3.1. Causes of sea-level rise

Recent and contemporary sea-level rise in our warming world is the result of a combination of a number of factors. The most important are: thermal expansion of seawater, the decay of mountain glaciers and small ice caps, melting of the Greenland Ice Sheet, and melting of the Antarctic Ice Sheet. Thermal expansion of seawater is based upon the simple premise that as seawater warms its density decreases and therefore it occupies more volume. For a given average ocean warming, sea-level rise depends upon where warming occurs. Because the expansion coefficient increases with water temperature, the greatest rise in sea-level will occur if warming is concentrated in regions where the ocean waters are already the warmest, in other words the upper few hundred metres of the sea at low latitudes.

The decay of glaciers and small ice caps is dependent on the relative rate of ice accumulation versus melting. Mountain glaciers and ice caps can be simply divided into two zones; an upper zone of accumulation where ice formed from new snowfall annually exceeds that lost due to melting, and a lower zone of ablation in which annual melting (ablation) exceeds annual accumulation. For a glacier to maintain its mass, the net mass loss from the ablation zone must be exactly balanced by a net mass gain in the accumulation zone. The two zones are separated by the equilibrium level altitude (ELA), the position of which determines whether a glacier will grow or decay. A rise in temperature, for example, will tend to raise the ELA which, if it rises above the highest point on the glacier, will cause it to eventually vanish entirely. Some glaciers are more sensitive to changes in the ELA than others and broadly speaking smaller glaciers or those with a greater mass throughput (higher rates of both accumulation and melting) will respond more rapidly to variations in the altitude of the ELA. The mountain glaciers and small ice caps that exist today are tiny remnants of those that covered upland areas between 15,000 and 20,000 years ago, and most are now confined to the Himalayas, Andes, Western Cordillera of North America and glaciated terrain in the Arctic and Antarctic. Even if all the mountain glaciers and small icecaps melt as a result of global warming, the total sea-level rise would still only be in the order of 50 cm ± 10 cm.

Future sea-level rise associated with melting of the polar ice caps is potentially an order of magnitude higher. Complete melting of the Greenland Ice Sheet, for example, would result in a rise of 7.4 m. Similarly, the collapse and melting of the West Antarctic Ice Sheet would cause sea-level to increase by 5 – 6 m.