They say emissions in the year 2100 could be five times as high as today's, twice the level previously predicted. Or, just possibly, they could be slightly lower than today's. The biggest unknown is how technology to minimise emissions will develop.

The warning comes in a draft special report on emissions scenarios by the UN's Intergovernmental Panel on Climate Change (IPCC). Due for publication in March, it will represent the consensus view of hundreds of scientists and economists.

Past reports from the IPCC have constructed a central "business-as-usual" scenario, representing likely emissions of greenhouse gases over the coming century if governments do nothing to cut them.

Climatologists estimated that this scenario would double the concentration of greenhouse gases in the atmosphere and increase global temperatures by between 2 and 4 �C.

The report's authors, chaired by Nebojsa Nakicenovic of the International Institute for Applied Systems Analysis in Austria, now warn that the uncertainties are such that calculating a business-as-usual scenario is not possible.

They have constructed 40 new scenarios based around four equally realistic assumptions about world population and economic and technological advances.

The previous single business-as-usual scenario put carbon dioxide emissions in the year 2100 at around 18 billion tonnes, or three times current annual emissions.

The new report includes predictions that range from 4.3 billion tonnes to 36.7 billion tonnes. The main scenarios from the four projected worlds range from 6 billion tonnes to 29 billion tonnes.

"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 says.

The authors say that their work "extends the range significantly towards higher emissions", which they say are more likely than suggested previously by the IPCC. But the good news is that soaring emissions are not inevitable.

Some scenarios involving a world population of 7 billion have higher CO2 emissions than some with a world population of 15 billion. And a doubling of global economic activity need not increase emissions.

The draft says: "Technology is at least as important a driving force of future greenhouse gas emissions as population and economic development."

Environmentalists have been quick to criticise the report for accepting the likelihood of high emissions. Aubrey Meyer of the London-based Global Commons Institute says emissions of the kind being suggested would trigger massive climate disruption, flooding of major cities and the death of the world's forests.

This article will appear in the September 18 issue of New Scientist New Scientist. Copyright 1999 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

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