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No time to lose in cutting CO₂ emissions
02 March 2008
From New Scientist Print Edition.
Jim Giles

We should not wait to cut back on burning fossil fuels until we have developed greener technology to supply our energy needs, despite what many economists are advising their respective governments. Such a waiting game may have deadly consequences.

The US administration often objects to emissions cuts on the grounds that it is cheaper to delay until low-carbon technologies are available. Now a study by some of the most respected climate-policy researchers has quantified the impact of every year of delay. It concludes that reducing greenhouse gas concentrations in subsequent decades will be far harder than we thought - not that anyone thought it was going to be easy in the first place. What's more, the team says that current delays mean the world is virtually certain to overshoot the limits of greenhouse gas concentration advocated by the European Union and many environmental groups. "It's a sobering assessment of where we are," says Bryan Mignone, a climate policy expert at Princeton University.

Mignone and his colleagues examined how levels of greenhouse gases in the atmosphere will change if emissions increase, flatten out sometime later this century and then decline. Most policy experts assume this will happen, but the effect on global temperatures will be determined by two key unknowns: how long it will take before emissions peak, and how quickly will they fall thereafter.

Even immediate and drastic cuts will not prevent dangerous temperature rises, the researchers warn. The EU, for example, hopes to limit global temperature increases to 2 °C - a rise that could avoid the more catastrophic consequences of climate change. To achieve that, carbon dioxide concentrations in the atmosphere must stabilise at about 450 parts per million - levels are currently at about 380ppm. But to keep concentrations under that limit, Mignone's analysis shows, global emissions would have to peak within a decade and then keep falling at 3 per cent every year - a rate many consider unfeasible. Experts have little faith in such a turnaround: "Even if aggressive mitigation were to begin today, 450ppm appears to be virtually impossible," say the authors (Climatic Change, DOI: 10.1007/s10584-007-9391-8).

A limit of 550ppm is seen by some as more realistic, but Mignone says that we are already on the verge of ruling that out, too. And emissions are still increasing, so delays will become more and more costly in the future. At current rates of emissions, a five-year delay before the peak would lead to an increase of 34ppm in CO₂ levels. If the peak has still not been reached 40 years from now, a five-year span of emissions at that time would result in a further leap of 54ppm. Even if new technology means emissions fall faster, the concentration of CO₂ may already be too great by then for some stabilisation targets to be met. To keep levels below 550ppm, for instance, emissions should start falling much sooner, probably within 20 years, the study concludes.

"One message to take away is that if our ability to reduce emissions is constrained to less than 3 per cent per year, then mitigation had better start soon, very soon," says Roger Pielke, a climate policy expert at the University of Colorado in Boulder. "Another message is that effective mitigation requires that we explore options to more rapidly reduce emissions." Pielke points out that the new model ignores the possibility that future technologies may allow for more rapid cuts. Direct removal of CO₂ from the atmosphere - a process known as air capture - might provide one such solution. Mignone also notes that his model does not deal with complexities that could impact the results, such as the amount of carbon taken up by land.

For some experts, Mignone's analysis is simply another indication that we have already fumbled our chance to limit greenhouse gases to some pre-determined level. "We may be at the point at which the luxury of choosing a target has disappeared," says Hugh Pitcher, an emissions modeller at the University of Maryland in College Park.

From issue 2645 of New Scientist magazine, 02 March 2008, page 14-15

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