Editing IPCC:AR6/SR15/TS (section)

=== Future Emissions in 1.5°C Pathways ===

'''Mitigation requirements can be quantified using carbon budget approaches that relate cumulative CO<sub>2</sub> emissions to global mean temperature increase.''' Robust physical understanding underpins this relationship, but uncertainties become increasingly relevant as a specific temperature limit is approached. These uncertainties relate to the transient climate response to cumulative carbon emissions (TCRE), non-CO<sub>2</sub> emissions, radiative forcing and response, potential additional Earth system feedbacks (such as permafrost thawing), and historical emissions and temperature. {2.2.2, 2.6.1}

'''Cumulative CO<sub>2</sub> emissions are kept within a budget by reducing global annual CO<sub>2</sub> emissions to net zero. This assessment suggests a remaining budget of about 420 GtCO<sub>2</sub> for a two-thirds chance of limiting warming to 1.5°C, and of about 580 GtCO<sub>2</sub> for an even chance (''medium confidence'').''' The remaining carbon budget is defined here as cumulative CO<sub>2</sub> emissions from the start of 2018 until the time of net zero global emissions for global warming defined as a change in global near-surface air temperatures. Remaining budgets applicable to 2100 would be approximately 100 GtCO<sub>2</sub> lower than this to account for permafrost thawing and potential methane release from wetlands in the future, and more thereafter. These estimates come with an additional geophysical uncertainty of at least ±400 GtCO<sub>2</sub>, related to non-CO<sub>2</sub> response and TCRE distribution. Uncertainties in the level of historic warming contribute ±250 GtCO<sub>2</sub>. In addition, these estimates can vary by ±250 GtCO<sub>2</sub> depending on non-CO<sub>2</sub> mitigation strategies as found in available pathways. {2.2.2, 2.6.1}

'''Staying within a remaining carbon budget of 580 GtCO<sub>2</sub> implies that CO<sub>2</sub> emissions reach carbon neutrality in about 30 years, reduced to 20 years for a 420 GtCO<sub>2</sub> remaining carbon budget (''high confidence'').''' The ±400 GtCO<sub>2</sub> geophysical uncertainty range surrounding a carbon budget translates into a variation of this timing of carbon neutrality of roughly ±15–20 years. If emissions do not start declining in the next decade, the point of carbon neutrality would need to be reached at least two decades earlier to remain within the same carbon budget. {2.2.2, 2.3.5}

'''Non-CO<sub>2</sub> emissions contribute to peak warming and thus affect the remaining carbon budget. The evolution of methane and sulphur dioxide emissions strongly influences the chances of limiting warming to 1.5°C. In the near-term, a weakening of aerosol cooling would add to future warming, but can be tempered by reductions in methane emissions (''high confidence'').''' Uncertainty in radiative forcing estimates (particularly aerosol) affects carbon budgets and the certainty of pathway categorizations. Some non-CO<sub>2</sub> forcers are emitted alongside CO<sub>2</sub>, particularly in the energy and transport sectors, and can be largely addressed through CO<sub>2</sub> mitigation. Others require specific measures, for example, to target agricultural nitrous oxide (N2O) and methane (CH4), some sources of black carbon, or hydrofluorocarbons (''high confidence''). In many cases, non-CO<sub>2</sub> emissions reductions are similar in 2°C pathways, indicating reductions near their assumed maximum potential by integrated assessment models. Emissions of N2O and NH3increase in some pathways with strongly increased bioenergy demand. {2.2.2, 2.3.1, 2.4.2, 2.5.3}