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.
This photo from the National Science Foundation shows the shear face of the massive B-15A iceberg stretches for 150 kms across McMurdo Sound after it broke off the Ross Ice Shelf in Antartica, 2001. UN scientists delivered their starkest warning yet about global warming, saying fossil fuel pollution would raise temperatures this century, worsen floods, droughts and hurricanes, melt polar sea ice and damage the climate system for a thousand years to come. (AFP/NSF-HO/File/Josh Landis)
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.
Global warming: the final warning
According to yesterday's UN report, the world will be a much hotter place by 2100. This will be the impact ...
+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.
Rising sea levels accelerate as the Greenland ice sheet tips into
irreversible melt, submerging atoll nations and low-lying deltas. In Peru,
disappearing Andean glaciers mean 10 million people face water shortages.
Warming seas wipe out the Great Barrier Reef and make coral reefs virtually
extinct throughout the tropics. Worldwide, a third of all species on the
planet face extinction
+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
Warming seas lead to the possible release of methane hydrates trapped in
sub-oceanic sediments: methane fireballs tear across the sky, causing
further warming. The oceans lose their oxygen and turn stagnant, releasing
poisonous hydrogen sulphide gas and destroying the ozone layer. Deserts
extend almost to the Arctic. "Hypercanes" (hurricanes of
unimaginable ferocity) circumnavigate the globe, causing flash floods which
strip the land of soil. Humanity reduced to a few survivors eking out a
living in polar refuges. Most of life on Earth has been snuffed out, as
temperatures rise higher than for hundreds of millions of years.
© Copyright 2007 Independent News and Media Limited