"The future. The polar ice caps have melted covering the Earth with water. Those who have survived have adapted to a new world". It's the start of the film Waterworld where we see the ice caps melt and the continents disappear under water. Could climate change lead to complete melting of the ice caps like we see in the film, how long will it take and what effect will it really have?

The major ice sheets are found at the poles (the Arctic and Antarctic), in Greenland and in the worlds glaciers. Floating ice does not raise the sea level when it melts. You can see this by taking a glass of water, adding some ice cubes and marking the water level. When the ice has all melted you will see that the water level is the same. Arctic ice is floating and so will not directly change the sea level even if it all melts. The start of Waterworld looks as though it only shows the arctic ice melting and so it looks a bit misleading.

Just a few meter sea level rise would cause major havoc to all our coastlines and many major cities. For example Bangladesh, the Netherlands as well as island archipelagos such as the Maldives would be threatened. However even with a 70m sea level rise, if all the ice melted, it would not cause all the land on Earth to be covered.

However large increases in sea level can happen if the seas are warming up. This can happen over long time due to global warming or if changes in oceanic circulation and convection take place. The oceans are so vast (deep) that the expansion produced by even a slight rise in the average temperature will produce a large change in the sea level.

The International Panel on Climate Change (IPCC) believes that the ice sheets are melting and that the seas are indeed warming. The average sea level rise currently amounts to about 1.8 mm / year although very recent data suggests its larger than this [4]. This figure has to be appreciated against a complicated set of natural long term periodic variations. In reality this average includes some parts of the ocean becoming lower while others will rise. In severe weather, and especially at high tides, large sea level surges are likely.

Currently, more than half of the average rise is due to expansion while the rest is due to ice melting. The ocean is a vast mass so it will take a long time for it to heat up significantly to any great depth. So very large permanent changes in sea level due to expansion probably won't happen for many 100's of years. Melting of the Greenland and massive Antarctic ice caps will require much longer timescales, however once started it may be impossible to stop – this is called a tipping point.

So overall in the short term (~100 years) thermal expansion will dominate the sea levels while on the much longer (~1000 years) melting of the Antarctic ices will become really significant.

There is an urgent debate [2, 3, 5] over the possible knock-on effect of melting ice on various critical mechanism in the oceans (e.g. the Atlantic conveyer). In addition enormous amounts of methane (a powerful greenhouse gas) are expected to be released if the warming causes permafrost to melt in places such as Siberia which will drive global warming further [3]. There are also vast reservoirs of methane hydrates (clathrates) in ocean floor sediments which, if they became unstable would also release methane gas.

The Waterworld scenario - where all the land is covered is unlikely to occur, even if global warming and ice melting continues unabated for 100s of years. But many fear that smaller changes in sea level will dramatically effect humans within 100 - 200 years.

References
[1] Waterworld, Universal Studios, 1995.
[2] IPCC web site e.g. chapter 5, "Observations: Oceanic Climate change and sea level"
[3] The Hot Topic, G Walker and D King, 2008, Bloomsbury. ISBN 978 0 7475 9395 9
[4] Wikipedia sea level page
[5] "We must shake off this inertia to keep sea level rises to a minimum", Stefan Rahmstorf, The Guardian, 3rd March 2009

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