To prevent Earth’s average temperature from rising more than 2 degrees Celsius above preindustrial levels, several teams of researchers suggest that policymakers limit cumulative carbon emissions to no more than 1 trillion metric tons.

The researchers suggested the goal and a possible road map during an April 27 teleconference and in the April 30 Nature. The task is daunting because human activity has already exhausted more than half that allotment since the Industrial Revolution began. Human activity will likely emit the rest of that budget in just a few decades, even if emissions are held at the current rate.

The two-degree limit comes from the most recent report by the Intergovernmental Panel on Climate Change as a way to reduce the risks of severe impacts from a warming climate.

The recent studies may be the first to suggest a limit on the total amount of anthropogenic carbon and therefore carbon dioxide emissions, rather than a particular atmospheric concentration of the greenhouse gas. Establishing a ceiling for human emissions of carbon dioxide “actually makes the problem simpler than it’s often portrayed … because it treats emissions as an exhaustible resource,” says David Frame, an atmospheric scientist at the University of Oxford and a coauthor of the Nature papers. “If you burn a ton of carbon today, then you can’t burn it tomorrow.”

During the past century, the global average temperature rose about 0.74 degrees C (SN: 2/10/07, p. 83). That increase, IPCC scientists say with 90 percent certainty, is linked to the rising concentrations of planet-warming carbon dioxide and other human-emitted greenhouse gases in the atmosphere. Carbon dioxide levels now sit above 380 parts per million (SN: 5/10/08, p. 18) and are rising about 2 ppm each year; before the Industrial Revolution, atmospheric concentrations of the gas averaged about 280 ppm.

Pinning down the precise relationship between global average temperature and the level of CO2 in the atmosphere is difficult, says Myles Allen, a climatologist at the University of Oxford in England and coauthor on the new Nature papers. While some researchers have suggested dire effects on climate if CO2 levels rise above 550 ppm, others have more recently hinted that 350 ppm — a threshold already passed — should be the ultimate target, he noted at the teleconference. Regardless of the level that’s chosen, he adds, the concentration never stabilizes. Levels rise and fall about 7 ppm each year as growing seasons come and go.

However, analyses suggest that there’s “a simple and predictable relation between the total amount of carbon we inject into the atmosphere and the peak projected warming in response,” Allen noted at the teleconference.

“If you want to limit the risk of exceeding 2 degrees C global warming to one in four, or 25 percent, then total CO2 emissions over the first half of the 21st century have be kept below 1,000 billion tons,” said Malte Meinshausen, a climatologist at the Potsdam Institute for Climate Impact Research in Germany and another coauthor. That level of emissions sounds substantial but actually isn’t: Between 2000 and 2006, human activities emitted about 236 billion tons of carbon dioxide, the researchers estimate. “Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming,” Meinshausen says.

Writing in a commentary in the same Nature issue, Gavin Schmidt of NASA’s Goddard Institute for Space Studies in New York City and David Archer of the University of Chicago suggest that, “unless emissions begin to decline very soon, severe disruption to the climate system will entail expensive adaptation measures and may eventually require cleaning up the mess by actively removing CO2 from the atmosphere.”

The researchers also estimate that limiting cumulative carbon dioxide emissions between now and 2050 to no more than 1 trillion tons would actually leave three-fourths of the world’s known reserves of oil, gas and coal in the ground unburned — unless techniques for capturing and sequestering carbon dioxide underground rather than dumping it into the atmosphere become commonplace in the future.

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