followed.

Sea ice

Any form of ice found at sea that has originated from the freezing of seawater. Sea ice may be discontinuous pieces (ice floes) moved on the ocean surface by wind and currents (pack ice), or a motionless sheet attached to the coast (land-fast ice). Sea ice less than one year old is called first-year ice. Multi-year ice is sea ice that has survived at least one summer melt season.

Thermokarst

The process by which characteristic landforms result from the thawing of ice-rich permafrost or the melting of massive ground ice (Van Everdingen, 1998).

Reference