June 2008

Sea-level rise and global climate change

by John Church*

This article first appeared in the WMO statement on the status of the global climate in 2007 (WMO-No. 1031)

Global averaged sea level continued to rise through 2006 and 2007. Modern satellite measurements reveal that since 1993, sea level has been rising at an average rate of about 3 mm per year, substantially faster than the average for the twentieth century of about 1.7 mm per year, estimated from coastal sea-level measurements.

These coastal measurements indicate that the 2006/2007 global averaged sea level is about 200 mm higher than in 1870 and since then there has been a significant increase in the rate of sea-level rise. Both data sets show interannual variability in the rate of rise. The increase in the rate of rise during the latter half of the 19th century and the early 20th century is confirmed by the few multi-century direct coastal sea-level measurements as well as by sea-level estimates from sediment cores in various parts of the world.

This is consistent with estimates made from sea-level benchmarks carved in rock in Tasmania, Australia, in 1840 and other data such as the height of ancient Roman fish tanks. Together, these results imply that there had been little net change in sea level from the first century ad to the 19th century ad and that the 20th century rate of rise is anomalous, relative to the recent Holocene.

The main contributions to sea-level change in the 20th and 21st centuries are the following:

  • Thermal expansion of the oceans (water expands as it warms);
  • The addition of mass to the oceans from the melting of glaciers and ice caps in areas such
    as the Himalayas, Alaska and Patagonia;
  • The exchange of mass with the ice sheets of the Antarctic and Greenland;
  • The exchange of mass with terrestrial water storages (groundwater, aquifers, dams, lakes).

Since 1960, the thermal expansion of the oceans and the melting of glaciers and ice caps are the largest contributions to sea-level rise. There has also been an increasing contribution from surface melt from the Greenland ice sheet over this period. These contributions are directly related to recent climate change.

Over the last decade, however, there are indications of a larger contribution from the movement of outlet glaciers of both the Greenland and West Antarctic ice sheets. If this were a sign of an acceleration in response to global warming, it would be a major concern, as the ice sheets contain enough water to raise sea level by 7 and 6 m, respectively, and any such dynamic response could raise sea level significantly faster than surface melting alone.

The impacts of sea-level rise will be felt through an increase in both mean sea level and the frequency of extreme sea-level events, for example storm surges, of a given level. Impacts include increased flooding, both in severity and frequency, of low-lying areas, erosion of beaches and damage to infrastructure and the environment, including wetlands, inter-tidal zones and mangroves, with significant impacts on biodiversity and ecosystem functioning. Millions of people in low-lying nations such as Bangladesh, the Mekong and other deltas, and Pacific islands such as Tuvalu, will have to respond to rising sea levels during the 21st century and beyond. 

Improved sea-level rise projections will contribute to more effective coastal planning and management. Adaptation measures include, for example, enhanced building codes, restrictions on where to build and developing infrastructure better able to cope with flooding.


* Marine and Atmospheric Research, Australian Commonwealth Scientific and Industrial Research Organisation and Chair, World Climate Research Programme Joint Scientific  Committee







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