HOW much more carbon dioxide can the atmosphere absorb before it triggers catastrophic climate change? That is the key question that climatologists working with the Intergovernmental Panel on Climate Change (IPCC) will be addressing in the coming months.
In Berlin this week, the US and a few other industrialised countries are defending their reluctance to commit themselves to future reductions in their emissions of greenhouse gases on the grounds that the scientists cannot yet offer an answer.
Legally, under the terms of the Climate Change Convention, they have a case. The signatories to the convention have agreed to stabilise greenhouse gases at concentrations 鈥渢hat would avoid dangerous anthropogenic interference with the climate system鈥�. The problem, the IPCC says, is that 鈥渋t is not yet possible to define what such an objective means in quantitative terms鈥�.
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Instead, IPCC chairman Bert Bolin told the meeting of signatories in Berlin that in its Second Assessment Report later this year, the IPCC would 鈥渧iew the issue in terms of an increased risk for damage鈥�. In essence, they will try to place bookmakers鈥� odds on various calamities.
This was reasonable, he said, because the convention requires signatories to 鈥渢ake precautionary measures鈥� and that 鈥渨here there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing such measures鈥�.
The basic numbers in the carbon conundrum are simple (see Diagram). About 200 years ago, before humans began to interfere seriously in the composition of the atmosphere, there were about 590 billion tonnes of carbon in the atmosphere in the form of CO2. There are now 760 billion tonnes, with most of the increase coming in the past 50 years. This change has increased the total solar energy absorbed by the Earth by 1 per cent.
Most current climate models predict what will happen when the concentration of CO2 in the atmosphere reaches twice preindustrial levels, or some 1.2 trillion tonnes of carbon. They suggest a world that is between 1.5 and 4.5 掳C warmer. But some regions would suffer worse climatic disruption, and there might be 200 000 deaths a year attributable to extra storms, droughts and heat waves.
And warming will not stop there. The IPCC鈥檚 鈥渂usiness as usual鈥� scenarios predict that there will be around 1.5 trillion tonnes of carbon in the air by the year 2100. And if all the known reserves of coal, oil and gas are burnt, the figure will eventually rise to more than 4 trillion tonnes. This assumes that the oceans and forests will carry on absorbing about 40 per cent of all the CO2 pushed out into the air.
Clearly, governments will have to call a halt at some point, and stabilise the amount of carbon in the atmosphere. Jeremy Leggett, a geologist who was once an oil industry consultant and now runs Greenpeace鈥檚 climate campaign, concludes that 鈥渨e have no option but to leave the vast majority of the remaining fossil-fuel reserves in the ground鈥�.
The IPCC has mapped out possible futures in which CO2 levels would be stabilised at anything from current levels to 1.6 trillion tonnes, to be reached at various times over the next 200 years. This year, it will try to assess the likely risk that we run from each scenario. The question, says Leggett, is 鈥淒o we feel lucky?鈥�.
John Houghton, chairman of the IPCC鈥檚 science working group, lends his personal support to the idea of stabilising CO2 in the atmosphere at twice preindustrial values, or around 1.2 trillion tonnes, by the end of the next century. The World Energy Council, an international industry body, says this could be done.
Houghton suggests this target could be met if developed countries cut their emissions by 20 per cent over the coming 25 years, while developing countries double theirs, to allow room for population growth and economic development. After that, serious cuts would be required to reduce global emissions to less than half today鈥檚 levels by 2100. This would probably be achieved mostly by switching to alternative forms of energy such as solar power.
This week, ministers in Berlin will have their first chance to embark on such a course by agreeing to start negotiations on making future reductions.
