To prevent Earth's average temperature from rising more than 2 degrees Celsius above pre-industrial levels, several teams of researchers say that cumulative carbon emissions must be limited to no more than 1 trillion metric tons. The findings, released April 30 in the journal Nature, are 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 target to reduce the impacts of climate change.

The ceiling for carbon dioxide emissions "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. That increase, IPCC scientists say with 90% certainty, is linked to the rising concentrations of carbon dioxide and other greenhouse gases. Carbon dioxide levels now sit above 380 parts per million and are rising about 2 ppm each year; before the Industrial Revolution, atmospheric concentrations of the gas averaged about 280 ppm.

"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 co-author. "Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming."

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 state 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. (Science News, April 29)

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