Global warming: rise of 4.5 C if pollution doubles, says draft report
Jan 30 9:44 AM US/Eastern
Earth's surface temperature could rise by 4.5 C (8.1 F) if carbon dioxide levels double over pre-industrial levels, but higher warming cannot be ruled out, according to a draft report under debate by the UN's top climate experts.
The draft -- being discussed line by line at the four-day meeting of the Intergovernmental Panel on Climate Change (IPCC) -- grimly states that the evidence for man-made influence on the climate system is now stronger than ever.
And carbon dioxide (CO2) pollution spewed out this century will stoke global warming and sea-level rise "for more than a millennium," given the time it takes for fossil-fuel pollution to degrade, it says.
Among other things, the document declares it "very likely" that heatwaves and pounding rain will become more frequent, snow cover is projected to contract -- and typhoons and hurricanes will become less frequent but more powerful.
Before the Industrial Revolution, levels of CO2, the principal greenhouse gas, stood at around 280 parts per million (ppm).
Today, CO2 concentrations are around 380 ppm and are rising between two and three ppm per year as big energy-gobbling countries, such as China and India, pursue their economic rise.
According to the draft, a copy of which was obtained by AFP Tuesday, the temperature has already risen by 0.74 C (1.33 F) over the last century.
It considers it "very likely" -- a probability of more than 90 percent -- that the rise since the mid-1900s was caused by man-made greenhouse gases. In its last report, in 2001, the IPCC said this probability was "likely," or 66 percent or less.
The report paints a bleak tableau of what has been happening to Earth's climate.
Since the 1970s, droughts have become intenser and longer, especially in the tropics and subtropics, while the maximum area covered by seasonally frozen ground in the northern hemisphere has retreated by seven percent since 1900.
Eleven of the last 12 years rank among the warmest years for which there are reliable records.
The average temperature of the global ocean has increased to depths of at least 3,000 metres (10,500 feet), showing that it is absorbing the heat from the atmosphere.
Warming the seas has caused them to expand, which accounts for 60-70 percent of the 1.8mm (0.07 of an inch) per year rise in global sea levels seen between 1961 and 2003. The rest of the rise is accounted for by shrinkage of the ice sheets of Antarctica and Greenland.
In 2001, the IPCC predicted global atmospheric temperatures would rise by between 1.4 and 5.8 C (2.52-10.4 F) by 2100 compared to 1990, depending on how much CO2 was in the air.
In this latest assessment, the draft forecasts what temperature rise can be expected according to the CO2 scenario, but without mentioning the 2100 timeframe.
With CO2 at 550ppm, average global temperatures would be between 2 and 4.5 C (3.6-8.1 F) higher than pre-industrial times, "with a best estimate of about 3 C (5.4 F)," says the report.
It warns, though, that "values substantially higher than 4.5 C (8.1 F) cannot be excluded" if CO2 concentrations also rise significantly.
These are among the forecast effects for this century:
-- snow cover will continue to shrink and the depth of thaw will accelerate over most regions with permafrost.
-- sea levels will rise by between 28 and 43 centimetres (11.2-17.2 inches), depending on the CO2 level. In the 2001 estimate, the range was 9-88 cms (3.5-35 inches).
-- sea-ice cover will shrink in both north and south poles. Some projections say summer sea ice in the Arctic "disappears almost entirely" by 2100.
-- hot extremes and heavy precipitation are very likely to become more frequent.
-- tropical cyclones will become less numerous but more intense, and storm tracks will move poleward.
-- the Gulf Stream, the warm Atlantic current which gives Western Europe its balmy climate despite its high latitude, will slow by a quarter during the 21st century, according to average projections.
But fears that Western Europe will be plunged into a regional Ice Age this century can be discounted. The Gulf Stream is "very unlikely" to undergo a brutal slowdown, and in any case, atmospheric temperatures will warm because of the greenhouse effect.
The IPCC report is the fourth assessment since the expert scientific panel was set up in 1988 to help guide policymakers. The Paris document, on the scientific basis for global warming, will be issued on Friday.
It is one of three that the IPCC will issue this year, the others being on the effects of climate change and how to cope with them.
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