But Dr Rogelj and his colleagues found that a significant cause of variation was simply a consequence of the different assumptions and methodologies inherent in such complex calculations.
So the researchers have re-examined both the options and the approaches, and have worked out a global figure that, they suggest, could be relevant to “real-world policy”.
It takes into account the consequences of all human activity, and it embraces detailed outlines of possible low-carbon choices. It also offers, they say, a 66% chance of staying within the internationally-agreed limit.
“We now better understand the carbon budget for keeping global warming below 2C,” Dr Rogelj says. “This carbon budget is very important to know because it defines how much carbon dioxide we are allowed to release into the atmosphere, ever.
“We have figured out that this budget is at the low end of what studies indicated before, and if we don’t start reducing our emissions immediately, we will blow it in a few decades.”