Carbon Dioxide Rate is at Highest Level for 650,000 Years

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.

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+2.4°: Coral reefs almost extinct

In North America, a new dust-bowl brings deserts to life in the high plains states, centred on Nebraska, but also wipes out agriculture and

cattle ranching as sand dunes appear across five US states, from Texas in the south to Montana in the north.

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+3.4°: Rainforest turns to desert

The Amazonian rainforest burns in a firestorm of catastrophic ferocity, covering South America with ash and smoke. Once the smoke clears, the interior of Brazil has become desert, and huge amounts of extra carbon have entered the atmosphere, further boosting global warming. The entire Arctic ice-cap disappears in the summer months, leaving the North Pole ice-free for the first time in 3 million years. Polar bears, walruses and ringed seals all go extinct. Water supplies run short in California as the Sierra Nevada snowpack melts away. Tens of millions are displaced as the Kalahari desert expands across southern Africa

+4.4°: Melting ice caps displace millions

Rapidly-rising temperatures in the Arctic put Siberian permafrost in the melt zone, releasing vast quantities of methane and CO2. Global temperatures keep on rising rapidly in consequence. Melting ice-caps and sea level rises displace more than 100 million people, particularly in Bangladesh, the Nile Delta and Shanghai. Heatwaves and drought make much of the sub-tropics uninhabitable: large-scale migration even takes place within Europe, where deserts are growing in southern Spain, Italy and Greece. More than half of wild species are wiped out, in the worst mass extinction since the end of the dinosaurs. Agriculture collapses in Australia

+5.4°: Sea levels rise by five metres

The West Antarctic ice sheet breaks up, eventually adding another five metres to global sea levels. If these temperatures are sustained, the entire planet will become ice-free, and sea levels will be 70 metres higher than today. South Asian society collapses due to the disappearance of glaciers in the Himalayas, drying up the Indus river, while in east India and Bangladesh, monsoon floods threaten millions. Super-El Niños spark global weather chaos. Most of humanity begins to seek refuge away from higher temperatures closer to the poles. Tens of millions of refugees force their way into Scandanavia and the British Isles. World food supplies run out

+6.4°: Most of life is exterminated

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