Global warming unpredictable, scientists say

This animation shows the ozone hole over Antarctica from Aug. 1-Nov. 30, 1997. The dark purple represents a thin layer of ozone, dark red represents a thick layer of ozone.

Climate scientists are changing their theory about greenhouse gas emissions and global warming and have come up with more pessimistic — and even optimistic — outlooks, according to the Sept. 16 issue of New Scientist.

Emissions in the year 2100 could be five times as high as today's, which is double the amount previously predicted. Or, emissions could be lower, the scientists say. The unknown factor that allows predictions to be good and bad is how technology to reduce emissions will be developed.

The warning was issued in a draft special report on emissions from the United Nations Intergovernmental Panel on Climate Change due out in March of 2000. The draft reflects the consensus view of hundreds of scientists and economists.

Past IPCC reports concentrated on one scenario that represented the likely emissions of greenhouse gases over the coming century if no action is taken to cut them. Climatologists estimate this scenario would be double the current concentrations of greenhouse gases and would increase global temperatures by 2 to 4 degrees Celsius.

The reports' authors now warn that uncertainties exist in calculating the scenario and have constructed 40 new scenarios based around four equally realistic assumptions about world population and economic and technological advances.

Climate scientists recently came up with optimistic and pessimistic views on global warming.

In the previous single scenario, carbon dioxide emissions for 2100 were around 18 billion tons, or three times current annual emissions. Predictions from the new report give a range of emissions from 4.3 billion tons to 36.7 billion tons. The report also says that an increase in emissions could be avoided. Some scenarios involving a world population of 7 billion have higher carbon dioxide emissions than some with a global population of 15 billion.

The report also says a doubling of worldwide economic activity will not increase emissions either.

"There can be no best guess — the future is inherently unpredictable and views will differ on which of the scenarios could be more likely," the report states.

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