Impact of short-lived non-CO 2 mitigation on carbon budgets for stabilizing global warming

Limiting global warming to any level requires limiting the total amount of CO 2 emissions, or staying within a CO 2 budget. Here we assess how emissions from short-lived non-CO 2 species like methane, hydrofluorocarbons (HFCs), black-carbon, and sulphates influence these CO 2 budgets. Our default case, which assumes mitigation in all sectors and of all gases, results in a CO 2 budget between 2011-2100 of 340 PgC for a >66% chance of staying below 2C, consistent with the assessment of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Extreme variations of air-pollutant emissions from black-carbon and sulphates influence this budget by about 5%. In the hypothetical case of no methane or HFCs mitigation-which is unlikely when CO 2 is stringently reduced-the budgets would be much smaller (40% or up to 60%, respectively). However, assuming very stringent CH 4 mitigation as a sensitivity case, CO 2 budgets could be 25% higher. A limit on cumulative CO 2 emissions remains critical for temperature targets. Even a 25% higher CO 2 budget still means peaking global emissions in the next two decades, and achieving net zero CO 2 emissions during the third quarter of the 21st century. The leverage we have to affect the CO 2 budget by targeting non-CO 2 diminishes strongly along with CO 2 mitigation, because these are partly linked through economic and technological factors.