Key Terms

📖 Key Terms

permafrost — Ground that remains permanently frozen (below 0°C) for at least two consecutive years. It underlies approximately 25% of Northern Hemisphere land. The active layer above thaws seasonally; permafrost beneath stays frozen year-round. Permafrost acts as an impermeable layer, causing waterlogging above it in summer.

active layer — The uppermost layer of soil above permafrost that thaws each summer and refreezes each winter. Typically 0.5–3 metres deep. Plant roots and soil organisms operate within the active layer only.

albedo — The proportion of incoming solar radiation reflected by a surface. Fresh snow and ice have an albedo of 0.80–0.90 (80–90% reflected). Open ocean has an albedo of about 0.06 (absorbs most radiation). High albedo is a key reason polar regions remain cold despite receiving summer sunlight.

ice-albedo feedback — A positive feedback loop in which cold temperatures maintain ice cover, which reflects solar radiation (high albedo), which prevents warming, which maintains the ice. When ice melts, it exposes dark ocean or land (low albedo), which absorbs more solar radiation, which warms the surface, which melts more ice. This feedback loop is responsible for the rapid acceleration of polar warming once it begins.

katabatic wind — A cold, dense wind that flows downslope from an elevated, cold surface. In Antarctica, cold air above the ice plateau sinks and accelerates down the ice slopes towards the coast, reaching speeds of up to 200 km/h. Katabatic winds are responsible for some of the most extreme wind conditions on Earth.

ice sheet — A thick mass of glacial ice covering a large land area (continental ice). Ice sheets rest on bedrock and are formed from compacted snowfall over thousands of years. The Antarctic Ice Sheet (26.5 million km³) and Greenland Ice Sheet are the world's only current ice sheets. Distinguished from sea ice, which floats on the ocean.

sea ice — Frozen ocean water, forming seasonally on the Arctic Ocean (and to a lesser extent around Antarctica). Sea ice is typically 2–4 m thick and forms when ocean surface temperature drops below −1.8°C. Arctic sea ice expands to ~15 million km² in winter and contracts to ~4–7 million km² in summer. Critical habitat for polar bears and ice algae.

ice shelf — A floating platform of glacial ice attached to a coastline, formed where glaciers and ice streams flow off the Antarctic continent and extend over the ocean. The Ross Ice Shelf is approximately the size of France. Ice shelves act as buttresses, slowing the flow of ice from land glaciers into the ocean.

tundra — The treeless biome found in Arctic regions, north of the boreal forest (taiga) and south of the permanent ice and snow zone. Characterised by permafrost, low-growing vegetation (mosses, lichens, sedges, dwarf shrubs), waterlogged soils in summer, and a short growing season of 60–90 days.

insolation — Incoming solar radiation reaching the Earth's surface. At polar latitudes, insolation is low because solar radiation arrives at an oblique angle, spreading energy over a larger surface area than at the equator.

polar night — The period at high latitudes when the sun does not rise above the horizon for 24 hours or more. At the poles, polar night lasts approximately 6 months (winter). During polar night, no solar energy reaches the surface, and temperatures plunge as the surface radiates heat into space with nothing to replace it.

midnight sun — The opposite of polar night: the period at high latitudes when the sun does not set below the horizon for 24 hours or more. At the poles, the midnight sun lasts approximately 6 months (summer). The midnight sun provides continuous solar energy, driving intense productivity in polar oceans and tundra.

continental ice — Ice resting directly on land (bedrock), as in the Antarctic Ice Sheet or Greenland Ice Sheet. Distinguished from sea ice, which floats on the ocean. Continental ice adds to sea level when it melts; sea ice does not (it is already displacing an equivalent volume of water).

counter-current heat exchange — A vascular adaptation in polar animals where arteries carrying warm blood and veins carrying cold blood run immediately alongside each other in opposite directions. The warm blood pre-heats the cold returning blood, minimising heat loss from extremities (feet, flippers) while preventing cold blood from chilling the animal's core temperature. Found in emperor penguins, polar bears, seals, and many Arctic fish.