Editing IPCC:AR6/WGI/Chapter-5 (section)

=== Remaining Carbon Budgets to Climate Stabilization ===

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'''There is a near-linear relationship between cumulative CO''' <sub>2</sub> '''emissions and the increase in global mean surface air temperature (GSAT) caused by CO''' <sub>2</sub> '''over the course of this century for global warming levels up to at least 2°C relative to pre-industrial''' ( ''high confidence'' ''').''' Halting global warming would thus require global net anthropogenic CO <sub>2</sub> emissions to become zero. The ratio between cumulative CO <sub>2</sub> emissions and the consequent GSAT increase, which is called the transient climate response to cumulative emissions of CO <sub>2</sub> (TCRE), ''likely'' falls in the 1.0°C–2.3°C per 1000 PgC range. The narrower range compared to the IPCC Fifth Assessment Report (AR5) is due to a better integration of evidence across the science in this assessment. Beyond this century, there is ''low confidence'' that the TCRE remains an accurate predictor of temperature changes in scenarios of very low or net negative CO <sub>2</sub> emissions because of uncertain Earth system feedbacks that can result in further warming or a path-dependency of warming as a function of cumulative CO <sub>2</sub> emissions. {5.4, 5.5.1}

'''Mitigation requirements over this century for limiting maximum warming to specific levels can be quantified using a carbon budget that relates cumulative CO''' <sub>2</sub> '''emissions to global mean temperature increase''' ( ''high confidence'' ''').''' For the period 1850–2019, a total of 655 ± 65 PgC (2390 ± 240 GtCO <sub>2</sub> , ''likely'' range) of anthropogenic CO <sub>2</sub> has been emitted. Remaining carbon budgets (starting from 1 January 2020) for limiting warming to 1.5°C, 1.7°C, and 2.0°C are 140 PgC (500 GtCO <sub>2</sub> ), 230 PgC (850 GtCO <sub>2</sub> ) and 370 PgC (1350 GtCO <sub>2</sub> ), respectively, based on the 50th percentile of TCRE. For the 67th percentile, the respective values are 110 PgC (400 GtCO <sub>2</sub> ), 190 PgC (700 GtCO <sub>2</sub> ) and 310 PgC (1150 GtCO <sub>2</sub> ). These remaining carbon budgets may vary by an estimated ± 60 PgC (220 GtCO <sub>2</sub> ) depending on how successfully future non-CO <sub>2</sub> emissions can be reduced. Since AR5 and the Special Report on Global Warming of 1.5°C (SR1.5), estimates have undergone methodological improvements, resulting in larger, yet consistent estimates. {5.5.2, 5.6; Figure 5.31; Table 5.8}

'''Several factors affect the precise value of remaining carbon budgets, including estimates of historical warming, future emissions from thawing permafrost, and variations in projected non-CO''' <sub>2</sub> '''warming.''' Remaining carbon budget estimates can increase or decrease by 150 PgC ( ''likely'' range; 150 PgC equals 550 GtCO <sub>2</sub> ) due to uncertainties in the level of historical warming, and by an additional ± 60 PgC (±220 GtCO, ''likely'' range) due to geophysical uncertainties surrounding the climate response to non-CO <sub>2</sub> emissions such as CH <sub>4</sub> , N <sub>2</sub> O, and aerosols. Permafrost thaw is included in the estimates, together with other feedbacks that are often not captured by models. Despite the large uncertainties surrounding the quantification of the effects of additional Earth system feedback processes, such as emissions from wetlands and permafrost thaw, these feedbacks represent identified additional amplifying risk factors that scale with additional warming and mostly increase the challenge of limiting warming to specific temperature thresholds. These uncertainties do not change the basic conclusion that global CO <sub>2</sub> emissions would need to decline to at least net zero to halt global warming. {5.4, 5.5.2}

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