The latest report from the Intergovernmental Panel on Climate Change (IPCC) for the first time sets a cap on the amount of carbon emissions we can allow into the atmosphere before calling a complete and permanent halt — if, that is, we are serious about keeping global warming below two degrees Celsius. That cap was set last month at a trillion tons of carbon. We are something over half way there, with time fast running out.
Thus, according to the World Resources Institute, a Washington-based think tank, the U.S. has since 1850 grabbed three times more carbon space in the atmosphere than China, despite today having only a quarter of China’s population. And the U.K., the first industrialized country, has occupied 6 per cent of the taken space, despite having less than 1 percent of the world’s population.
To some government delegations in Stockholm — China and Brazil were most vocal — it looked like the rich nations were using science to legitimize a back-door cap on their future emissions. Knutti, who spent the five hours successfully arguing against attempts to water down his text, denies this. “The trillion tons is a carbon budget. How it is allocated we didn’t consider, because that is political. But of course some policymakers don’t like it.”
Leaving aside the politics for a moment, what does this mean for global emissions? A web site set up by climate researchers at Oxford University, England, is counting the tons in real time: www.trillionthtonne.org. The world has now emitted over 574 billion tons, it says. “Based on emission trends over the past 20 years, we expect the trillionth ton will be emitted on Sunday, 25 November, 2040 .” This presumes that the oceans and terrestrial ecosystems will continue to absorb roughly half of our emissions — a big assumption, many climate scientists believe. But it is the best guess we have.
According to the web site, to prevent the trillionth ton ever being emitted, our emissions have to fall by 2.47 percent a year, and to keep on falling at that rate until they reach zero.
If holding global warming below two degrees requires a trillion-ton cap, then the next question is how we might share out the remaining atmospheric space. And here things get really hard. Logically, there should be some sense of fair shares. Most people believe that should be based on population.
Meyer says we should ignore the past and start sharing the remaining space from today. An American geophysicist, Raymond Pierrehumbert of the University of Chicago, assumed a start date of 2000 and found that a fair shares approach would require North America to cease emissions completely by 2025.
But many developing nations, as was evident in Stockholm, do not think the past can be ignored.
In the end, political agreements will mean nothing unless there are practical means of achieving them. So what needs to be done? Some used to think we might run out of oil and other fossil fuels in time to stave off climate disaster. That no longer seems likely. Discoveries of unconventional reserves like shale gas and tar sands — and advances in technology for extracting them — have transformed predictions of how much fossil fuel we can tap.
There is at least another trillion tons of carbon lying in economically viable reserves of fossil fuels, and we have room for less than half that, says Allen. We have to prevent it from getting into the atmosphere. That, of course, means getting our energy in new ways.
Another option is to stick with some share of fossil fuels but to ensure that the carbon emissions are captured from the stack and put out of harm’s way — down old oil wells, coal mines or other geological spaces such as salt mines.
We have options, but the stakes are high. And the worst thing may be that there is no guarantee that keeping below the trillion-ton target will be enough. The IPCC admits it does not allow for other emissions of greenhouse gases, and particularly for the danger that the planet’s ecosystems might react to climate change in dangerous ways — for instance by bubbling methane out of melting Arctic permafrost.