| Summary: | 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 2°C, 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.
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