Page last updated at 13:28 GMT, Monday, 7 December 2009
The key effects of climate change



A sizeable proportion of the world's population have made their home in coastal regions.

For many low-lying areas, scientists warn that the coming century is likely to see sea level rise that will change the shape of coastlines around the globe.

While industrialised nations are able to spend billions on flood protection schemes, many small island nations are at risk of disappearing beneath the waves.

But by how much will the waters rise in the coming century, and what are the factors driving the increase?

Infographic about sea level rises
Glaciers: If the world's mountain glaciers and icecaps melt, sea levels will rise by an estimated 0.5m
Thermal expansion: The expansion of warming oceans was the main factor contributing to sea level rise in the 20th Century, and currently accounts for more than half of the observed rise in sea levels
Ice sheets: These vast reserves contain billions of tonnes of frozen water - if the largest of them (the East Antarctic Ice Sheet) melts, the global sea level will rise by an estimated 64m

The majority of the current global average sea level rise of about 3mm each year is from the thermal expansion of the oceans.

As greenhouse gases become more concentrated, more heat energy is trapped in the atmosphere. This energy is absorbed by the world's oceans, causing it to warm and expand.

graph

Another contributor is melt water from mountain glaciers. Data shows that, on average, snow and ice cover in the world's mountain ranges have declined.

The run-off increases the volume of water flowing into rivers and lakes, which in turn ends up in the seas.

However, there are big question marks over how much the vast polar ice sheets, which have the potential to have a catastrophic impact, will contribute to future sea level rise.

The world's three ice sheets - Greenland, West Antarctic and East Antarctic - are vast bodies of ice, containing billions of tonnes of frozen water.

At present, their contribution to average sea level rise is relatively small. However, they are projected to become key drivers.

In its benchmark Fourth Assessment Report (AR4), the Intergovernmental Panel on Climate Change (IPCC) projected a sea level rise of up to 43cm by 2100.

However, it acknowledged that it could not predict how the ice sheets would respond to a warming world.

Leading up to the publication of the AR4, researchers had gathered evidence of glaciers in Greenland and parts of the Antarctic were flowing more quickly, feeding more ice into the oceans, which could translate into faster sea level rise.

Since 2007, there have been more much more research into the dynamics of the ice sheets, resulting in a number of updated projections.

Map of Antarctica (Image: BBC)

One of the latest assessments suggest that sea levels are likely to rise by about 1.4m (4ft 6in) globally by 2100 as polar ice melts.

A major review of climate change in Antarctica by an international team of researchers said that warming seas were accelerating melting in the west of the continent.

ANTARCTIC CLIMATE CHANGE

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By the end of the century, it projected, the sheet will probably have lost enough ice alone to raise sea levels globally by "tens of centimetres".

It added that the Antarctic Peninsula - the strip of land that points towards the southern tip of South America - has warmed by about 3C over the last 50 years, the fastest rise seen anywhere in the southern hemisphere.

But the rest of the continent has remained largely immune from the global trend of rising temperatures.

Indeed, the continent's largest portion, East Antarctica, appears to have cooled, bringing a 10% increase in the sea ice extent since 1980.

Other observers project a global average sea level increase of about one metre by 2100.

But there is a scientific consensus that the IPCC's 2007 projection of 43cm was too conservative.

However, for many people the debate over the extent of future rises are academic.

Leaders of small island nations - especially in the South Pacific - are fearful for the fate of their populations.

Even a small increase will result in the small islands disappearing beneath the waves.



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