Editing IPCC:AR6/WGI/Chapter-5 (section)

===== 5.5.1.2.3 Reversibility and Earth system feedbacks =====

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There are relatively few studies that have assessed how the TCRE is expected to change in scenarios of declining emissions followed by net negative annual CO <sub>2</sub> emissions. Conceptually, the literature suggests that the small lag of about a decade between CO <sub>2</sub> emissions and temperature change ( [[#Ricke--2014|Ricke and Caldeira, 2014]] ; [[#Zickfeld--2015|Zickfeld and Herrington, 2015]] ) would result in more warming at a given amount of cumulative emissions in a scenario where that emissions level is first exceeded and then returned to by deploying negative emissions (referred to as an ‘overshoot’, as is often the case in scenarios that aim to limit radiative forcing in 2100 to 2.6 or 1.9 W m <sup>–2</sup> ( [[#Riahi--2017|Riahi et al., 2017]] ; [[#Rogelj--2018a|Rogelj et al., 2018a]] ). [[#Zickfeld--2016|Zickfeld et al. (2016)]] showed this to hold across a range of scenarios, with positive emissions followed by negative emissions, whereby the TCRE increased by about 10% across the transition from positive to negative emissions as a result of the thermal and carbon inertia of the deep ocean. However, CMIP6 results for the SSP5-3.4-overshoot scenario show diverging trends across various ESMs (Figure 5.30). In an idealized CO <sub>2</sub> -concentration-driven setting, [[#Tachiiri--2019|Tachiiri et al. (2019)]] also reported an increase in TCRE. Exploring pathways with emissions rates and overshoots closer to mitigation pathways considered over the 21st century (in this case up to about 300 PgC), a recent emissions-driven EMIC experiment showed pathway independence of TCRE ( [[#Tokarska--2019a|Tokarska et al., 2019a]] ). Furthermore, also in absence of net negative emissions, warming would not necessarily remain perfectly constant on time scales of centuries and millennia, but could decrease or increase ( [[#Frölicher--2015|Frölicher and Paynter, 2015]] ; R.G. [[#Williams--2017|Williams et al., 2017]] a; [[#Hajima--2020b|Hajima et al., 2020b]] ). These additional changes in global mean temperature increase at various time scales are known as the ZEC (C.D. [[#Jones--2019|]] [[#Jones--2019|Jones et al., 2019]] ; [[#MacDougall--2020|MacDougall et al., 2020]] ), assessed in [[IPCC:Wg1:Chapter:Chapter-4#4.7.1.1|Section 4.7.1.1]] , and have to be integrated when using TCRE to estimate warming or remaining carbon budgets in overshoot scenarios.

The AR5-assessed (W.J. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ) TCRE range was based in part on the ESMs available at the time, which did not include some potentially important Earth system feedbacks. Since then, a number of studies have assessed the importance of permafrost carbon feedbacks, in particular on remaining carbon budgets ( [[#MacDougall--2015|MacDougall and Friedlingstein, 2015]] ; [[#MacDougall--2015|MacDougall et al., 2015]] ; [[#Burke--2017b|Burke et al., 2017b]] ; [[#Gasser--2018|Gasser et al., 2018]] ; [[#Lowe--2018|Lowe and Bernie, 2018]] ), a development highlighted and assessed in the IPCC Special Report on Global Warming of 1.5°C ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). [[#MacDougall--2015|MacDougall and Friedlingstein (2015)]] reported a TCRE increase of about 15% when including permafrost carbon feedbacks. The overall linearity of the TCRE during the 21st century was not affected, but they also found that permafrost carbon feedbacks caused an increase in TCRE on multi-century time scales under declining CO <sub>2</sub> emissions rates. In addition, other processes that are not regarded, or are only partially considered in individual or all ESMs, could cause a further increase or decrease of TCRE ( [[#Matthews--2020|Matthews et al., 2020]] ). These are discussed in detail in [[#5.4|Section 5.4]] , but their quantitative effects on TCRE have not yet been explored by the literature.

Whether TCRE remains an accurate predictor of CO <sub>2</sub> -induced warming when annual CO <sub>2</sub> emissions reach zero and are followed by net carbon dioxide removal (also referred to as TCRE reversibility) therefore hinges on contributions of slow components of the climate system that cause unrealized warming from past CO <sub>2</sub> emissions. Such slow components can arise from either physical climate (i.e., ocean heat uptake) or carbon cycle (i.e., ocean carbon uptake and permafrost carbon release) processes. The combined effect of these processes determines the magnitude and sign of the ZEC ( [[#MacDougall--2020|MacDougall et al., 2020]] ), which in turn impacts TCRE reversibility. As discussed in [[IPCC:Wg1:Chapter:Chapter-4#4.7.1.1|Section 4.7.1.1]] , recent model estimates of the ZEC suggest a range of ±0.19°C centred on zero ( [[#MacDougall--2020|MacDougall et al., 2020]] ). This suggests ''low agreement'' among models as to the reversibility of the TCRE in response to net-negative CO <sub>2</sub> emissions. Furthermore, most models used for ZEC assessments to date do not represent permafrost carbon processes, although understanding their contribution is essential to quantify the TCRE contribution. There is therefore ''limited evidence'' that quantifies the impact of permafrost carbon feedbacks on the reversibility of TCRE, leading to ''low confidence'' that the TCRE remains an accurate predictor of temperature changes in scenarios of net-negative CO <sub>2</sub> emissions on time scales of more than a half a century.

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