“There are not sufficient observations of the uptake of heat, particularly into the deep ocean, that will be one of the possible mechanisms that would explain this warming hiatus,” Stocker said.
However, the most controversial finding of the report was its “carbon budget.” Participants told the Guardian that this was the last part of the summary to be decided and the subject of hours of heated discussions in the early hours of Friday.
The scientists found that to hold warming to 2°C, total emissions cannot exceed 1,000 gigatonnes of carbon. Yet by 2011, more than half of that total “allowance” — 531 gigatonnes — had already been emitted.
To ensure the budget is not exceeded, governments and businesses may have to leave valuable fossil fuel reserves unexploited.
“There’s a finite amount of carbon you can burn if you don’t want to go over 2°C,” Stocker told the Guardian. “That implies if there is more than that [in fossil fuel reserves], that you leave some of that carbon in the ground.”