Where ice melts without warmth

The ice caps of Greenland (left) and Antarctica (right) flow towards the coast, where monumental icebergs calve into the ocean. Figure: Based on material from K. Steffen, CIRES / University of Canada

Antarctica is renowned for its cold. The mountains in the middle of Antarctica are buried under several kilometres of ice that never melts. The highest temperature ever recorded at the South Pole is -13.6°C and the yearly average is a frigid -50°C. So how could anyone be worried that a few degrees of warming could make the ice melt?

The answer lies at the coast. Antarctica’s ice creeps slowly towards the north, but speeds up in the steeper terrain farther north. When it reaches the sea, the ice spreads out across hundreds of kilometres, lying atop the water. The Antarctic continent is surrounded by floating glaciers several hundred metres thick. In this way it differs from Greenland, where the ice flows out into fjords, just like the Svalbard glaciers we know so well (see the figure).

The climate at the coast – especially along the Antarctic Peninsula – is much milder than in the continent’s altitudinous inland. Temperatures around freezing are relatively common in the summer months. In addition, these more northerly areas have seen a particularly strong warming trend in the past decades. This has led to increased calving from the floating ice. Several ice shelves have collapsed and disappeared altogether. When they are gone, the ice lying over solid ground flows more quickly. The damming effect of the ice shelf has been eliminated.

This leads to a domino effect. Melting at the coast triggers accelerated outflow of ice from the constantly sub-zero areas farther inland. Faster ice flow leads to more calving from the ice shelves. The resulting icebergs drift northward and melt, ultimately influencing sea level. This explains the apparent physical impossibility: ice can melt without warmth.