The global carbon budget is really the balance between what animals emit – in this context, the word animals includes humans with cars and aeroplanes and factories – and what plants and algae can absorb. So the calculations are bedevilled by uncertainties about forests, grasslands and oceans.
To make things simpler, climate scientists translate the target into the billions of tons of carbon dioxide that, ideally, may be released into the atmosphere from 2015 onwards. Even these, however, are estimates.
There is general agreement that a limit of 590 bn tons would safely keep the world from overheating in ways that would impose ever greater strains on human society. The argument is about the upper limit of such estimates.
Dr Rogelj says: “In order to have a reasonable chance of keeping global warming below 2C, we can only emit a certain amount of carbon dioxide, ever. That’s our carbon budget.
“This has been understood for about a decade, and the physics behind this concept are well understood, but many different factors can lead to carbon budgets that are either slightly smaller or slightly larger. We wanted to understand these differences, and to provide clarity on the issue for policymakers and the public.
“This study shows that, in some cases, we have been overestimating the budget by 50 to more than 200%. At the high end, this is a difference of more than 1,000 billion tons of carbon dioxide.”
The same study takes a closer look at why estimates of the “safe” level of emissions have varied so widely.
One complicating factor has been, of course, uncertainty about what humans might do, and another has been about the other more transient greenhouse gases, such as methane and the oxides of nitrogen.
Although short-lived and released in smaller quantities, some of these are potentially far more potent than carbon dioxide as an influence on planetary temperatures.