The creeping menace

The Greenland Ice Cap has the potential to raise sea levels by about 7m. and is presently loosing over 250 gigatonnesof ice per annum. 1 gigatonne = 1 cubic kilometre of water.

The cap is melting at all its margins because global warming has caused melting of sea and land based snow and ice, with the result that solar energy is being absorbed, heating land on which the ice cap rests. In addition, increasing atmospheric CO2 is radiating solar energy reflected by the ice cap back on to it.

The result is that snow falling on the ice cap is melting rather than compacting into new ice. In the south precipitation previously falling as snow, is more frequently falling as rain. Combined, these cause the formation of lakes on the surface of the ice cap. This water eventually erodes the lake bed, seeping through it until it reaches bed-rock on which the ice cap rests, providing lubricant enabling the ice to move, become stressed, crack, break-up and discharge into glaciers draining into the sea.

The effect is that a growing number of Greenland glaciers have less impeded access to the sea enabling them to move more rapidly, draining greater volumes of ice from the interior of the ice cap. In recent years an increase of over 7 percent per annum in the volume of ice discharged into the sea has been recorded by the GRACE satellites.

The West Antarctic Ice Cap has the potential to raise sea level by 6-9m. and is currently loosing over 130 gigatonnes of ice per annum and accelerating.

Over 400 glaciers on the Antarctic Peninsula are in retreat and in summer, snow covering large areas of adjacent land melt permitting absorption of solar radiation. Collapse of the Larsen B. ice shelf in 2002 has enabled many glaciers draining the Peninsula ice cap to significantly increase in speed and discharge much greater quantities of ice into the Southern Ocean.

The West Antarctic ice cap is a marine ice sheet largely grounded on the sea bed and is highly susceptible to rapid melting both on its surface and underside. The latter will result in erosion and weakening of the ice anchoring the cap to the sea floor. When this occurs it will cause large portions of the cap to break-away at the margins and float, precipitating rapid sea level rise.

Glaciers draining the largest portion of the West Antarctic Ice Cap are blocked from direct access to the sea by the massive Rönne and Ross ice shelves, slowing the amount of ice they discharge into the sea. The seaward edges of both ice shelves show significant susceptibility to warming temperatures, causing the calving of massive icebergs.

These ice shelves will eventually collapse permitting more rapid discharge of glaciers and melting of the ice cap. Evidence for this is seen in the massive increase in the flow of glacier discharge into the Amundsen Sea, particularly from the Pine Island Glacier. Stability of the ice cap over the next 20 years is far from certain and gives rise to speculation that the entire ice cap could rapidly melt causing it to disintegrate within a few hundred years, rather than millennia, contributing to relatively rapid rise in sea level.

The East Antarctic Ice Cap, has the potential to raise sea levels by 55-70m. It is currently losing 50-70 gigatonnes per annum. That rate is increasing.

The East Antarctic Ice Cap is entirely land based. Until a few months ago, it was widely accepted that temperatures were far too low to permit any part of the ice cap to melt. At the very worst, the ice cap was unaffected by global warming and at best it was actually growing.

Given that precipitation over Antarctica is very low producing very little new ice and that much of its coastline is exposed to warming sea and air temperatures, those conclusions were always optimistic and have now been proven wrong by GRACE satellite measurements. Global warming is already melting the ice cap at its fringes and traces of lakes having formed on the surface of the ice cap and draining away through the ice have been found, indicating similar dynamics to those evident in Greenland.