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

==== 4.6.3.2 Climate Response to Mitigation by Carbon Dioxide Removal ====

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CDR options include afforestation, soil carbon sequestration, bioenergy with carbon capture and storage (BECCS), wet land restoration, ocean fertilization, ocean alkalinisation, enhanced terrestrial weathering and direct air capture and storage (see [[IPCC:Wg1:Chapter:Chapter-5#5.6.2|Section 5.6.2]] and Table 5.9 for a more complete discussion). [[IPCC:Wg1:Chapter:Chapter-8|Chapter 8]] (Section 8.4.3) assesses the implications of CDR for water cycle changes. The potential of different CDR options in terms of the amount of CO <sub>2</sub> removed per year from the atmosphere, costs, co-benefits and side effects of the CDR approaches are assessed in SR1.5 ( [[#de%20Coninck--2018|de Coninck et al., 2018]] ), the AR6 WGIII Report (see [[IPCC:Wg1:Chapter:Chapter-7|Chapters 7]] and [[IPCC:Wg1:Chapter:Chapter-12|12]] ), and in several review papers ( [[#Fuss--2018|Fuss et al., 2018]] ; [[#Lawrence--2018|Lawrence et al., 2018]] ; [[#Nemet--2018|Nemet et al., 2018]] ). In the literature, CDR options are also referred to as ‘negative CO <sub>2</sub> emissions technologies’.

Deployment of CDR will lead to a reduction in atmospheric CO <sub>2</sub> levels only if uptake by sinks exceeds net CO <sub>2</sub> emissions. Hence, there could be a substantial delay between the initiation of CDR and net CO <sub>2</sub> emissions turning negative ( [[#van%20Vuuren--2016|van Vuuren et al., 2016]] ), and the time to reach net negative CO <sub>2</sub> emissions and the evolution of atmospheric CO <sub>2</sub> and climate thereafter would depend on the combined pathways of anthropogenic CO <sub>2</sub> emissions, CDR, and natural sinks. The cooling (or avoided warming) due to CDR would be proportional to the cumulative amount of CO <sub>2</sub> removed from the atmosphere by CDR ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Zickfeld--2016|Zickfeld et al., 2016]] ), as implied by the near-linear relationship between cumulative carbon emissions and GSAT change (Section 5.5).

Emissions pathways that limit globally averaged warming to 1.5°C or 2°C by the year 2100 assume the use of CDR approaches in combination with emissions reductions to follow net negative CO <sub>2</sub> emissions trajectory in the second half of this century. For instance ''',''' in SR1.5, all analysed pathways limiting warming to 1.5°C by 2100 with no or limited overshoot include the use of CDR to some extent to offset anthropogenic CO <sub>2</sub> emissions and the median of CO <sub>2</sub> removal across all scenarios was 730 GtCO <sub>2</sub> in the 21st century ( [[#Rickels--2018|Rickels et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ). Affordable and environmentally and socially acceptable CDR options at scale well before 2050 are an important element of 1.5°C-consistent pathways especially in overshoot scenarios ( [[#de%20Coninck--2018|de Coninck et al., 2018]] ). The required scale of removal by CDR can vary from 1–2 GtCO <sub>2</sub> yr <sup>–1</sup> year from 2050 onwards to as much as 20 GtCO <sub>2</sub> yr <sup>–1</sup> ( [[#Waisman--2019|Waisman et al., 2019]] ). In the SSP class of scenarios, net CO <sub>2</sub> emissions turn negative from around 2050 in SSP1-1.9 and around 2070 in SSP1-2.6 and in the overshoot scenario SSP5-3.4-OS ( [[#O’Neill--2016|O’Neill et al., 2016]] ). Thus, CDR would play a pivotal role in limiting climate warming to 1.5°C or 2°C ( [[#Minx--2018|Minx et al., 2018]] ). In stark contrast, however, two extensive reviews ( [[#Lawrence--2018|Lawrence et al., 2018]] ; [[#Nemet--2018|Nemet et al., 2018]] ) conclude that it is implausible that any CDR technique can be implemented at the scale needed by 2050.

When CDR is applied continuously and at scales as large as currently deemed possible, under RCP8.5 as the background scenario, the widely discussed CDR options such as afforestation, ocean iron fertilization and surface ocean alkalinisation are individually expected to be relatively ineffective, with limited (8%) warming reductions relative to the scenario with no CDR option ( [[#Keller--2014|Keller et al., 2014]] ). Hence, the potential role that CDR will play in lowering the temperature in high-emissions scenarios is limited ( ''medium confidence'' ). The challenges involved in comparing the climatic effects of various CDR options has also been recognized in recent studies ( [[#Sonntag--2018|Sonntag et al., 2018]] ; [[#Mengis--2019|Mengis et al., 2019]] ). For instance, due to compensating processes such as biogeophysical effects of afforestation (warming from albedo decrease when croplands are converted to forests) more carbon is expected to be removed from the atmosphere by afforestation than by ocean alkalinisation to reach the same global mean cooling.

The climate response to CDR-caused net negative CO <sub>2</sub> emissions has been studied in Earth system models by prescribing idealized ramp-down of CO <sub>2</sub> concentrations (MacDougall,2013; [[#Zickfeld--2016|Zickfeld et al., 2016]] ; [[#Schwinger--2018|Schwinger and Tjiputra, 2018]] ), CO <sub>2</sub> concentrations of RCP scenarios that have net negative CO <sub>2</sub> emissions (C.D. [[#Jones--2016|Jones et al., 2016]] b), and idealized net negative CO <sub>2</sub> emissions scenarios ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ). The Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) uses multiple ESMs to explore the climate response, effectiveness of CO <sub>2</sub> removal, and challenges of CDR options ( [[#Keller--2018|Keller et al., 2018]] ). Idealized CDRMIP simulations increase CO <sub>2</sub> concentrations at 1% per year from the level in the pre-industrial control run (piControl) to 4×CO <sub>2</sub> <sub></sub> and subsequently decrease at the same rate to the piControl level. This section assesses the lag in climate response to CDR-caused negative emissions; climate ‘reversibility’ is assessed in [[#4.7.2|Section 4.7.2]] . The ramp-down phase, though unrealistic, represents the ‘net negative CO <sub>2</sub> emissions’ phase.

Figure 4.37 illustrates the first results from CDRMIP ( [[#Keller--2018|Keller et al., 2018]] ). Other studies that use similar (Zickfeld et al.,2016; [[#Schwinger--2018|Schwinger and Tjiputra, 2018]] ; [[#Jeltsch-Thömmes--2020|Jeltsch-Thömmes et al., 2020]] ) or other idealized scenarios ( [[#MacDougall--2013|MacDougall, 2013]] ) or more realistic net negative CO <sub>2</sub> emissions scenarios such as RCP2.6 (C.D. [[#Jones--2016|Jones et al., 2016]] b) and scenarios that limit warming to 2°C or less after different levels of overshoot ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ) arrive at similar conclusions. Changes in key climate variables substantially lag behind the decline in CO <sub>2</sub> (Figure 4.37). The precipitation increase at the beginning of the ramp-down phase agrees with the increase in precipitation for an abrupt decline in CO <sub>2</sub> ( [[#Cao--2011|Cao et al., 2011]] ). Notwithstanding a decline in atmospheric CO <sub>2</sub> , global mean thermosteric sea level would continue to rise. When atmospheric CO <sub>2</sub> returns to the piControl level, global mean thermosteric sea level is higher than its value at peak CO <sub>2</sub> (Figure 4.37), and it is ''likely'' that thermosteric global sea level would not return to piControl levels for over 1000 years after atmospheric CO <sub>2</sub> is restored to piControl concentrations ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Ehlert--2018|Ehlert and Zickfeld, 2018]] ). Therefore, there is ''high confidence'' that sea level rise will not be reversed by CDR at least for several centuries ( [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.6.3.5). A comparison of different models shows recovery of AMOC intensity during net negative CO <sub>2</sub> emissions, but the results are model dependent – strengthening with an overshoot in most models ( [[#Jackson--2014|Jackson et al., 2014]] ) and strengthening but not reaching the initial state in some models ( [[#Sgubin--2015|Sgubin et al., 2015]] ). The overall lag in response is qualitatively similar to the lagged climate system response in the overshoot scenario SSP5-34-OS where CO <sub>2</sub> rises until 2062 and decreases thereafter (Figure 4.34). The lag in climate response to CDR causes hysteresis between key climate variables such as temperature, precipitation, AMOC and sea level, and atmosphere CO <sub>2</sub> with the hysteresis characteristics dependent on the rate of CDR and climate sensitivity ( [[#MacDougall--2013|MacDougall, 2013]] ; [[#Jeltsch-Thömmes--2020|Jeltsch-Thömmes et al., 2020]] ).

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[[File:3711ae45733f8a3f5cb71f1a85e01b3d IPCC_AR6_WGI_Figure_4_37.png]]

'''Figure 4.37''' '''|''' '''Delayed climate response to carbon dioxide removal (CDR)-caused net negative CO''' <sub>2</sub> '''emissions.''' Multi-model simulated response in global and annual mean climate variables for a ramp-up followed by ramp-down of CO <sub>2</sub> . Atmospheric CO <sub>2</sub> increases from the pre-industrial level at a rate of 1% yr <sup>–1</sup> to 4×CO <sub>2</sub> , then decreases at the same rate to the pre-industrial level and then remains constant. The ramp-down phase represents the period of net negative CO <sub>2</sub> emissions. '''(a)''' Normalized ensemble mean anomaly of key variables as a function of year, including atmospheric CO <sub>2</sub> , surface air temperature, precipitation, thermosteric sea level change (see Glossary), global sea ice area, Northern Hemisphere sea ice area in September, and Atlantic meridional overturning circulation (AMOC); '''(b)''' surface air temperature; '''(c)''' precipitation; '''(d)''' September Arctic sea ice area; '''(e)''' AMOC; '''(f)''' thermosteric sea level; five-year running means are shown for all variables except the sea level change. In (b, f), red lines represent the phase of CO <sub>2</sub> ramp-up, blue lines represent the phase of CO <sub>2</sub> ramp-down, brown lines represent the period after CO <sub>2</sub> has returned to pre-industrial level, and black lines represent the multi-model mean. For all of the segments in (b, f), the solid coloured lines are CMIP6 models, and the dashed lines are other models (i.e., EMICs, CMIP5-era models). Vertical dashed lines indicate peak CO <sub>2</sub> and when CO <sub>2</sub> again reaches pre-industrial value. The number of CMIP6 and non-CMIP6 models used is indicated in each panel. The time series for the multi-model means (b, f) and the normalized anomalies (a) are terminated when data from all models are not available, in order to avoid the discontinuity in the time series. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Termination of CDR refers to a sudden and sustained discontinuation of CDR deployment (see [[#4.6.3.3|Section 4.6.3.3]] for termination effects of SRM). The literature on the termination effects of CDR is limited, mostly considering scenarios where CDR implementation is explicit and does not result in net negative CO <sub>2</sub> emissions ( [[#Keller--2014|Keller et al., 2014]] ; [[#González--2018|González et al., 2018]] ). In simulations where CDR is applied on the RCP8.5 scenario at scales as large as currently deemed possible, the increase in global mean warming rates following CDR termination are relatively small in comparison to SRM termination ( [[#Keller--2014|Keller et al., 2014]] ). The exception is artificial ocean upwelling where surface cooling is mainly caused by bringing cold water from the deep ocean; upon termination this causes larger rates of surface warming ( [[#Oschlies--2010|Oschlies et al., 2010]] ). When background emissions are as high as in RCP8.5, termination of a large global-scale application of CDR such as ocean alkalinisation for multiple decades could also result in large regional warming rates (up to 0.15°C per year) that are comparable to those caused by termination of SRM ( [[#González--2018|González et al., 2018]] ). In such cases, large amounts of CO <sub>2</sub> would be removed from the atmosphere before termination, and termination would cause a temporal trajectory of atmospheric CO <sub>2</sub> that is parallel to the high-emissions scenario but from an atmosphere with much lower CO <sub>2</sub> levels. Because CO <sub>2</sub> radiative forcing is a logarithmic function of CO <sub>2</sub> concentration, large regional warming rates are simulated in such terminations. Thus, there is ''high confidence'' that the climate effect of CDR termination would depend on the amount CO <sub>2</sub> removed by CDR prior to termination and the rate of background CO <sub>2</sub> emissions at the time of termination. See also Chapter 5, Table 5.9, which summarizes the termination effects of individual CDR options.

In summary, there is ''high confidence'' that, due to the near-linear relationship between cumulative carbon emissions and GSAT change, cooling or avoided warming due to a CDR option would depend on the cumulative amount of CO <sub>2</sub> removed by that CDR option. The climate system response to the deployment of CDR is expected to be delayed by years (e.g., in temperature, precipitation, sea ice extent) to centuries (e.g., sea level and AMOC) ( ''high confidence'' ). The climate response to a sudden and sustained CDR termination would depend on the amount of CDR-induced cooling prior to termination and the rate of background CO <sub>2</sub> emissions at the time of termination ( ''high confidence'' ).

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