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Professor John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany told The Guardian recently that even a small increase in temperature could trigger several climatic tipping points.
Unfortunately, though, the model shows that the Earth's temperature will be 2.6 degrees higher in 2100 than in pre-industrial times and, worse, it will still be rising. More sophisticated models indicate that there may be only a fifty-fifty chance of not exceeding the 2 degree target at the 450 ppm concentration. The Stern Review produced for the British government put the chance of staying below it at only 22 per cent.
This uncertainty about the temperature rise is inevitable since the atmosphere is a complex system and, as with all complex systems, it is impossible to state categorically how it will behave as a result of a change to one component, such as the concentration of a single gas. All climate modellers can hope to do, therefore, is to estimate the probability that the temperature rise will stay below the 2 degree figure at various concentrations. The lower the concentration, the higher the probability of staying below the limit will be.
It's the uncertainty that makes the 450 target unacceptable. The risks of exceeding a 2 degree rise are too great. As the Australian ecologist Philip Sutton says, we "wouldn't fly in a plane that had more than a 1 per cent chance of crashing. We should be at least as careful with the planet."
So let's go back to the C-LEARN model and plug in a 99 per cent drop in fossil emissions by 2050, a complete cessation of deforestation and a massive tree-planting drive to take up a billion tonnes of carbon a year (that's 3.7 billion tonnes of CO2, about a seventh of current fossil fuel emissions) from the air and see if we can hit the 2 degree target at the end of the century. The answer is that we can and we can also achieve an atmospheric concentration of 356ppmv by then. This is about 8 per cent less than the current concentration and the fall would continue as the years passed, bringing the temperature down to nearer its current level.
The fast-growing miscanthus, or elephant grass, can produce a high yield of biomass fuel
Rather than 450 ppmv a target of 350 ppmv or below is advocated by the smaller, poorer nations, by James Hansen and by the UN's chief climate scientist, Rajendra Pachauri. "As chairman of the Intergovernmental Panel on Climate Change, I cannot take a position because we do not make recommendations," Pachauri told an AFP journalist this August when asked if he supported the 350 limit, “but as a human being I am fully supportive of that goal. What is happening, and what is likely to happen, convinces me that the world must be really ambitious and very determined at moving toward a 350 target." He added later in the interview that “things are going to get substantially worse than what we had anticipated."
When I spoke to Pachauri in Dublin in 2007, he told me that an increase of 1.5 degree C might be the upper limit of what is tolerable but I've been unable to find an estimate of the chances that 350 ppm would achieve that. Hansen supports the figure only as an interim target which would be re-assessed in the light of new data. He points out that the Arctic sea-ice began to melt decades ago and his presentations suggest that 300-325 ppm might be necessary for the ice to increase again.
Other scientists think that almost any increase over the pre-industrial level might be too much. Professor John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany, told The Guardian recently that even a small increase in temperature could trigger several climatic tipping points.
“Nobody can say for sure that 330ppm is safe,’ he said. “Perhaps it will not matter whether we have 270ppm or 320ppm, but operating well outside the [historic] realm of carbon dioxide concentrations is risky as long as we have not fully understood the relevant feedback mechanisms.”
What all this means is that, for there to be any chance of maintaining a livable planet, fossil fuel emissions have to be reduced extremely rapidly and there needs to be a massive programme to use plants and the soils to get CO2 out of the air. The Copenhagen meeting will be a failure if it does not make it clear that both need to happen and fails to get time-tabled undertakings from governments to bring them about.
Extracting and burying some of the CO2 in the air is likely to prove the easier task, as it only takes a relatively small increase in the amount of carbon held in the soil and the plants growing on it to achieve the desired result. The UN Environmental Programme estimates that terrestrial ecosystems store about 600 billion tonnes of carbon in living organisms and decaying material and 1,500 billion tonnes in soil organic matter. The total, 2,100 billion tonnes, is almost three times the amount currently in the atmosphere. Consequently, reducing the atmospheric concentration of CO2 from 387 to 350 ppmv only involves increasing the amount in plants and soils by 3 per cent. Returning to a really safe level demands a 9 per cent increase.
Another encouraging fact is that each year's flow of carbon into and out of the terrestrial stock is huge. It is only necessary to reduce the outflow and/or increase the inflow by a small amount to achieve the 9 per cent increase over, say, the next fifty years. These are the figures: each year, plants take in roughly 120 billion tonnes of carbon from the air each year – about 19 times the amount released from fossil fuels – but about half of the amount they capture is released again when they respire at night, and a similar amount when they burn or decay.
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Issue 1, Vol 5 Out Now
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