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Global Warming Is Happening Faster: Effects To Last For Millenia

Shanghai: January 20, 2001: A new scientific assessment by the Intergovernmental Panel on Climate Change (IPCC) concludes that global warming of approximately 0.6 C during the 20th century was likely the largest increase in temperature in any century during the past 1000 years (1).

As a result of the 20th century rise in temperature, the Panel reports that there has been:

probably a 10% decrease in the extent of worldwide snow cover since the late 1960s;

a two-weeks reduction in the annual duration of lake- and river-ice cover at the mid and high northern latitudes;

widespread retreat of mountain glaciers in non-polar regions;

a 10-15% reduction in Northern Hemisphere spring and summer sea-ice extent since the 1950s;

something like a 40% decline in Arctic sea-ice thickness during late summer to early autumn;

a 10 to 20 cm rise in global average sea level;

significant changes in precipitation, cloud cover, temperature extremes and drought frequency.

The Panel attributes most of the 20th century increase in temperature to human activity, in particular those activities causing increases in atmospheric concentrations of the greenhouse gases carbon dioxide, methane, nitrous oxide and halocarbons.

The Next Hundred Years
For the future, the IPCC predicts that warming will continue at an accelerated pace, with global mean surface temperature climbing by somewhere between 1.4 and 5.8 C during the 21st century. The primary cause will again be human activity, particularly fossil fuel use and deforestation leading to further increases in atmospheric carbon dioxide concentration.

Currently, atmospheric carbon dioxide concentration is 31% higher than in 1750, a level that has not been exceeded during the last 420,000 years. The Panel predicts that by the end of the present century, atmospheric carbon dioxide concentration will be 90 to 250% higher than in 1750, when it will account for between 0.05 and 0.125% of total atmospheric pressure.

The range of variation in the Panel's estimate of warming over the next century arose from running each of seven climate models with 35 different scenarios concerning the amounts of atmospheric greenhouse gases and particulates that could affect global climate. The different scenarios reflect different assumptions about the amounts of carbon dioxide, sulphate aerosols and other emissions to the atmosphere, which in turn, depend on different assumptions about such factors as global living standards and modes of energy use.

The Panel projects that the 21st century warming will be:
much larger than that observed during the 20th century and larger than the increase in any century during the last 10,000 years;

faster over land than over the oceans;

unevenly distributed over land areas, with North America and northern Central Asia warming 40% faster than the global mean, while southeast Asia in summer and South America in winter will warm more slowly.

Anticipated effects of the 21st century warming include:
a rise in global mean sea level of 10 to 88 cm;

further decreases in Northern Hemisphere snow cover and sea-ice extent;

continued retreat of glaciers and icecaps;

an increase in the mass of the Antarctic ice sheet because of greater precipitation, but a decrease in mass of the Greenland ice sheet;

a weakening of the ocean thermohaline circulation which transports heat from the tropics to high latitudes of the Northern Hemisphere.

After 2100
For the 22nd century and beyond, the IPCC predicts that:
increases in atmospheric concentrations of greenhouse gases will take centuries to reverse, even if emissions cease entirely;

even if atmospheric greenhouse gas concentrations are stabilized at present levels, global mean surface temperatures will rise at a rate of a few tenths of a degree per century for hundreds of years;

rising global mean temperature will result in a continual rise in sea level due to thermal expansion of the ocean;

ice sheets will react to climate warming and contribute to sea level rise for thousands of years after the climate has been stabilized;

specifically, a 3 C local rise in temperature, sustained for millenia, would result in the melting of virtually the entire Greenland ice sheet causing a 7 metre rise in sea level;

a warming over Greenland of 5.5 C for 1000 years would likely result in a 3 metre rise in sea level due to melting of the ice sheet;

melting of the West Antarctic ice sheet could contribute up to 3 metres to sea level rise over the next 1000 years.

The Panel further predicts that warming during the 21st century may result in the complete, and possibly irreversible, shut down of the thermohaline circulation. Cessation of this transoceanic energy conveyor could cause sharp cooling of northern Europe (2).


(1) Climate Change 2001: The Scientific Basis (Summary for Policy Makers).

(2) A New European Ice Age? naturalSCIENCE Cover Story, November 1997.


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