SciDev.Net

Summary

Rising sea-level is one of the most widely-discussed and potentially significant problems associated with global warming. Not only does a large proportion of the global population live in areas likely to be affected by sea-level rise, but the long adjustment time of the world’s oceans means that, in principle, the process will be difficult to reverse.

Furthermore the impact of sea-level rise will also take a considerable period of time before it is fully felt, although remaining within the not-too-distant future. Sea-level changes at the shoreline due to tidal motion take place over a period of hours; those changes that are due to movements of continental plates take place over millennia. Between these two extremes, the timescale on which rises in sea-level attributed to climate change are likely to occur will be decades to centuries.

Sea-level rise due to climate change occurs primarily because - like most materials - water expands as it warms up. Another contribution comes from the melting of glaciers and other land ice, which add to the rise in sea-level by adding water to the oceans. In practice, the contribution from large ice masses in Greenland and Antarctica is expected to be small over the coming decades. But it may become larger in future centuries.

Sea-level rise can be offset up by irrigation, the storage of water in reservoirs, and other land management practices that reduce run-off of water into the oceans. Changes in land-levels due to coastal subsidence or geological movements can also affect local sea-levels.

How fast have sea levels been rising?
Averaged out over the globe, the sea-level rose by between 10 and 20 centimetres during the twentieth century. This rise has therefore been at an average rate of 1 to 2 millimetres a year, and there is no evidence that this rate accelerated during the century.

The calculation of global average sea-level is based on data from more than 1800 locations worldwide of tide gauges. These compare the level of water with a reference mark on coastal land.

The longest records of sea-level in a handful of northern European locations extend back 200 to 300 years, and indicate that the sea-level rose faster in the twentieth century than during the previous century. The latest report by Working Group I of the Intergovernmental Panel on Climate Change concludes that “it is very likely that the twentieth century warming has contributed significantly to the observed sea-level rise”.

Sea levels have risen dramatically in the past. Longer records of sea-level from geological data indicate a rise of 120 metres since the last glacial maximum (or ‘Ice Age’) 20 000 years ago. This has been the result of the melting of ice sheets covering the continental land masses.

But the rate of rise has varied. Global sea-level rose about 10 millimetres a year between 15 000 and 6000 years ago, then 0.5 millimetres a year between 6000 and 3000 years ago. Over the last 3000 years, global sea-level rose 0.1 to 0.2 millimetres a year - about a tenth the rate of rise in the last 100 years.

Regional variations
By the end of the current century, scientists expect the global sea-level to have risen due to global warming by 15 to 100 centimetres compared to the average level in the period 1961-1990. But regional changes in sea-level will not necessarily be the same as the global average.

This is the result of regional differences in warming, as well as to different coastline features, salinities, winds, ocean currents and the fact that warm water expands more than cold water per degree change in temperature.

Regions to the east of Australia and Japan, for example, are expected to experience sea-level rise that is greater than the global average. In contrast, areas around the southern oceans are expected to experience a smaller rise.

Deep water expansion
The warming of the ocean surface is not the only factor leading to sea-level raise. Energy from global warming is being mixed downwards to the ocean depths, a process that will cause an increasing amount of water to expand - and thus further increase sea-level.

Sea-level rise will eventually slow if atmospheric greenhouse gas concentrations are stabilised. But the slow rate of change of deep ocean temperatures means that we are likely to be committed to sea-level rise for many centuries to come.

Dangers of sea-level rise
Even a small rise in sea-level increases the risk of storm surges - sudden rises in sea-level during high tides that flood low-lying coastal areas. These risks would be further increased if intense storm activity increases due to the enhanced greenhouse effect.

Computer modelling at the UK Met Office’s Hadley Centre suggest that sea-level rise resulting from unmitigated emissions of greenhouse gases could result in the number of people being flooded each year rising by many millions. Most of those flooded would be along coasts in southern Asia (from Pakistan, through India, Sri Lanka and Bangladesh to Burma) and South East Asia (from Thailand to Vietnam, Indonesia and the Philippines).

Substantial numbers of people might also be flooded in eastern Africa (from South Africa to the Red Sea, including Madagascar), the Mediterranean (from Turkey to Algeria) and western Africa (from Morocco to Namibia).

In addition to an increase in flooding, sea-level rise could also result in the permanent flooding of low-lying small island states in the Caribbean, the Indian Ocean and the Pacific Ocean. The threat of flooding is reduced under scenarios where greenhouse gas concentrations are stabilised or reduced.

Sea-level rise also threatens coastal wetlands, including salt marsh habitats and mangroves, adding to threats to these ecologically valuable areas by human actions. However computer models suggest that, if the rate of sea-level rise is reduced by the stabilisation or reduction of greenhouse gas emissions, wetlands could survive by accretion or migration.

Without this, as well as increases in the risk of flooding and in the destruction of coastal wetlands, sea-level rise could also increase coastal erosion and salinisation of soil.

Finally, even more dramatic events might occur. The discussion above has focussed on the impacts of the likely, slow warming of the oceans and slow melting of land ice due to global warming. But there is a risk - admittedly small at present - that regional warming in the Antarctic could lead to the disintegration of the west Antarctic ice shelf. If this did happen, it could raise global sea-level by several metres. But even this would take more than a century to occur.

Simon Torok and Mike Hulme are, respectively, external communications manager and director of the Tyndall Centre for Climate Change Research at the University of East Anglia.