The usefulness of a fixed carbon budget to keep global warming below a prescribed temperature level is questioned in two Comment pieces published online this week in Nature Geoscience. The authors argue that instead of focusing on a single number, it would be more fruitful to focus on emissions ranges and political realities in climate policy discussions.

The concept that the level of warming ultimately only depends on total carbon emissions, and not on the evolution of emissions over time, was introduced in 2009 in the run-up to the Copenhagen Climate Summit. As a result, a carbon budget can be calculated for given temperature levels, such as the 1.5 and 2 °C targets written into the Paris Agreement.

In the first piece, Glen Peters argues that carbon budgets are of limited use for policymakers for two main reasons. First, their scale is inherently global, whereas policy decisions are made on a country-by-country scale; and second, they are highly uncertain, because of incomplete knowledge of the carbon cycle and because carbon budget estimates also depend on user and societal choices. He suggests that there is a need to openly discuss these uncertainties and focus on the goal to reach zero emissions in the second half of the century.

In the second Comment, Oliver Geden notes that since its introduction in 2009, the concept of carbon budgets has only influenced climate policy talk, but not climate policy actions. He suggests that a different framing of the communication from the climate scientists would be more aligned with initiating action under political realities. Instead of continuing with a narrative that it is ‘five minutes to midnight’, scientists could point out that achieving climate targets of 1.5 or 2 °C is implausible, unless stringent action is taken.