On screen chemistry with Jonathan Hare

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 polar ice caps like we see in the film, how long will it take and what effect will it really have?

Meltdown at the Poles

An ice cap breaking apart

Source: Shutterstock

The major ice sheets are found at the Poles (the Arctic and Antarctic), in Greenland and in the world’s glaciers. Floating ice does not raise the sea level when it melts. You can demonstrate this by adding some ice cubes to a glass of water and marking the water level. When the ice has 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.

Around 60 per cent of all the fresh water on the Earth’s surface is locked up in the enormous continental Antarctic ice sheet. If this ice melted it is estimated that it would cause the sea level to rise by ca 60 m, while the Greenland ice sheet would produce around a 7 m rise. Some of the Antarctic ice sheet is more stable than other parts. The west Antarctic ice sheet (WAIS) is thought to be most at risk from collapse and melting which, if it all went, would lead to around a 5 m rise in sea level.

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

Hot water

Large increases in sea level can happen if the seas warm up – above 4 ºC water expands with increasing temperature, taking up a larger volume. This can occur over a long time as a result of 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 Intergovernmental Panel on Climate Change (IPCC) has reported that the ice sheets are melting and that the seas are warming. The average sea level rise currently amounts to ca 1.8 mm per year, though recent data suggest its larger than this. 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 rise.

Currently, more than half of the average rise is the result of thermal expansion of water while the rest is from ice melting. Since the oceans are a vast mass they will take a long time to heat up significantly to any great depth. So large permanent changes in sea level caused by thermal expansion probably won’t happen for many hundreds of years. Melting of the Greenland and massive Antarctic ice sheet will require much longer timescales, however once started it may be impossible to stop this process.

Overall, in the short term (~100 years) thermal expansion will dominate the changes in sea levels while over the longer term (~1000 years) melting of the Antarctic ice sheet will become more significant. The Waterworld scenario where all the land is covered by water is unlikely to occur, even if global warming and ice melting continues unabated for hundreds of years. But many fear that smaller changes in sea level will dramatically effect humans within the next 100–200 years.

Originally published by InfoChem