Impact of short-lived non-CO₂mitigation on carbon budgets for stabilizing global warming

Limiting global warming to any level requires limiting the total amount of CO₂emissions, or staying within a CO₂budget. Here we assess how emissions from short-lived non-CO₂species like methane, hydrofluorocarbons (HFCs), black-carbon, and sulphates influence these CO₂budgets. Our default case, which assumes mitigation in all sectors and of all gases, results in a CO₂budget between 2011-2100 of 340 PgC for a >66% chanceofstaying below 2°C, consistent with the assessment of the Fifth Assessment Report of the Intergovernmental Panelon Climate Change. Extreme variations of air-pollutant emissions from black-carbon and sulphates influence this budgetby about ±5%. Inthe hypothetical case of nomethane or HFCs mitigation-whichis unlikely when CO₂is stringently reduced-the budgets wouldbe much smaller (40%or upto 60%, respectively). However, assuming very stringent CH₄mitigationasa sensitivity case, CO₂budgets could be25% higher. Alimit on cumulative CO₂emissions remains critical for temperature targets. Evena25% higher CO₂budget still means peaking global emissions in the next two decades, and achieving net zero CO₂emissions during the third quarter of the 21st century. The leverage we have to affect the CO₂budget by targeting non-CO₂diminishes strongly along with CO₂mitigation, because these are partly linked through economic and technological factors.