Glaciers and ice sheets.

Step 1 of 6

Snow that outlasts the summer

A glacier starts as snow that does not melt. Snow buried under the next years’ snow is squeezed: the flakes round into grains, the grains weld into firn, and some tens of metres down the firn turns into solid glacial ice, with the old air trapped in it as bubbles.

The Scene is an ice sheet on a northern continent, cut through its summit, 2,800 km across, with heights drawn $200$⁠ times their true size. The climate warms steadily to the south, on the left. For each climate the Scene shows the ice sheet whose yearly snowfall and melt balance. The Summer temperature is the temperature on the coast.

Step 2 of 6

A budget of snow

Every glacier keeps a budget. Up high, in the accumulation zone, snowfall wins and the ice thickens. Lower down, in the ablation zone, the summer melts more than fell. The line between them, where a year’s snow and melt just balance, is the equilibrium line. In the Unit on the atmosphere we saw that air cools about 6.5 °C for each kilometre, so the high middle of an ice sheet stays cold even in summer. The Summer temperature is now −1 °C.

Step 3 of 6

Solid ice flows

Step Look under the ice to Flow inside. The arrows give the ice’s speed at five depths; the dots are parcels of snow moving with the ice, white when the snow is fresh, blue after thousands of years and amber after tens of thousands. On the left they come back to the surface in the ablation zone and melt; on the right they reach the sea in icebergs.

Step 4 of 6

The ground sinks

The Earth’s crust floats on the mantle, which over thousands of years flows like a very thick fluid. Look under the ice is at Under the ice, and the ground appears beneath the sheet.

Step 5 of 6

Water missing from the sea

Every tonne of ice on land is a tonne of water taken from the sea. It is now an Ice age, −2 °C on the coast.

Step 6 of 6

Floating ice

Today the sea is rising by about 4 mm a year: roughly half from melting glaciers and ice sheets, and most of the rest because sea water expands as it warms. But not all melting ice raises the sea.