What is climate change and why should we be concerned about it?

<---Sea levels will rise if the gigantic ice sheets in Greenland and west Antarctica start to break up

The Earth's climate has always varied, so the term climate change is now generally used to describe the changes caused by human activity - specifically, greenhouse emissions such as carbon dioxide and methane which build up in the atmosphere anda trap heat.

As human activity increases the concentration of these gases in the atmosphere fay beyond their naturals levels, much more heat is trapped. The term climate change is often used interchangeably with global warming.

Measurements at the Earth´s surface show that average temperatures have risen by some 0.4C since the 1970s. Scientists are confident this change can be blamed on human emissions because the increase is too big to be explained by natural causes. Although natural factors such as changes in the sun and large volcanic eruptions are known to have warmed and cooled tle planet in the past, these effects are not powerful enough to explain the rapid warming seen recently. Only an increased greenhouse effect caused by higher amounts of heat-trapping gases in the atmosphere can explain it.

What future temperature rise is expected? Scientists say continued emissions will cause the planet to heat up further. To work out how much, they use computers models based on the programs used to predict the weather. These models are not perfect, and struggle to simulate come features of the climate system such as clouds. To get around this, the scientists run many differents versions and pool the results. The computer models predict that if emissions continue to rise at the present rate, average temperatures will most likely increase by 4C by 2100.

There are uncertainties, though-for example, the planet´s oceans, forests and soils could release their massive stocks of carbon as the world warms, leading to much greater temperature rises than human emissions alone would cause.

Why are warmer temperatures bad? Most plants and animals have evolved to live in a fairly narrow ecological niche. Some will move to find their desired conditions, others will be able to adapt. Those that cannot move or adapt will perish. Some animals, such as the polar bear, have nowhere to move to.

A warmer climate will affect agriculture and water availability. Increased temperatures are also expected to limit rainfall in some regions and bring more extreme weather events such as storms to others.

Sea levels will rise--gradually at first as the extra warmth works its way into the oceans and makes them expand; more quickly if the gigantic ice sheets in Greenland and west Antarctica start to break up.