The carbon budget might be smaller than you think: non-CO2 contributions to the quantification of remaining carbon budgets in line with the Paris Agreement

Carbon budgets are quantifications of the total amount of carbon dioxide that can ever be emitted while keeping global warming below specific temperature limits. They are widely used to interpret and inform climate change mitigation actions, be it by countries or corporates. Estimates of these budgets for limiting warming to 1.5 °C and well-below 2 °C include assumptions about how much warming can be expected from non-CO2 emissions. These assumptions, however, are often poorly understood by users of carbon budget information. In this study, we clarify the non-CO2 emissions assumptions that underlie the remaining carbon budget estimates by the Intergovernmental Panel on Climate Change and quantify the implication of the world pursuing alternative higher or lower non-CO2 emissions trajectories. We consider contributions of methane, nitrous oxide, fluorinated gases, and aerosols and show how pursuing inadequate methane emission reductions causes remaining carbon budgets compatible with the Paris Agreement temperature limits to be exhausted today. A decision not to reduce non-CO2 emissions hence effectively puts the achievement of the Paris Agreement out of reach.