3 February 2007The Independent
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 278 ppm, and a range over the previous 650,000 years of between 180 and 300 ppm, the report says.
Present levels of carbon dioxide - which continue to rise inexorably 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 1.4 ppm 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 1.9 ppm 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 man-made 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. One isa 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 sea ice and snow cover is shrinking at the poles and on mountains, leading to a further increase in local temperatures.
