Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.6.2.1 Climate Change Under Overshoot ====

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The SR1.5 ( [[#IPCC--2018b|IPCC, 2018b]] ) concluded with ''high confidence'' that overshoot trajectories ‘result in higher impacts and associated challenges compared to pathways that limit global warming to 1.5°C with no or limited overshoot’. The degree and duration of overshoot affects the risks and impacts likely to be experienced ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) and the emissions pathway required to achieve it ( [[#Akimoto--2018|Akimoto et al., 2018]] ). Consequences relating to ice sheets and climatic extremes have been found to be greater at 2°C of global warming than at 1.5°C ( [[#Schleussner--2016|Schleussner et al., 2016]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) but even on recovery to lower temperatures, these effects may not reverse. Overshoot has been found to lead to irreversible changes in thermosteric sea level ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Palter--2018|Palter et al., 2018]] ; [[#Tachiiri--2019|Tachiiri et al., 2019]] ), AMOC ( [[#Palter--2018|Palter et al., 2018]] ), ice sheets, and permafrost carbon (Sections 4.7.2 and 5.4.9) and to long-lasting effects on ocean heat ( [[#Tsutsui--2006|Tsutsui et al., 2006]] ). Abrupt changes and tipping points are not well understood, but the higher the warming level and the longer the duration of overshoot, the greater the risk of unexpected changes ( [[#4.7.2|Section 4.7.2]] ). Non-reversal of the hydrological cycle has also been found in some studies with an increase in global precipitation following CO <sub>2</sub> decrease being attributed to a build-up of ocean heat ( [[#Wu--2010|Wu et al., 2010]] ), and to a fast atmospheric adjustment to CO <sub>2</sub> radiative forcing ( [[#Cao--2011|Cao et al., 2011]] ).

Global temperature is expected to remain approximately constant if emissions of CO <sub>2</sub> were to cease ( [[#4.7.1.1|Section 4.7.1.1]] ), and so reductions in GSAT are only possible in the event of net negative global CO <sub>2</sub> emissions. We assess here results from an overshoot scenario (SSP5-3.4-OS; [[#O’Neill--2016|O’Neill et al., 2016]] ), which explores the implications of a peak and decline in forcing during the 21st century. Reversibility under more extreme and idealized carbon dioxide removal (CDR) scenarios is assessed in [[#4.6.3|Section 4.6.3]] . In SSP5-3.4-OS, CO <sub>2</sub> peaks at 571 ppm in the year 2062 and reverts to 497 ppm by 2100 – approximately the same level as in 2040. SSP5-3.4-OS has strong net negative emissions of CO <sub>2</sub> , exceeding those in SSP1-2.6 and SSP1-1.9 from 2070 onwards and reaching –5.5 PgC yr <sup>–1</sup> (–20 GtCO <sub>2</sub> yr <sup>–1</sup> ) by 2100. While this causes global mean temperature to decline, changes in climate have not fully reversed by 2100 under this reversal of CO <sub>2</sub> concentration (Figure 4.34). Quantities are compared for 2081–2100 relative to a 20-year period (2034–2053) of the same average CO <sub>2</sub> . Differences between these two periods of the same CO <sub>2</sub> are: GSAT: 0.28 ± 0.30°C (mean ± standard deviation); global land precipitation: 0.026 ± 0.011 mm day <sup>–1</sup> ; September Arctic sea ice area: –0.32 ± 0.53 million km <sup>2</sup> ; thermosteric sea level: 12 ± 0.8 cm. As assessed in Section 9.3.1.1, Arctic sea ice area is linearly reversible with GSAT. Although these climate quantities are not fully reversible, the overshoot scenario results in reduced climate change compared with stabilisation or continued increase in greenhouse gases ( [[#Tsutsui--2006|Tsutsui et al., 2006]] ; [[#Palter--2018|Palter et al., 2018]] ; [[#Tachiiri--2019|Tachiiri et al., 2019]] ) ( ''high confidence'' ).

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[[File:61bb4e57a44ee1dcc8c841ad587e826e IPCC_AR6_WGI_Figure_4_34.png]]

'''Figure 4.34''' '''|''' '''Simulated changes in climate indices for SSP5-3.4-OS plotted against atmospheric CO''' <sub>2</sub> '''concentration (ppm) from 480 up to 571 and back to 496 by 2100. (a)''' Global surface air temperature change; '''(b)''' Global land precipitation change; '''(c)''' September Arctic sea ice area change; '''(d)''' Global thermosteric sea level change. Plotted changes are relative to the 2034–2053 mean which has same CO <sub>2</sub> as 2081–2100 mean (shaded grey bar). Red lines denote changes during the period up to 2062 when CO <sub>2</sub> is rising, blue lines denote changes after 2062 when CO <sub>2</sub> is decreasing again. Thick line is multi-model mean; thin lines and shading show individual models and complete model range. Numbers in square brackets indicate number of models used in each panel. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

The transient climate response to cumulative CO <sub>2</sub> emissions, TCRE, allows climate policy goals to be associated with remaining carbon budgets as global temperature increase is near-linear with cumulative emissions (Section 5.5). Research since AR5 has shown that the concept of near-linearity of climate change to cumulative carbon emissions holds for measures other than just GSAT, such as regional climate ( [[#Leduc--2016|Leduc et al., 2016]] ) or extremes ( [[#Harrington--2016|Harrington et al., 2016]] ; [[#Seneviratne--2016|Seneviratne et al., 2016]] ). However, ocean heat and carbon uptake do exhibit path dependence, leading to deviation from the TCRE relationship for levels of overshoot above 300 PgC ( [[#Zickfeld--2016|Zickfeld et al., 2016]] ; [[#Tokarska--2019|Tokarska et al., 2019]] ). Sea level rise, loss of ice sheets, and permafrost carbon release may not reverse under overshoot and recovery of GSAT and cumulative emissions ( [[#4.7|Section 4.7]] ). TCRE remains a valuable concept to assess climate policy goals and how to achieve them but given the non-reversibility of different climate metrics with CO <sub>2</sub> and GSAT reductions, it has limitations associated with evaluating the climate response under overshoot scenarios and CO <sub>2</sub> removal ( ''medium confidence'' ).

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