Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.6.1.4 Comparing Long-lived with Short-lived Greenhouse Gases ====

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Since AR5 there have been developments in how to account for the different behaviours of short-lived and long-lived compounds. Pulse-based emissions metrics for short-lived GHGs with lifetimes less than 20 years are very sensitive to the choice of time horizon (e.g., [[#Pierrehumbert--2014|Pierrehumbert, 2014]] ). Global surface temperature changes following a pulse of CO <sub>2</sub> emission are roughly constant in time (the principle behind TCRE; [[IPCC:Wg1:Chapter:Chapter-5#5.5.1|Section 5.5.1]] and Figure 7.21b) whereas the temperature change following a pulse of short-lived GHG emission declines with time. In contrast to a one-off pulse, a step change in short-lived GHG emissions that is maintained indefinitely causes a concentration increase that eventually equilibrates to a steady state in a way that is more comparable to a pulse of CO <sub>2</sub> . Similarly the resulting change in global surface temperature from a step change in short-lived GHGs (Figure 7.21a) after a few decades increases only slowly (due to accumulation of heat in the deep ocean) and hence its effects are more similar to a pulse of CO <sub>2</sub> ( [[#Smith--2012|Smith et al., 2012]] ; [[#Lauder--2013|Lauder et al., 2013]] ; [[#Allen--2016|Allen et al., 2016]] , 2018b). The different time dependence of short-lived and long-lived compounds can be accounted for exactly with the CO <sub>2</sub> forcing equivalent metric ( [[#Wigley--1998|Wigley, 1998]] ; [[#Allen--2018b|Allen et al., 2018b]] ; [[#Jenkins--2018|Jenkins et al., 2018]] ) that produces a CO <sub>2</sub> emissions time profile such that the radiative forcing matches the time evolution of that from the non-CO <sub>2</sub> emissions. But other metric approaches can approximate this exact approach.

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[[File:4392da8f1bc131e9b8e504983581c657 IPCC_AR6_WGI_Figure_7_21.png]]

'''Figure 7.21''' '''|''' '''Emissions metrics for two short-lived greenhouse gases: HFC-32 and methane (CH''' <sub>4</sub> '''; lifetimes of 5.4 and 11.8 years).''' The temperature response function comes from Supplementary Material 7.SM.5.2. Values for non-CO <sub>2</sub> species include the carbon cycle response ( [[#7.6.1.3|Section 7.6.1.3]] ). Results for HFC-32 have been divided by 100 to show on the same scale. '''(a)''' Temperature response to a step change in short-lived greenhouse gas emissions. '''(b)''' Temperature response to a pulse CO <sub>2</sub> emission. '''(c)''' Conventional GTP metrics (pulse vs pulse). '''(d)''' Combined GTP metric (step versus pulse). Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

The similarity in behaviour of step changes in short-lived GHG emissions and pulses of CO <sub>2</sub> emissions has recently been used to formulate new emissions metric concepts ( [[#Collins--2020|Collins et al., 2020]] ). For short-lived GHGs, these new concepts use a step change in the rate of emissions, in contrast to an instantaneous pulse in a given year that is typically used (e.g., [[#Myhre--2013b|Myhre et al., 2013b]] ). Metrics for step emissions changes are denoted here by a superscript ‘ <sup>S</sup> ’ (e.g., ''AGT'' ''P'' <sup>S</sup> X is the absolute global surface temperature-change potential from a unit step change in emissions of species “ ''X'' ”). These can be derived by integrating the more standard pulse emission changes up to the time horizon. The response to a step emissions change is therefore equivalent to the integrated response to a pulse emission ( ''AGT'' ''P'' <sup>S</sup> X = ''iAGT'' ''P'' X ); and the radiative forcing response to a step emissions change ''AGF'' ''P'' <sup>S</sup> X is equivalent to the integrated forcing response ''iAGF'' ''P'' X which is the AGWP. The step metric for short-lived GHGs can then be compared with the pulse metric for CO <sub>2</sub> in a ratio ''AGT'' ''P'' <sup>S</sup> X / ''AGT'' ''P'' CO 2 ( [[#Collins--2020|Collins et al., 2020]] ). This is referred to as a combined GTP (CGTP) in [[#Collins--2020|Collins et al. (2020)]] , and has units of years (the standard GTP is dimensionless). This CGTP shows less variation with time than the standard GTP (comparing Figure 7.21c with Figure 7.21d) and provides a scaling for comparing a change in emissions rate (in kg yr <sup>–1</sup> ) of short-lived GHGs with a pulse emission or change in cumulative CO <sub>2</sub> emissions (in kg). Cumulative CO <sub>2</sub> equivalent emissions are given by CGTP × emissions rate of short-lived GHGs. The CGTP can be calculated for any species, but it is least dependent on the chosen time horizon for species with lifetimes less than half the time horizon of the metric ( [[#Collins--2020|Collins et al., 2020]] ). Pulse-step metrics can therefore be useful where time dependence of pulse metrics, like GWP or GTP, complicates their use (see Box 7.3).

For a stable global warming from non-CO <sub>2</sub> climate agents (gas or aerosol) their effective radiative forcing needs to gradually decrease ( [[#Tanaka--2018|Tanaka and O’Neill, 2018]] ). [[#Cain--2019|Cain et al. (2019)]] find this decrease to be around 0.3% yr <sup>–1</sup> for the climate response function in AR5 ( [[#Myhre--2013b|Myhre et al., 2013b]] ). To account for this, a quantity referred to as GWP* has been defined that combines emissions (pulse) and changes in emissions levels (step) approaches ( [[#Cain--2019|Cain et al., 2019]] ; [[#Smith--2021|Smith et al., 2021]] ). <sup>[[#footnote-000|2]]</sup> The emissions component accounts for the need for emissions to decrease to deliver a stable warming. The step (sometimes referred to as flow or rate) term in GWP* accounts for the change in global surface temperature that arises from a change in short-lived GHG emissions rate, as in CGTP, but here approximated by the change in emissions over the previous 20 years.

Cumulative CO <sub>2</sub> emissions and GWP*-based cumulative CO <sub>2</sub> equivalent GHG emissions multiplied by TCRE closely approximate the global warming associated with emissions time series (of CO <sub>2</sub> and GHG, respectively) from the start of the time series ( [[#Lynch--2020|Lynch et al., 2020]] ). Both the CGTP and GWP* convert short-lived GHG emissions rate changes into cumulative CO <sub>2</sub> equivalent emissions, hence scaling these by TCRE gives a direct conversion from short-lived GHG emissions to global surface temperature change. By comparison expressing methane emissions as CO <sub>2</sub> equivalent emissions using GWP-100 overstates the effect of constant methane emissions on global surface temperature by a factor of 3–4 ( [[#Lynch--2020|Lynch et al., 2020]] , their Figure 5), while understating the effect of any new methane emission source by a factor of 4–5 over the 20 years following the introduction of the new source ( [[#Lynch--2020|Lynch et al., 2020]] , their Figure 4).

Figure 7.22 explores how cumulative CO <sub>2</sub> equivalent emissions estimated for methane vary under different emissions metric choices and how estimates of the global surface air temperature (GSAT) change deduced from these cumulative emissions compare to the actual temperature response computed with the two-layer emulator. Note that GWP and GTP metrics were not designed for use under a cumulative carbon dioxide equivalent emissions framework ( [[#Shine--1990|Shine et al., 1990]] , 2005), even if they sometimes are (e.g., [[#Cui--2017|Cui et al., 2017]] ; [[#Howard--2018|Howard et al., 2018]] ) and analysing them in this way can give useful insights into their physical properties. Using these standard metrics under such frameworks, the cumulative CO <sub>2</sub> equivalent emissions associated with methane emissions would continue to rise if methane emissions were substantially reduced but remained above zero. In reality, a decline in methane emissions to a smaller but still positive value could cause a declining warming. GSAT changes estimated with cumulative CO <sub>2</sub> equivalent emissions computed with GWP-20 matches the warming trend for a few decades but quickly overestimates the response. Cumulative emissions using GWP-100 perform well when emissions are increasing but not when they are stable or decreasing. Cumulative emissions using GTP-100 consistently underestimate the warming. Cumulative emissions using either CGTP or GWP* approaches can more closely match the GSAT evolution ( [[#Allen--2018b|Allen et al., 2018b]] ; [[#Cain--2019|Cain et al., 2019]] ; [[#Collins--2020|Collins et al., 2020]] ; [[#Lynch--2020|Lynch et al., 2020]] ).

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[[File:edc4d79264e4f65065419bb746d8b187 IPCC_AR6_WGI_Figure_7_22.png]]

'''Figure 7.22''' '''|''' '''Explores how cumulative carbon dioxide equivalent emissions estimated for methane vary under different emissions metric choices and how estimates of the global surface air temperature (GSAT) change deduced from these cumulative emissions compare to the actual temperature response computed with the two-layer emulator (solid black lines).''' Panels '''(a)''' and '''(b)''' show the SSP4-6.0 and SSP1-2.6 scenarios respectively. The panels show annual methane emissions as the dotted lines (left axis) from 1750 to 2100. The solid lines can be read as either estimates of GSAT change or estimates of the cumulative carbon dioxide equivalent emissions. This is because they are related by a constant factor, the TCRE. Thus, values can be read using either of the right-hand axes. Emissions metric values are taken from Table 7.15. The GWP* calculation is given in ( [[#7.6.1.4|Section 7.6.1.4]] . The two-layer emulator has been calibrated to the central values of the Report’s assessment (see Supplementary Material 7.SM.5.2). Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

In summary, new emissions metric approaches such as GWP* and CGTP are designed to relate emissions changes in short-lived GHGs to emissions of CO <sub>2</sub> as they better account for the different physical behaviours of short- and long-lived gases. Through scaling the corresponding cumulative CO <sub>2</sub> equivalent emissions by the TCRE, the GSAT response from emissions over time of an aggregated set of gases can be estimated. Using either these new approaches, or treating short- and long-lived GHG emissions pathways separately, can improve the quantification of the contribution of emissions to global warming within a cumulative emissions framework, compared to approaches that aggregate emissions of GHGs using standard CO <sub>2</sub> equivalent emissions metrics. As discussed in Box 7.3, there is ''high confidence'' that multi-gas emissions pathways with the same time-dependence of aggregated CO <sub>2</sub> equivalent emissions estimated from standard approaches, such as weighting emissions by their GWP-100 values, rarely lead to the same estimated temperature outcomes.

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