Rising sea levels
Global sea level rose on average by 10-20mm a decade during the 20th century. Sea level is predicted to rise by 100-900mm in the next century.
More than 300 million people live within 1m of average sea level and one-third of the world's population lives near the coast.
Many heavily populated areas, for example Bangladesh and the Netherlands, are near or below sea level. Some large cities such as New Orleans are below sea level and require large dams to keep the sea at bay.
The main cause of sea-level rise is thermal expansion of the oceans. As water heats up its molecules move further apart and so its volume increases. Water at higher temperature or under greater pressure (deep waters) expands more for a given heat input from the atmosphere, so the global average expansion of the oceans is affected by the distribution of heat within the ocean. This is a very difficult problem for climate modellers.
There is a considerable delay before temperature increases in the atmosphere translate to rises in sea level. As a result, global average sea level will continue to rise for centuries after atmospheric greenhouse gases have stabilised.
Another cause of sea-level rise is melting glaciers in the Alps, Andes, Himalayas and Rockies.
The Greenland ice sheet could begin to melt if average local temperatures rise by 3°C. If Greenland's ice melted completely sea level would rise by 7m, though this process would take many centuries. The Arctic is one of the fastest warming regions on the planet.
Warming of Antarctica is of concern because the ice sheets contain vast reserves of water and increased melting of this ice could contribute to global sea-level rise.
Global climate model predictions of how the Antarctic climate may change over the next 100 years differ in detail from model to model. Most models indicate relatively modest temperature rises around Antarctica over the next 50 years and, over this time period, snowfall is likely to increase over the continent, and this effect may partially offset the rise in sea level. However, there are parts of the continent Antarctic Peninsula and West Antarctic Ice Sheet where recent observations have indicated an ongoing loss of ice. The mechanisms responsible for those losses are the focus of ongoing research, but there is a significant possibility that they could accelerate over the next 100 years and mean that the Antarctic as a whole becomes a significant contributor to sea-level rise, adding to the other sources; thermal expansion of the oceans and melting of icecaps and glaciers elsewhere in the world.