To avoid catastrophic climate change, the world will need to emit less than one trillion metric tons of carbon between now and 2050, according to two new papers published in Nature today. In other words, there is only room in the atmosphere to burn or vent less than one quarter of known oil, natural gas and coal reserves.

Atmospheric concentrations of carbon dioxide
have reached 386 parts per million—and rising, because every year, human activity spews more than 30 billion metric tons of CO2. So far, that's led to warming of roughly 0.8 degree Celsius (1.4 degrees Fahrenheit). The question is: How much more can we safely emit? The two new papers attempt an answer.

"There is a simple and predictable relationship between the total amount of carbon injected into the atmosphere and peak projected warming," says physicist Myles Allen of the University of Oxford in England and lead author of one of the studies. "Releasing a trillion [metric] tons of carbon into the atmosphere may cause a most likely peak warming of 2 degrees C [3.6 degrees F], which many identify as the danger point." An average temperature rise of 2 degrees C or lower has been adopted by the European Union and other countries—110 in all—as a goal for any treaty to control climate change, and has been identified by scientists, including the authors of the U.N. Intergovernmental Panel on Climate Change report, as a point at which most climate changes become damaging.

But CO2—and the carbon at its molecular core—is not the only greenhouse gas. Others—ranging from methane to hydrofluorocarbons (HFCs)—"could contribute as much as 10 to 40 percent of the warming induced by CO2 alone," says climatologist Malte Meinshausen of the Potsdam Institute for Climate Impact Research (PIK) in Germany, lead author of the second study appearing in Nature. That drops the overall budget for atmospheric emissions to roughly 750 billion metric tons of carbon between the years 2000 and 2050. "To limit the risk to a one-in-four chance [of 2 degree C warming], then total CO2 in the first half of the 21st century has to be kept below [one trillion] metric tons."

Put another way: humanity can only afford to burn and vent less than one quarter of known oil, natural gas and coal reserves. Already, between 2000 and 2006, the world emitted roughly 234 billion metric tons of CO2—and roughly one third of the total trillion metric ton "budget" has already been spent to date. "We can burn less than a quarter of known economically recoverable fossil fuel reserves between now and 2050," says co-author and climatologist William Hare, also of the PIK. "Not much at all of coal reserves can be burnt and still keep warming below the 2 degree [C] limit."

Meeting that target will require global emissions to peak and begin to decline before 2020—unless countries desire drastic cuts later in the century. "If you burn a [metric] ton of carbon today then you can't burn it tomorrow, you've got a finite stock," says co-author and physicist David Frame of Oxford. "How would you apportion that finite stock of carbon?" That is a decision the world's governments will have to make in coming months and years.

Global greenhouse gas emissions will need to be at least 50 percent below 1990 levels by 2050—that means cuts by industrialized countries such as the U.S. of more than 90 percent—and on a path to zero emissions. A market for carbon—charging emitters for the ability to pollute—could help. A $50 price for a metric ton of CO2 by 2020 might do the trick based on economic modeling, Frame says, allowing emissions to peak some 25 percent above 1990 levels. "After that the carbon price would need to progressively increase to push carbon emissions down globally to where you would stay within this budget," he adds. "The 80 percent reduction [by 2050 pledge] from the U.S. is a good start but it's not enough to limit warming in order to meet [the] 2 degree [C increase] with high confidence."

Of course, 2 degrees C warming—a further 1.2 degrees C from the current level of heating—will in and of itself have a host of unpleasant effects, such as ongoing sea-level rise that will swamp coastlines and island states. Since 1750 humanity has added 520 billion metric tons of greenhouse gases to the atmosphere and we're on pace to add that much again within 40 years—a scenario that is likely to result in catastrophic climate change.

"The longer we let [emissions] rise, the harder and costlier reductions become," Allen notes.