It is certainly true that sea levels are rising – they have been for the last 18,000 years. Since the peak of the last ice age 18.000 years ago, seas have risen by 130 metres. During the current interglacial period, sea levels were at their highest about 140,000 years ago when they were briefly 6 metres higher than now. They have been higher in the far distant past but changed landforms make comparisons meaningless.
For most of the last 3,000 years, seas have been rising at between 1 and 2 millimetres a decade. Since 1900, the rate of sea level rise has been about 1.7 centimetres a decade – ten times faster than before the industrial era. Since the mid-1990s, sea levels have been rising at about 3.1 centimetres per decade.
The causes of rising sea levels are:
- Water expanding as it becomes warmer.
- Melting glaciers.
- Melting ice on Greenland.
- The diminishing Antarctic ice sheet which is still contracting since the last ice age.
Other factors cause sea levels to drop:
- Increased snowfall in Greenland and Antarctica because there is more moisture in a warmer atmosphere and
- Water being stored in dams and aquifers.
Of course, melting ocean ice has no effect on sea levels.
Thermal expansion accounts for around half of the current sea level rise – about 1.6 mm a year. Expansion from the atmospheric warming that has already happened will continue for hundreds more years. If carbon dioxide levels double, it will cause sea levels to rise by between 0.5 and 2 metres over 500 years.
Melting glaciers are currently an important component and are expected to contribute about 1.1 mm a year during the 21st century. Glaciers near the coast of Greenland are melting but snowfall in the interior has increased, resulting in a small net lowering of sea levels from Greenland.
Sustained warming over Greenland by 5.5 degrees Celsius, which is possible with current projections, would melt enough of the Greenland ice sheet to cause sea levels to rise by about 3 metres, over a period of 1,000 to 1,500 years. Although there is sufficient ice on Greenland to raise sea levels by 7.2 metres, temperatures would have to rise by more than 30 degrees Celsius to melt all of it.
Melting of ice in West Antarctica (the peninsula that points up towards South America) is contributing about 0.4 mm a year to sea level rise. Much of this is thought to be due to long term factors that have been going on for thousands of years. The pattern is complex, with the more northerly glaciers retreating, while many southerly glaciers are advancing. The line between retreating and advancing glaciers has generally moved south with increasing global temperature. Since much of the ice currently melting is over the sea, it is not expected to cause significant sea level rise during this century. If the entire West Antarctic ice sheet was to melt, it would raise sea levels by 4 to 5 metres, probably over a period of 2,000 to 3,000 years but these estimates are still very uncertain.
The larger part of Antarctica, West Antarctica, is far too cold for any melting of ice. Increased snowfall there is currently reducing sea levels.
Overall, the IPCC predicts that sea levels will rise by between 22 and 44 centimetres during this century – many experts predict that it will be towards the upper end of this range.
About 100 million people who live at less than one metre above sea level and could be directly affected this century. Higher sea levels also mean that populated areas will be affected by storm surges more frequently. An increase of 20 centimetres in sea levels, which is predicted by around 2050, would mean that storm surges which now occur once every 100 years, would happen every 40 years. In Australia, this would effect all of the tropical coastline. In other areas, such as New South Wales, where the formation of the continental shelf protects against storm surges, coastal erosion would be a greater problem.
(Most of the information in this posting comes from papers produced by the CSIRO.)