What is climate change?

The planet's climate is constantly changing. The global average temperature is currently in the region of 15C. Geological and other evidence suggests that, in the past, this average may have been as high as 27C and as low as 7C.

But scientists are concerned that the natural fluctuation has been overtaken by a rapid human-induced warming that has serious implications for the stability of the climate on which much life on the planet depends.


What is the "greenhouse effect"?

The "greenhouse effect" refers to the role played by a layer of gases which effectively trap the heat from the Sun in the Earth's atmosphere. Without them, the planet would be too cold to sustain life as we know it.

These gases include carbon dioxide, methane and nitrous oxide, which are released by modern industry, agriculture and the burning of fossil fuels.

Their concentration in the atmosphere is increasing - the concentration of carbon dioxide has risen by more than 30% since 1800.

The majority of scientists accept the theory that an increase in these gases will cause a rise in the Earth's temperature.


What is the evidence of warming?

Temperature records go back to the late 19th Century and show that global average temperature increased by about 0.6C in the 20th Century.

Sea levels have risen 10 - 20cm - thought to be due mainly to the expansion of warming oceans. Most of the recorded non-polar glaciers are in retreat and records show Arctic sea-ice has thinned by 40% in recent decades in summer and autumn.

There are anomalies however - parts of the Antarctic appear to be getting colder, and there are discrepancies between trends in surface temperatures and those in the troposphere.


How much will temperatures rise?

If nothing is done to reduce emissions, current climate models predict a global temperature increase of 1.4 - 5.8°C by 2100.

To put this in context, global temperatures are thought to have fluctuated by only one degree Celsius since the dawn of human civilisation.

Even if we cut greenhouse gas emissions dramatically now, scientists say the effects would continue because parts of the climate system, particularly large bodies of water and ice, can take hundreds of years to respond to changes in temperature.

Some scientists say it is possible that we have already irrevocably committed the Greenland ice sheet to melting.

This would take centuries - if not millennia - but would cause an estimated seven metre rise in sea level.


How will the weather change?

Globally, we can expect more extreme weather events, with heat waves becoming hotter and more frequent. Scientists predict more rainfall overall, but say the risk of drought in inland areas during hot summers will increase.

More flooding is expected from storms and rising sea levels.

There are, however, likely to be very strong regional variations in these patterns, and these are difficult to predict.


What will the effects be?

The potential impact is huge, with predicted freshwater shortages, sweeping changes in food production conditions, and increases in deaths from floods, storms, heat waves and droughts.

Poorer countries, which are least equipped to deal with rapid change, will suffer most.

Plant and animal extinctions are predicted as habitats change faster than species can adapt, and the World Health Organisation has warned that the health of millions could be threatened by increases in malaria, water-borne disease and malnutrition.


What don't we know?

We don't know exactly how much warming is caused by human activities or what the knock-on effects of the warming will be.

Global warming will cause some changes which will speed up further warming, such as the release of large quantities of the greenhouse gas methane as permafrost melts.

Other factors may mitigate warming - such as plants taking more CO2 from the atmosphere as their growth rate is increased by warmer conditions.

Scientists are sure how the complex balance between these positive and negative feedback effects will play out.


What about the sceptics?

Most global warming sceptics do not deny that the world is getting warmer, but they do doubt that human activity is the cause.

Some say the changes now being witnessed are not extraordinary - similar, rapid changes can be seen at other times in Earth's history when humans did not exist.

Some point to the Sun's present high activity as the prime influence on recent temperature trends.

Nevertheless, there is a growing scientific consensus that, even on top of the natural variability of the climate, something out of the ordinary is happening and humans are to blame.


What is the international community doing?

An international agreement, the Kyoto Protocol, commits industrialised countries to specific targets for reducing their greenhouse gas emissions.

It must be ratified by a certain number of countries before it becomes binding. The protocol suffered a huge blow when the US - responsible for a quarter of global emissions - pulled out in 2001.

The agreement will now only come into force if Russia ratifies it.

While many countries have taken steps to reduce their emissions, the Kyoto targets are just a fraction of the emissions reductions thought necessary to slow global warming significantly.