When human injection of carbon into the atmosphere reaches 1 trillion tons, dangerous climate change with average global warming of more than 2 Celsius degrees will likely occur, a new analysis finds.
And humans are hurrying toward that 1 trillion mark. So far, We’ve added about 520 billion tons of carbon to the atmosphere. With the addition of an estimated 9 billion tons of carbon a year — a number that’s been growing since 1850 — dangerous warming is likely to occur within half a century.
That’s the message from a new paper in the journal Nature, which — along with half a dozen other papers in the issue — provides a simpler way of looking at the climate change problem. What matters is the total amount of carbon that we release into the atmosphere, and focusing on that number as a budget can shape the way policymakers look at the problem, argues Myles Allen, lead author of one of the papers and a climatologist at the University of Oxford.
“The important thing about the cumulative budget is that a ton of carbon is a ton of carbon. If we release it now, it’s a ton we can’t release in 40 years’ time. Every ton we put out is using up a ton of that atmospheric capacity,” Allen told Wired.com. “Reducing emissions steadily over 50 years is much cheaper and easier and less traumatic than allowing them to rise for 15 years and then reducing them violently for 35 years.”
Previous climate change efforts have tried to find the correct “stabilization level” for which to aim. Policymakers would try to craft scenarios showing that the world’s people should aim to peg the concentration of carbon dioxide at 350 or 450 or 550 parts per million. Beyond the scientific complexity of finding what that number should be — which Myles called “a nightmare” — the esoteric nature of those numbers made the climate problem difficult to communicate to populations across the world.
Allen hopes his team’s new analysis, along with a similar paper lead-authored by Malte Meinshausen of the Potsdam Institute for Climate Impact Research in Germany, will let people look at the problem square on.
The numbers presented in their research are probabilistic. They look at different levels of carbon and carbon dioxide in the atmosphere and try to assign the likelihood that a certain emissions level would equate to a temperature change across the Earth. The two papers use different periods of analysis and base cases, but they are broadly consistent in their findings that it’s the total amount of carbon added to the atmosphere that will determine the peak warming of the globe.
Where Allen’s team found that adding 480 billion tons of carbon from here on out would push the risk of 2 degrees of warming to over 50 percent, Meinshausen’s team found even more alarming results. The German team estimates that 310 billion tons is all that would be needed. Without policy changes, that means humans would hit dangerous warming levels in 20 years (Meinshausen) to 40 years (Allen) .
“The bottom line? Dangerous change, even loosely defined, is going to be hard to avoid,” write Gavin Schmidt of the NASA Goddard Institute for Space Science and David Archer, a geoscientist at the University of Chicago, in an accompanying commentary in Nature. “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.”
Forcing emissions to decline will require changing the way the world uses fossil fuels. In Allen’s view, humans can pull a trillion tons of carbon-rich fossil fuels out of the ground and burn them with risks that have been deemed acceptable by most people. But it’s the second trillion tons of fossil fuels, largely in the form of coal and oil shale, that will determine how recklessly humans play with the climate system.
“From all the incredible arcane arguments that go on, in the end, it’s really a very simple question: what are we going to do with the second trillion tons?” Allen asked.
Fossil-fuel–reserve estimates vary. While it’s clear that there is a lot of coal and oil shale on Earth, there is intense debate over how much of that fossil fuel will be economical to mine. Allen’s group used the World Energy Council’s estimates, which show nearly 6 trillion tons of fossil fuels still left to be mined. Other scientists believe that fossil fuel reserves could be much lower.