Editing IPCC:AR6/SYR/SPM (section)

=== Carbon Budgets and Net Zero Emissions ===

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'''B.5 Limiting human-caused global warming requires net zero CO '''''2''''' emissions. Cumulative carbon emissions until the time of reaching net zero CO '''''2''''' emissions and the level of greenhouse gas emission reductions this decade largely determine whether warming can be limited to 1.5°C or 2°C '''''(high confidence)''''' . Projected CO '''''2''''' emissions from existing fossil fuel infrastructure without additional abatement would exceed the remaining carbon budget for 1.5°C (50%) '''''(high confidence)''''' . Links to longer report 2.3, 3.1, 3.3, Table 3.1'''
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B.5.1 From a physical science perspective, limiting human-caused global warming to a specific level requires limiting cumulative CO '''2''' emissions, reaching at least net zero CO '''2''' emissions, along with strong reductions in other greenhouse gas emissions. Reaching net zero GHG emissions primarily requires deep reductions in CO '''''2''''' , methane, and other GHG emissions, and implies net-negative CO '''2''' emissions [[#footnote-018|39]] . Carbon dioxide removal (CDR) will be necessary to achieve net-negative CO '''''2''''' emissions (see B.6). Net zero GHG emissions, if sustained, are projected to result in a gradual decline in global surface temperatures after an earlier peak. ''(high confidence) Links to longer report 3.1.1, 3.3.1, 3.3.2, 3.3.3, Table 3.1, Cross-Section Box.1''

B.5.2 For every 1000 GtCO '''2''' emitted by human activity, global surface temperature rises by 0.45°C (best estimate, with a ''likely'' range from 0.27°C to 0.63°C). The best estimates of the remaining carbon budgets from the beginning of 2020 are 500 GtCO '''2''' for a 50% likelihood of limiting global warming to 1.5°C and 1150 GtCO '''2''' for a 67% likelihood of limiting warming to 2°C [[#footnote-017|40]] . The stronger the reductions in non-CO '''2''' emissions the lower the resulting temperatures are for a given remaining carbon budget or the larger remaining carbon budget for the same level of temperature change [[#footnote-016|41]] . Links to longer report 3.3.1

B.5.3 If the annual CO '''2''' emissions between 2020-2030 stayed, on average, at the same level as 2019, the resulting cumulative emissions would almost exhaust the remaining carbon budget for 1.5°C (50%), and deplete more than a third of the remaining carbon budget for 2°C (67%). Estimates of future CO '''2''' emissions from existing fossil fuel infrastructures without additional abatement [[#footnote-015|42]] already exceed the remaining carbon budget for limiting warming to 1.5°C (50%) ''(high confidence)'' . Projected cumulative future CO '''2''' emissions over the lifetime of existing and planned fossil fuel infrastructure, if historical operating patterns are maintained and without additional abatement [[#footnote-014|43]] , are approximately equal to the remaining carbon budget for limiting warming to 2°C with a likelihood of 83% [[#footnote-013|44]] ''(high confidence)'' . Links to longer report 2.3.1, 3.3.1, Figure 3.5

B.5.4 Based on central estimates only, historical cumulative net CO '''2''' emissions between 1850 and 2019 amount to about four-fifths [[#footnote-012|45]] of the total carbon budget for a 50% probability of limiting global warming to 1.5°C (central estimate about 2900 GtCO '''2''' ), and to about two thirds [[#footnote-011|46]] of the total carbon budget for a 67% probability to limit global warming to 2°C (central estimate about 3550 GtCO '''2''' ). Links to longer report 3.3.1, Figure 3.5

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