Floating ice shelves act as brakes on the glaciers behind them
An ice shelf is a slab of glacier ice, up to a kilometre thick, floating on the sea while still attached to land. It looks passive, but it holds back the glaciers feeding it. When a shelf breaks up, those glaciers can speed toward the ocean and shed more ice than snowfall replaces.
Shelves form where glaciers flow off the coast into water too deep for the ice to rest on the seabed. The line where grounded ice starts floating is the grounding line; the seaward edge is the calving front, where chunks periodically break away. Gravity pushes ice outward from the land, so the front creeps forward for years or decades between major calving events, while snow adds mass on top and seawater melts it from below. Unlike sea ice, which freezes from seawater and is usually under 3 metres thick, a shelf ranges from about 100 to 1,000 metres.
Physics sets how much shows. Cold seawater weighs about 1,028 kilograms per cubic metre and glacier ice somewhat less, typically well under 920, so at least a ninth of a floating shelf rides above the waterline, more if bubbles or loose firn make it lighter. Nearly all of the world's ice shelves fringe Antarctica, where together they cover more than 1,550,000 square kilometres; the Ross and Filchner-Ronne shelves are the largest.
In a balanced state, Antarctic shelves lose roughly half their mass to melting underneath and half to calving. In recent decades they have lost more than they gain. Parts of the Larsen Ice Shelf shattered into hundreds of small fragments in 1995 and 2002, and Larsen C released an enormous ice island in 2017. Losing that buttress lets the glaciers behind accelerate, a speed-up already observed on the Antarctic Peninsula.
The Arctic tells a similar story. Every Canadian ice shelf is attached to Ellesmere Island north of 82 degrees. The M'Clintock shelf broke up between 1963 and 1966, the Ayles in 2005 and the Markham in 2008, and the Milne lost a large section in August 2020. In the Russian Arctic, the 222-square-kilometre Matusevich Ice Shelf in Severnaya Zemlya ceased to exist in 2012.
Source: Ice shelf
