CONCENTRATIONS of carbon dioxide in the atmosphere are at their highest levels for at least 650,000 years, and this rise began with the birth of the Industrial Revolution 250 years ago, according to the Intergovernmental Panel on Climate Change (IPCC).

Carbon dioxide is the principal greenhouse gas responsible for global warming and, in 2005, concentrations stood at 379 parts per million (ppm). This compares to a pre-industrial level of 278ppm, and a range over the previous 650,000 years of between 180 and 300ppm, the report says.

Present levels of carbon dioxide - which continue to rise each year - are unprecedented for the long period of geological history that scientists are able to analyse from gas samples trapped in the frozen bubbles of deep ice cores.

However, the IPCC points to a potentially more sinister development: the rate of increase of carbon dioxide in the atmosphere is beginning to accelerate. Between 1960 and 2005, the average rate at which carbon dioxide concentrations increased was 1.4ppm per year. But when the figures are analysed more closely, it becomes apparent that there has been a recent rise in this rate of increase to 1.9ppm per year between 1995 and 2005.

It is too early to explain this accelerating increase, but one fear is that it may indicate a change in the way the earth is responding to global warming. In other words, climate feedbacks that accelerate the rate of change may have kicked in.

The IPPC's report points out that, as the planet gets warmer, the natural ability of the land and the oceans to absorb carbon dioxide from the atmosphere begins to get weaker.

It is estimated that about half of all the manmade emissions of carbon dioxide have been taken out of the air and absorbed by natural carbon "sinks" on the land and in the sea. Many computer models of the climate predict that, as the earth continues to get warmer, these sinks will become less able to absorb carbon dioxide from the atmosphere.

This means that more carbon dioxide will be left in the air to exacerbate the greenhouse effect, so leading to further temperature rises and more global warming, which in turn will make the natural carbon sinks of the earth even less efficient.

As the IPCC's summary says: "Warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, increasing the fraction of anthropogenic [man-made] emissions that remain in the atmosphere."

This is just one of several "positive feedbacks" that could quickly accelerate the rate of global warming over the coming century. Another scenario is that a warmer world is causing more evaporation from the oceans and a rise in water vapour - a powerful greenhouse gas - in the lower atmosphere. Another is that sea ice and snow cover is shrinking at the poles and on mountains, leading to a further increase in local temperatures.

»"Best guesses" are that the global temperature will rise by 1.8°C to 4°C over the next century, depending on level of population and industrial activity. Local figures would be higher in high latitudes, such as Ireland's.

»Worst-case scenario is that with high fossil fuel use and strong economic growth, rise could be 6.4°C, again with higher rises nearer the poles.

»Sea-level rise worst-case scenario is 59cm by 2100, less than predicted in 2001, but this might be much higher when climate system feedbacks are factored in, and ice discharge from Greenland and Antarctica rises.

»Stabilising CO 2levels in the atmosphere at 550ppm - which some experts think the world should aim at - would itself probably mean a rise of 3°C, and possibly 4.5°C.

»Temperatures in the next two decades are likely to rise by 0.4°C.

»That global climate change is occurring is "unequivocal".

»That human beings are responsible for it is "at least a nine out of 10 chance".