Editing IPCC:AR6/WGI/TS (section)

==== TS.3.3.3 Relating Different Forcing Agents ====

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'''When including other GHGs, the choice of emissions metric affects the quantification of net zero GHG emissions and their resulting temperature outcome (''high confidence''). Reaching and sustaining net zero GHG emissions typically leads to a peak and decline in temperatures when quantified with the global warming potential over a 100-year period (GWP-100). Carbon-cycle responses are more robustly accounted for in emissions metrics compared to AR5 (''high confidence''). New emissions metric approaches can be used to generate equivalent cumulative emissions of CO <sub>2</sub> for short-lived greenhouse gases based on their rate of emissions. Links to chapters 7.6.2''' '''Over 10- to 20-year time scales, the temperature response to a single year’s worth of current emissions of short-lived climate forcers (SLCFs) is at least as large as that of CO <sub>2</sub> , but because the effects of SLCFs decay rapidly over the first few decades after emission, the net long-term temperature response to a single year’s worth of emissions is predominantly determined by cumulative CO <sub>2</sub> emissions.'''

'''Emissions reductions in 2020 associated with COVID-19 containment led to small and positive global ERF; however, global and regional climate responses to the forcing are undetectable above internal variability due to the temporary nature of emissions reductions. Links to chapters 6.6, Cross-Chapter Box 6.1'''

The relative climate effects of different forcing agents are typically quantified using emissions metrics that compare the effects of an idealised pulse of 1 kg of some climate forcing agent against a reference climate forcing agent, almost always CO <sub>2</sub> . The two most prominent pulse emissions metrics are the global warming potential (GWP) and global temperature change potential (GTP) (see Glossary). The climate responses to CO <sub>2</sub> emissions by convention include the effects of warming on the carbon cycle, so for consistency these also need to be determined for non-CO <sub>2</sub> emissions. The methodology for doing this has been placed on a more robust scientific footing compared to AR5 (''high confidence''). Methane from fossil fuel sources has slightly higher emissions metric values than those from biogenic sources since it leads to additional fossil CO <sub>2</sub> in the atmosphere (''high confidence''). Updates to the chemical adjustments for CH <sub>4</sub> and N <sub>2</sub> O emissions (Section TS.3.1) and revisions in their lifetimes result in emissions metrics for GWP and GTP that are slightly lower than in AR5 (''medium confidence''). Emissions metrics for the entire suite of GHGs assessed in the AR6 have been calculated for various time horizons. Links to chapters 7.6.1, Table 7.15, Table 7.SM.7

New emissions metric approaches, such as GWP* and Combined-GTP (CGTP), relate changes in the emissions rate of short-lived greenhouse gases to equivalent cumulative emissions of CO <sub>2</sub> (CO <sub>2</sub> -e). Global surface temperature response from aggregated emissions of short-lived greenhouse gases over time is determined by multiplying these cumulative CO <sub>2</sub> -e by TCRE (see Section TS.3.2.1). When GHGs are aggregated using standard metrics such as GWP or GTP, cumulative CO <sub>2</sub> -e emissions are not necessarily proportional to future global surface temperature outcomes (''high confidence'') Links to chapters 7.6.1, Box 7.3

Emissions metrics are needed to aggregate baskets of gases to determine net zero GHG emissions. Generally, achieving net zero CO <sub>2</sub> emissions and declining non-CO <sub>2</sub> radiative forcing would halt human-induced warming. Reaching net zero GHG emissions quantified by GWP-100 typically leads to declining temperatures after net zero GHGs emissions are achieved if the basket includes short-lived gases, such as CH <sub>4</sub> . Net zero GHG emissions defined by CGTP or GWP* imply net zero CO <sub>2</sub> and other long-lived GHG emissions and constant (CGTP) or gradually declining (GWP*) emissions of short-lived gases. The warming evolution resulting from net zero GHG emissions defined in this way corresponds approximately to reaching net zero CO <sub>2</sub> emissions, and would thus not lead to declining temperatures after net zero GHG emissions are achieved but to an approximate temperature stabilization (''high confidence''). The choice of emissions metric hence affects the quantification of net zero GHG emissions, and therefore the resulting temperature outcome of reaching and sustaining net zero GHG emissions levels (''high confidence''). Links to chapters 7.6.1.4, 7.6.2, 7.6.3

As pointed out in AR5, ultimately, it is a matter for policymakers to decide which emissions metric is most applicable to their needs. This Report does not recommend the use of any specific emissions metric, as the most appropriate metric depends on the policy goal and context (see Chapter 7, [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]]). A detailed assessment of GHG metrics to support climate change mitigation and associated policy contexts is provided in the WGIII contribution to the AR6.

The global surface temperature response following a climate change mitigation measure that affects emissions of both short- and long-lived climate forcers depends on their lifetimes, their ERFs, how fast and for how long the emissions are reduced, and the thermal inertia in the climate system. Mitigation, relying on emissions reductions and implemented through new legislation or technology standards, implies that emissions reductions occur year after year. Global temperature response to a year’s worth of current emissions from different sectors informs about the mitigation potential (Figure TS.20). Over 10- to 20-year time scales, the influence of SLCFs is at least as large as that of CO <sub>2</sub> , with sectors producing the largest warming being fossil fuel production and distribution, agriculture, and waste management. Because the effects of the SLCFs decay rapidly over the first few decades after emission, the net long-term temperature effect from a single year’s worth of current emissions is predominantly determined by CO <sub>2</sub> . Fossil fuel combustion for energy, industry and land transportation are the largest contributing sectors on a 100-year time scale (''high confidence''). Current emissions of CO <sub>2</sub> , N <sub>2</sub> O and SLCFs from East Asia and North America are the largest regional contributors to additional net future warming on both short (''medium confidence'') and long time scales (10 and 100 years, respectively) (''high confidence''). Links to chapters 6.6.1, 6.6.2, Figure 6.16

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'''Figure TS.20 |''' '''Global surface temperature change 10 and 100 years after a one-year pulse of present-day emissions.''' ''The intent of this figure is to show the sectoral contribution to present-day climate change by specific climate forcers, including carbon dioxide (CO'' 2 '') as well as short-lived climate forcers (SLCFs).'' The temperature response is broken down by individual species and shown for total anthropogenic emissions (top) , and sectoral emissions on 10-year (left) and 100-year time scales (right) . Sectors are sorted by (high-to-low) net temperature effect on the 10-year time scale. Error bars in the top panel show the 5–95% range in net temperature effect due to uncertainty in radiative forcing only (calculated using a Monte Carlo approach and best estimate uncertainties from the literature). Emissions for 2014 are from the Coupled Model Intercomparison Project Phase 6 (CMIP6) emissions dataset, except for hydrofluorocarbons (HFCs) and aviation H 2 O, which rely on other datasets (see Section 6.6.2 for more details). CO <sub>2</sub> emissions are excluded from open biomass burning and residential biofuel use. Links to chapters 6.6.2, Figure 6.16

COVID-19 restrictions led to detectable reductions in global anthropogenic emissions of nitrogen oxides (NO x) (about 35% in April 2020) and fossil CO <sub>2</sub> (7%, with estimates ranging from 5.8% to 13.0%), driven largely by reduced emissions from the transportation sector (''medium confidence''). There is ''high confidence'' that, with the exception of surface ozone, reductions in pollutant precursors contributed to temporarily improved air quality in most regions of the world. However, these reductions were lower than what would be expected from sustained implementation of policies addressing air quality and climate change (''medium confidence''). Overall, the net global ERF from COVID-19 containment was likely small and positive for 2020 (with a temporary peak value less than 0.2 W m <sup>–2</sup>), thus temporarily adding to the total anthropogenic climate influence, with positive forcing (warming influence) from aerosol changes dominating over negative forcings (cooling influence) from CO <sub>2</sub> , NO ''x'' and contrail cirrus changes. Consistent with this small net radiative forcing, and against a large component of internal variability, Earth system models show no detectable effect on global or regional surface temperature or precipitation (''high confidence''). Links to chapters Cross Chapter Box 6.1

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