Editing IPCC:AR6/WGI/Chapter-6 (section)

=== 6.6.1 Implications of Lifetime on Temperature Response Time Horizon ===

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The effect over time on GSAT following a mitigation effort affecting emissions of LLGHGs or SLCFs depends on the lifetimes of the LLGHGs and SLCFs, their radiative efficiencies, how fast the emissions are reduced, how long reductions last (limited time or sustained reduction), and the inertia of the climate system itself. Mitigation of SLCFs is often implemented through new legislation or technology standards for the different emissions sectors and components, implying that reductions are sustained over time.

It is often perceived that the full climatic response following mitigation of SLCFs will occur almost immediately. However, the inertia of the climate system strongly modifies the short-term and long-term response. SLCFs with lifetimes shorter than the time scales for inter-hemisheric mixing (1–2 years) can cause a more spatially heterogeneous forcing than LLGHGs and thus different regional patterns of the climate response (Section 6.4.3). The temporal response in GSAT to a radiative forcing can be quantified using linear impulse response functions (Cross-Chapter Box 7.1; Geoffroy et al. , 2013; [[#Olivié--2013|Olivié and Peters, 2013]] ; Smith et al. , 2018) . Figure 6.15 shows the GSAT response for sustained step reduction in emissions of idealised SLCFs with different lifetimes. The response is relative to a baseline with constant emissions, so effects of emissions before the step reduction is not shown. For SLCFs with lifetimes shorter than a few years, the concentrations quickly reach a new steady state and the response time is primarily governed by the thermal inertia and thus the time scales of the climate system. For compounds with lifetime on the order of 10 years (e.g., methane), there is about a 10-year delay in the response during the first decades, compared to compounds with lifetimes less than one year. However on longer time scales the response is determined solely by the time scales of the climate system itself. For CO <sub>2</sub> (dashed line in Figure 6.15) the temporal response is very different due to the long time scale for mixing into the deep ocean and therefore a substantial fraction of atmospheric CO <sub>2</sub> is only removed on millenium time scales. This means that for SLCFs including methane, the rate of emissions drives the long-term stabilisation, as opposed to CO <sub>2</sub> where the long-term effect is controlled by cumulative emissions ( [[#Allen--2018b|Allen et al., 2018b]] ). Methods to compare rates of SLCF emissions with cumulative CO <sub>2</sub> emissions are discussed in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] ( [[IPCC:Wg1:Chapter:Chapter-7#7.6.1.4|Section 7.6.1.4]] ).

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'''Figure 6.15 |''' '''Global mean surface air temperature (GSAT) response to an abrupt reduction in emissions (at time t=0) of idealized climate forcing agents with different lifetimes.''' All emissions are cut to give a radiative forcing of –1 W m <sup>–2</sup> at a steady state (except for CO <sub>2</sub> ). In other words, if the yearly emissions are E <sub>0</sub> before the reduction, they will have a fixed lower value E <sub>year&gt;0</sub> = (E <sub>0</sub> – δ E) for all succeeding years. For comparison, the GSAT response to a sustained reduction in CO <sub>2</sub> emissions resulting in an RF of –1 W m <sup>–2</sup> in year 100 is included (dashed line). The temperature response is calculated using an impulse response function (Cross-Chapter Box 7.1) with a climate feedback parameter of –1.31 W m <sup>–2</sup> °C <sup>–1</sup> . Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

As a consequence, in idealized ESM studies that assume an instantaneous removal of all anthropogenic or fossil fuel-related emissions, a rapid change in aerosol levels occurs leading to large increases in GSAT with the rate of warming lasting for several years. Similarly, the thermal inertia causes the pulse emissions (Figure 6.15) of SLCFs to have a significant effect on surface temperature even after 10 years.

In summary, for SLCFs with short lifetime (e.g., months), the response in surface temperature occurs strongly as soon as a sustained change in emissions is implemented and continues to grow for a few years, primarily due to thermal inertia in the climate system ( ''high confidence'' ). Near its maximum, the response slows down but will then take centuries to reach equilibrium ( ''high confidence'' ). For SLCFs with longer lifetimes (e.g., a decade), a delay equivalent to their lifetimes comes in addition to the delay due the thermal inertia ( ''high confidence'' ).

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