As a report warns that the pace of climate change could be faster than previously thought, Nick Morrison looks at the latest thinking on what global warming means - and why it won't see us basking in year-round sunshine

IT was a year of seemingly endless natural disaster. From the turmoil left by the Asian tsunami to the devastation of Hurricane Katrina, floods in China and India to hundreds killed by heatwaves across Europe, famine in Africa to the earthquake in Pakistan - 2005 was a year when calamity piled on top of catastrophe.

An aberration it may have been, but it seems as though the last 12 months could be just a dress rehearsal for what is to come. And while the timescale could be decades or even centuries, in scale it could dwarf the events of the last year.

Global warming has long been the shape on the horizon. At first indistinct, its very existence seemed in doubt. Then, while it was generally accepted, it was still distant enough and formless enough not to cause too much anxiety. But now it seems it is both bigger and moving towards us faster than we thought.

A report published yesterday suggests that the effect of rising concentrations of greenhouse gases may be more serious than previously believed. The document, Avoiding Dangerous Climate Change, is the result of a conference hosted by the UK Meteorological Office last year and brings together the latest thinking on the subject by experts from around the world.

Among the more worrying of its conclusions is that there is a strong likelihood of the Greenland ice sheet - two kilometres thick and the second largest in the world - melting, and that this could cause sea levels to rise by seven metres over 1,000 years. And it claims that we are rapidly approaching a tipping point, a stage beyond which the changes will be irreversible.

For the world, that will mean disaster. Entire ecosystems will be wiped out, animals and plants becoming extinct. Low-lying towns and cities will be underwater. Billions of people will face water shortages. Tropical diseases will become widespread. Those whose lives are already precariously balanced will be pushed over the edge.

For us in Britain the consequences will also be severe, but not necessarily as straightforward. Parts of the country may indeed become warmer - with the water shortages that implies - but disruption of the ocean currents, which keep us warmer than we should be, may counterbalance this, although not without its own side-effects.

The principal behind global warming is that gases trap energy from the Sun in the Earth's atmosphere. Without them, the Earth would be too cold to sustain life. Among these gases are carbon dioxide, methane and nitrous oxide, gases produced by industry, agriculture and burning fossil fuels.

The theory is that an increase in these gases will cause the Earth's temperature to rise, as more of the Sun's energy is trapped. Analysis of Antarctic ice cores shows the level of carbon dioxide over the last 500,000 years has ranged from between 200 parts per million (ppm) during ice ages to 270ppm. Today it stands at about 380ppm and is predicted to reach 400ppm within a decade.

Records show the global average temperature increased by about 0.6 degrees Celsius in the last century. Over the next century, it is forecast to rise by anything from 1.4 to 5.8 degrees.

One of the principal effects will be to melt the polar ice caps. Arctic sea ice has thinned by 40 per cent in recent decades, and last September set a new record minimum for ice cover. On September 19, the area covered by ice was 5.35 million square miles, the lowest since records began in 1978, and 20 per cent less than the average for the years 1978-2000. At the existing rate of shrinkage, there will be no ice at all during the summer of 2060.

There are also signs that the west Antarctic ice sheet is melting. In a further twist, as ice reflects the Sun's radiation and water absorbs it, as more ice becomes water the pace of change will increase. It is not all one way, however. There are signs the much larger east Antarctic ice sheet is becoming thicker, although there is no conclusive reason why.

The most immediate effect of the retreat of the ice is the loss of habitats for animals including polar bears, penguins, seals and caribou. Where possible, they will have to migrate to find new sources of food. Where this is not possible they will become extinct.

But the vast increase in amount of water will have far-reaching consequences. As well as the rise in sea levels, the addition of such huge quantities of freshwater will make the oceans more acidic, threatening fish populations. The melting of glaciers which provide freshwater supplies in Asia and South America will lead to widespread drought.

Although none of the scenarios are favourable, the extent of the effects will depend on the size of the increase in global temperatures, according to Jim Hall, professor of Earth systems engineering at Newcastle University. While rich countries could cope with an increase of up to two degrees, it is poorer countries which will be vulnerable.

"Even for quite low changes there are many ecosystems which will be threatened or may become extinct and, in some circumstances, they have got nowhere to go: the aquatic ecosystem of the Antarctic will just die," he says.

"Tropical rainforests will also be threatened and while Europe is among the least vulnerable, poorer countries will be badly hit, with increased temperature, increased flooding and increased drought."

A temperature rise of between two and three degrees will see these effects become more widespread, exacerbated by problems of migration as both animals and humans move to more hospitable climes. The Amazon and other rainforests will suffer a massive decline, and on top of the loss of the ecosystems themselves, the world will be deprived of what is increasingly being seen as a valuable source of medicines.

Increased temperatures will also release greenhouse gases stored in the permafrost and in the sediment at the bottom of the ocean, accelerating the warming effect. Globally, weather will become more extreme, with heatwaves hotter and more frequent, rainfall higher but the risk of drought in inland areas greater.

Higher than three degrees in temperature enters the realm of what scientists refer to as abrupt climate change, significant changes to the Earth's climate over relatively short periods.

One of these could be the disruption of the thermohaline circulation in the North Atlantic. This system, of which the Gulf Stream is a part, keeps Western Europe warmer than it should be, given it is on the same latitude as Newfoundland in Canada. The addition of billions of gallons of cold water from the melting Arctic could see this decline or even switch off altogether.

It is believed that this could lower temperatures in western Europe to balance out the increase from global warming, but the uncertainty means it is perhaps even more worrying. "This will be an unprecedented change in the Earth's ecosystem, and even if it looks as though there will be some beneficial effects, we don't want to go there," says Prof Hall.

"We are testing the Earth's system in a way which has never - at least not since the last ice age - been done before, and there is a potential for consequences on a huge scale. There are really significant grounds for concern and everyone accepts that. It is just a question of what we're going to do about it."