International negotiators at a United Nations-sponsored climate conference ending today in Bangkok repeatedly underscored the goal of keeping the amount of global warming in this century to no more than 2˚C. But results from a Canadian government climate modeling study published last month suggest that “it is unlikely that warming can be limited to the 2˚C target,” the scientists who wrote the study say.

The paper finds that reaching that goal would require that greenhouse emissions “ramp down to zero immediately” and that scientists deploy means, starting in 2050, to actively remove greenhouse gases from the atmosphere. Previous modeling efforts have already highlighted the difficulty of reaching the 2˚C goal. But the new study is unique in several ways. Most important, it relies on the first published results from the latest generation of so-called Earth System climate models, complex programs that run on supercomputers and seek to simulate the planet’s oceans, land, ice, and atmosphere. The model in this study, Canadian Earth System Model 2, also incorporates updated data on volcanic eruptions, and it simulates in a more sophisticated way the biosphere’s ability to take in or emit carbon.

In the study, scientists with Environment Canada, a government agency, fed their model various scenarios of future greenhouse gas concentrations out to the year 2100. In the scenario with the most carbon emissions, the concentration of CO2 in the atmosphere skyrocketed from its current level, about 390 parts per million, to 920 ppm, and global land surface temperature rose by 4.9˚C above 2005 levels. But even in a scenario in which emissions cuts caused CO2 levels to peak at 450 ppm in 2050, temperatures rose by 2.3˚C by the end of the century, above the 2˚C goal.

In one figure in the paper, the highest-emissions pathway was depicted with an orange line, with the lowest-emissions line in blue. “In terms of emissions, right now we’re more likely on the orange line than on the blue, “ said co-author Ken Denman, an oceanographer at the University of Victoria in Canada who is affiliated with Environment Canada. Much higher temperatures may await humanity if emissions aren’t reduced quickly, and the difficulty of reaching the 2˚C goal may be irrelevant, he says.

Climate modeler Myles Allen of the University of Oxford in the United Kingdom says that the paper’s findings suggest a way to buy time while we reduce CO2 emissions: cut emissions of short-lived gases that warm the atmosphere aggressively but persist in the atmosphere for decades instead of centuries, like CO2. “I wouldn’t see this as hopeless,” he says. “Methane, for example, is relatively short-lived. We have time to invent the technologies required ... to deal with it, in contrast to CO2.”

As for the paper’s conclusion that removing atmospheric carbon is necessary in order to achieve the 2˚C target, climate scientist Richard Moss of the Pacific Northwest National Laboratory's Joint Global Change Research Institute in College Park, Maryland, says that’s a nearly impossible goal “with what we know about today.” But later in the century, carbon-removing techniques, such as burning biofuels while capturing their carbon emissions or developing carbon-sucking machines, may be feasible.

“We can’t give up” on emissions cuts, says Denman, although it’s “probably already too late” to limit warming to 2˚C. “But maybe we’ll have to live with 3 or 4 degrees of warming.”

by Eli Kintisch on 8 April 2011, 6:02 PM