Thread by @Oliver_Geden

The #IPCC #AR6 WG3 report includes a comprehensive assessment of Carbon Dioxide Removal (CDR), its role in mitigation strategies and long-term pathways, but also a techno-economic assessment of ~10 CDR methods
An ongoing 🧵
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For Carbon Dioxide Removal, it's still early days in #climate policy, although there are already established methods (mainly forestry-related and soil carbon sequestration, not necessarily done to remove CO2)
In #AR6 reports, there aren't chapters dealing solely with CDR
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There was quite some CDR coverage already in the #AR6 Special Reports on 1.5°C (#SR15) and on land (#SRCCL). In WG I, CDR was mainly assessed in chapter 5 ('Biogeochemical Cycles'), and a bit in chapter 4

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In #AR6 WG III, chapter 12 (Cross-Sectoral Perspectives) is the 'hub' for the CDR assessment, mainly section 12.3 (report.ipcc.ch/ar6wg3/pdf/IPC…). Many land-based biological CDR methods are covered in ch. 7 (AFOLU). And of course, CDR is also included in ch. 3 on 'Scenarios'
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Unsurprisingly, CDR is widely covered in Technical Summary and SPM. In the latter, there's a 'CDR only' Headline Statement C11 (for which @CowieAnnette and I were responsible) and longer sub-statement C3.5 on CDR in scenarios. Many mentions of CDR methods in sections D & E
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Most important sentence on CO2 Removal in SPM:
"The deployment of CDR to counterbalance hard-to-abate residual emissions is unavoidable if net zero CO2 or GHG emissions are to be achieved"
Read: 'Net zero' = (some) CDR. Otherwise you'd simply set a 'zero' emissions target
6/n

But where to start in the longer #IPCC #AR6 WG III report? Probably in the cross-chapter box on CDR, on which you will find a lot of familiar names (even @NASA's chief scientist)
report.ipcc.ch/ar6wg3/pdf/IPC…
[7/n]

Okay, it's more of a conceptual box, not entirely 'rocket science'... But it sets overall tone for CDR in #IPCC WG III

1⃣ 'net zero' = some CDR
➡️but from there on, it'll mainly be the result of actors' choices & preferences: IPCC doesn't 'demand' certain numbers & methods
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2⃣ CDR vs. (net) negative emissions
➡️#IPCC WG III with deliberate terminology choice. To highlight practice of 'removal', CDR used as primary term, 'negative emissions' only (with 'net') on systems level.
@CowieAnnette, @ReisingerAndy and I acted as WG- wide consistency👮‍♂️
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CDR box in #IPCC WG III also introduces a taxonomy of methods, building on Minx et al 2018 & #AR6 WG I assessment, focusing on two dimensions
1⃣ Removal process
2⃣ Timescale of storage
It avoids common, but rather meaningless categorisations like 'nature-based vs. tech CDR'
10/n

'Meaningless' sounds a little harsh, and in fact there's often quite some meaning attached to NbS or techCDR. But within same category (e.g., land-based biological CDR) options often differ considerably in terms of mitigation potential, cost, risk & co-benefits, and maturity
11/n

Two more clarifications in CDR box
1⃣ CDR requires deliberate action ('anthropogenic sink enhancement')
2⃣ CCS & CCU as such aren't CDR. If coupled with fossil fuels, CCS/CCU can only reduce emissions. For removal, CO2 must come from atmosphere, via biomass or ambient air
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Finally, the box looks at 3 complementary roles of CDR in ambitious mitigation strategies at global and national levels (it's a stylised figure!)
1⃣ accelerate near-term mitigation
2⃣ counterbalancing residual emissions for 'net zero'
3⃣ achieving 'net negative' emissions
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What's important in this figure: the basic elements, not exact shape or timing.
To understand role of CDR before reaching net-zero, it's important to look at gross emissions & gross removals, even for land use.
'Negative emissions' (mis)leads many to think 'after net-zero'
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Zooming in, figure also shows that under standard GWP-100 metric, net-zero CO2 to be reached far earlier than net-zero GHG. Depending on composition of residual emissions in 1.5°C scenarios, timelag can be 10-40 years.
At net-zero GHG, CO2 emissions are already net-negative!
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The relationship between net-zero COs and net-zero GHG is being explored in an own, quite extensive cross-chapter box in #IPCC #AR6 WG III, covering all aspects, including timing, metrics, regional and sectoral charecteristics, and the role of CDR.
report.ipcc.ch/ar6wg3/pdf/IPC…
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Box shows differentiation in net-zero years for CO2 & all GHGs (for GWP-100) across scenario categories
[if net zero reached after 2100, scenario database doesn't report exact years anymore]
At time of net-zero GHG, annual CO2 removal often at >5 Gt
report.ipcc.ch/ar6wg3/pdf/IPC…
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Also detectable in figure SPM.5: for 1.5°C with no/limited overshoot (C1), net-zero GHG emissions to be reached much later (2095-2100) than for net-zero CO2 (2050-2055)
On this, quite some explaining to do for #IPCC #AR6 WG III (but not here, since not mainly due to CDR)
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Which now brings us to CDR numbers in #IPCC #AR6 WG III...
In the SPM scenarios table, there's a column for cumulative 'net negative emissions' until 2100. For all 1.5°C scenarios with no/limited overshoot it's 220 Gt.
report.ipcc.ch/ar6wg3/pdf/IPC…
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For sub-categories of 1.5°C no/limited overshoot the cumulative 'net-negative' by 2100 volumes are remarkable
➡️360 Gt if net-zero GHG reached by 2100 (C1a)
➡️50 Gt if net-zero GHG not reached by 2100 (C1b)
Here: more net negative = earlier net zero
report.ipcc.ch/ar6wg3/pdf/IPC…
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So what about gross CDR volumes? Obvs, these would have to be much higher than the net negative values?
➡️Short answer: yes
➡️Long answer: yes, but there are no cumulative CDR volumes, for methodological reasons
Why? IAM reporting issues for CDR from AFOLU, mainly forestry
[21/n]

But what about these CDR numbers in #AR6 SPM?
Read carefully: C3.5 has gross CDR volumes only for BECCS & DACCS, for AFOLU only net negative
Hence, there is no total CDR number, different from #SR15 (which had similar AFOLU reporting issues, so better forget the 100-1000 Gt)
22/n

But aren't there CDR tables in the full #IPCC #AR6 WG III report that give total CDR volumes, like table 3.4 referred to in the SPM?
Indeed, they're in the chapter version uploaded yesterday. But this is the draft from Nov 2021, after which corrigenda still could be made...
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So what happened here? Undue government intervention, as often suspected? Not at all
Authors detected AFOLU CDR reporting issues & submitted corrigenda in week before SPM approval
That's why current versions have watermarks & corrigenda lists upfront
report.ipcc.ch/ar6wg3/pdf/IPC…
24/n

But what about the CDR table 3.5 in the full #IPCC #AR6 WG III? It does give total CDR volumes
Indeed, they're in the Nov 2021 chapter version uploaded April 4 - but table 3.5 has the same issues with AFOLU-CDR reporting as described above. Corrigenda apply here as well...
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Authors detected AFOLU CDR reporting issues & submitted corrigenda in week before SPM approval
That's why current versions have watermarks & corrigenda lists upfront
report.ipcc.ch/ar6wg3/pdf/IPC…
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While #AR6 WG III can't report total CDR volumes, it's possible to look at different methods & their representation in IAMs - here with time series showing all scenarios for 1.5-2°C
Black lines shows median, colored lines show illustrative pathways
report.ipcc.ch/ar6wg3/pdf/IPC…
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➡️BECCS still dominates CDR in #IPCC #AR6 IAM scenarios
➡️AFOLU-CDR (mainly forestry) is substantial, and without reporting issues described above ('net' accounting, baselines), AFOLU CDR numbers would be higher
➡️DACCS now in, median volumes modest
report.ipcc.ch/ar6wg3/pdf/IPC…
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But one shouldn't get hung up on detailed CDR numbers without scenario-dependent context.
Plus: some models are over-/underrepresented in what is not a 'statistically controlled' sample
See section 3.2 in ch. 3 (mainly done by @Peters_Glen)
report.ipcc.ch/ar6wg3/pdf/IPC…
[29/n]

A different approach regarding CDR's role in #IPCC #AR6 scenarios is taken with 'Illustrative Mitigation Pathways' (Ied by @DetlefvanVuuren). Here, individual marker scenarios illustrate different mitigation strategies, with only one (IMP-Neg) showing very high BECCS volumes
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And here's a comparison of CDR methods in all scenarios for 1.5 and below 2°C (C1-C3) and the IMPs, as time series and cumulatively.
Again, the AFOLU-CDR numbers would be higher under 'common sense' reporting conventions
report.ipcc.ch/ar6wg3/pdf/IPC…
[31/n]

So what's a different way of assessing the potential role of CDR in #climate policy? Bottom-up techno-economic analysis of 13 methods, as done in ch 7 (land-based biological) and ch 12 (DACCS, Enhanced Weathering, ocean fertilisation & alkalinisation)
report.ipcc.ch/ar6wg3/pdf/IPC…
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CDR methods were analysed according to similar criteria
➡️Maturity
➡️Cost
➡️Mitigation Potential
➡️Risks & Impacts
➡️Co-benefits
➡️Trade-offs and adverse side-effects
➡️Role in mitigation pathways
report.ipcc.ch/ar6wg3/pdf/IPC…
[33/n]

This assessment of >10 individual CDR methods is summarized in table 12.6 (as well as in TS). It's based on the available literature, often existing review studies but also more recent articles. The ones below are more deeply explored in ch. 7 (AFOLU)
report.ipcc.ch/ar6wg3/pdf/IPC…
34/n

Assessing >10 CDR methods (coming with many implementation options), based on different strands of literature is obviously challenging, and can't follow entirely harmonized criteria
(Overview table below is the more condensed version TS.7)
report.ipcc.ch/ar6wg3/pdf/IPC…
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Some CDR methods are already being demonstrated or deployed, hence considered in the underlying sectoral chapters and integrated into "Cost & Potentials" analysis in ch. 12
Below the AFOLU subpanel of Figure SPM.7, showing near-term (2030) net emissions reduction potentials
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Here the energy subpanel of Figure SPM.7, showing near-term (2030) net emissions reduction potentials, including from #BECCS in power sector (hence "bioelectricity", since there are other ways to do BECCS).
Based on "Cost & Potentials" analysis in ch 12, led by @KornelisBlok
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During the #IPCC WG3 SPM approval session, CCS played an important role in the discussions, not only in relationship to CDR (as BECCS or DACCS), but also in the context of its deployment in traditional sectors like energy (ch. 6) and industry (ch11)
report.ipcc.ch/ar6wg3/pdf/IPC…
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Geological CO2 storage is not a major topic in #IPCC #AR6 WG III, mainly covered in chapter 6.
It is often overlooked that there was already an IPCC Special Report on #CCS, back in 2005
report.ipcc.ch/ar6wg3/pdf/IPC…
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Risks, co-benefits, feasibility of CDR deployment obviously depend on contextual factors (scale, location, implementation options) and should preferably be assessed for distinct pathways.
Alas, #IPCC #AR6 WG III assessment is largely method-focused
report.ipcc.ch/ar6wg3/pdf/IPC…
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Method-focused CDR feasibility assessment was part of broader analysis done for mitigation options across sectors in #AR6 WG III, along the feasibility dimensions established already in #SR15 and used in WG II as well.
Overview in Technical Summary
report.ipcc.ch/ar6wg3/pdf/IPC…
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In the Summary for Policymakers' section on Sustainable Development there is a specific statement on co-benefits and risks of deploying terrestrial CDR options, including #biochar (the latter got a lot of coverage in #IPCC #AR6 WG3).
report.ipcc.ch/ar6wg3/pdf/IPC…
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When assessing mitigation options for (in)compatibility with Sustainable Development Goals (#SDG), the devil is obviously in the details...
Below #IPCC #AR6 WG3 Figure SPM.8, including one CDR method and some components of several other CDR methods
report.ipcc.ch/ar6wg3/pdf/IPC…
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Finally, there's section 12.3.3 on CDR governance & policies, decidedly not overly conceptual (based on high-level principles or the mis-representation of CDR in modelling) but more geared towards literature analysing real-world policymaking processes
report.ipcc.ch/ar6wg3/pdf/IPC…
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Not surprisingly, focus here again is on countries with net-zero targets, where core governance question isn't whether CDR should be mobilised or not, but which CDR methods govs want to see deployed by whom, by when, at which volumes and in which ways
report.ipcc.ch/ar6wg3/pdf/IPC…
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To make the assessment on CDR governance & policymaking more real-worldish, we included a case-study box on the UK, having been the global frontrunner for quite some time (with the US now catching up). Writing led by @stv_smth
ipcc.ch/report/ar6/wg3…
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Finally, there's not that much on international governance of CDR in chapter 12, but a lot more in chapter 14 on "International Cooperation", with a focus on already existing regimes (CBD, LP/LC) and a tiny bit on equity considerations
ipcc.ch/report/ar6/wg3…
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And now there's the final, copyedited, typeset and graphically polished version of the #IPCC AR6 WG III report. All the corrigenda mentioned above now implemented. And even @Peters_Glen's signature CDR figure has changed (well, only the colors)
ipcc.ch/report/ar6/wg3…
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Also now released, and a bit hidden on the #IPCC AR6 WG III report website, the earlier chapter drafts (first, second, final).
Here a snapshot from ch12, FOD. I had already forgotten we used the term #StateofCDR here
ipcc.ch/report/ar6/wg3…
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If you're really into it, you can now compare chapter versions, and check expert & government review comments on #CDR - and author responses. You can try to reconstruct governments' positions - or simply look up how we dealt with your review comments
ipcc.ch/report/ar6/wg3…
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