Zeke Hausfather (@hausfath): The IPCC report makes it clear that limiting warming to well-below 2C requires "immediate and deep" greenhouse gas reductions in all sectors.

But they also feature carbon dioxide reduction as a "key element" to reach achieve these goals. Lets explore CDR in the WG3 report!

1/ https://twitter.com/hausfath/status/1512195746426105858/photo/1

Zeke Hausfather (@hausfath): The report makes it extremely clear that CDR is not a replacement for mitigation. There is no world where we can continue to anything close to current levels of GHGs without blowing past 1.5C and 2C this century. Delaying reductions puts 1.5C and increasingly 2C out of reach. 2/

Zeke Hausfather (@hausfath): However, even with deep near-term emissions reductions big challenges remain: while we can cut non-CO2 GHG emissions (methane and nitrous oxide) by around two thirds, the remaining third will be extremely difficult to fully eliminate and will have to be counterbalanced by CDR. 3/ https://twitter.com/hausfath/status/1512195755334807572/photo/1

Zeke Hausfather (@hausfath): Similarly, there will be a small portion of existing CO2 emissions that will be too costly or difficult to fully eliminate – though just how far we can get toward zero CO2 will depend on how well we can change behaviors and develop technologies for hard-to-decarbonize sectors. 4/

Zeke Hausfather (@hausfath): As the report notes, "At the point of global net zero CO2 emissions, 3-12 GtCO2 of emissions from some sectors are compensated for by net negative CO2 emissions in other sectors." 4.5/

Zeke Hausfather (@hausfath): Finally, while in an ideal world we would cut emissions by ~50% globally in the next decade, there is a real risk that slower action results in the world overshooting our temperature goals. In that case, the only way to permanently bring global temperature back down is CDR. 5/

Zeke Hausfather (@hausfath): To put it simply, to reduce future temperatures, CO2 will have to be actively removed from the atmosphere through human activity. 6/

Zeke Hausfather (@hausfath): As the report notes, CDR “is a necessary element of mitigation portfolios to achieve net-zero CO2 and GHG emissions both globally and nationally, counterbalancing residual emissions from ‘hard-to-transition’ sectors such as industry, transport and agriculture”. 7/

Zeke Hausfather (@hausfath): It is “a key element in scenarios likely to limit warming to 2C or lower, regardless of whether global emissions reach near-zero, net-zero or net-negative levels”. 8/

Zeke Hausfather (@hausfath): At the same time, the report cautions that the role of CO2 removal can at times be “inflate[d]” in model pathways because of “insufficient representation of variable renewables”, such as wind and solar, limited use of demand-side options and high discount rates. 9/

Zeke Hausfather (@hausfath): CDR can come in different forms, including afforestation/reforestation, bioenergy with carbon capture and storage (BECCS), direct air capture (DAC), enhanced weathering, biochar, wetland restoration, soil carbon, bio-CO2 use in industry and ocean alkalinity enhancement. 10/

Zeke Hausfather (@hausfath): Most scenarios employing CDR in WG3 primarily focus on afforestation/reforestation and BECCS, though some models include the deployment of DACCS, which was largely absent in previous generations of models. 11/

Zeke Hausfather (@hausfath): This largely reflects modelling choices rather than the feasibility or preferability of these or alternative approaches to CDR. Most models only include a few different CDR options. Here is the portion of scenarios run using models supporting different CDR tech: 12/ https://twitter.com/hausfath/status/1512195781716979720/photo/1

Zeke Hausfather (@hausfath): In practice, however, just because models can support different CDR approaches does not mean they do. IAMs are optimization models, and tend to swing toward favoring what they assume to be cheapest in the future. In most cases thats afforestation/reforestation and BECCS. 13/

Zeke Hausfather (@hausfath): Here are the portion of scenarios that deploy different types of permanent (non-biosphere-based) CDR in WG3. All models deploy BECCS, while only a third have DAC and almost none have enhanced weathering (or other approaches): 14/ https://twitter.com/hausfath/status/1512195791317741569/photo/1

Zeke Hausfather (@hausfath): The median scenario limiting warming to 1.5C deploys around 1 billion tons of CO2 removal by 2030, 6 billion tons by 2050, and 13 to 17 billion tons by 2100 in scenarios limiting warming to 1.5C or likely (>66% chance) below 2C: 15/ https://twitter.com/hausfath/status/1512195796166356994/photo/1

Zeke Hausfather (@hausfath): There numbers likely reflect an underestimate of CDR deployment, as some models do not report any land-use-related CDR until all land-use emissions go net-negative. There is also a wide range of CDR deployment, from as low as zero to as high as 26 GtCO2 in 2100. 16/

Zeke Hausfather (@hausfath): If we break it down by tech, we see most scenarios are still dominated by BECCS (in part due to not simulating other options, and not accounting for secondary impacts of BECCS), but some models include lots of DACs deployment (which generally wasn't the case in the SR15): 17/ https://twitter.com/hausfath/status/1512195805821644837/photo/1

Zeke Hausfather (@hausfath): Overall there is slightly less CDR deployment in the median scenario in the new WG3 report than there was in the 2018 SR15 report for the same scenarios. This is somewhat surprising, given that four years of continuing high emissions have reduced remaining carbon budgets: 18/ https://twitter.com/hausfath/status/1512195812930990099/photo/1

Zeke Hausfather (@hausfath): It turns out that a number of scenarios run for WG3 purposefully explore low CDR outcomes (e.g. constrain CDR deployment in the model). The inclusion of these low-CDR scenarios in the database reduces the median deployment compared to the SR15. 19/

Zeke Hausfather (@hausfath): Or as the report notes, "Since AR5, there has been fervent debate on the large-scale deployment of Bioenergy with Carbon Capture and Storage (BECCS) in scenarios. Hence, many recent studies explore mitigation pathways with limited BECCS deployment." 20/

Zeke Hausfather (@hausfath): The report highlights that CDR via afforestation/reforestation and soils can be reversed and "is vulnerable to climate change impacts. CO2 stored in geological and ocean reservoirs (via BECCS, DACCS, ocean alkalinisation) and in biochar is less vulnerable to reversal." 21/

Zeke Hausfather (@hausfath): So what are the takeaways from all of this? First, CDR at a gigaton scale will be essential to meet our climate goals. At the same time, we can limit its use by more rapid emissions reductions, and finding solutions to more deeply cut non-CO2 GHGs and hard-to-decarb sectors. 22/

Zeke Hausfather (@hausfath): Second, the representation of different CDR technologies in models is still quite rudimentary, with BECCS (a somewhat problematic technology) effectively serving as a proxy for all permanent carbon removal. 23/

Zeke Hausfather (@hausfath): Third, there is a danger of reading too much into IAMs in terms of how much CDR we need. Ultimately it will depend on our speed of mitigation, the degree to which costs fall, and the political will to draw down emissions after (very likely) overshooting 1.5C. 24/

Zeke Hausfather (@hausfath): You can ultimately justify any number ranging from a few GtCO2/yr needed to counterbalance residual positive emissions to many tens of tons of GtCO2/yr needed to deal with high overshoot. 25/

Zeke Hausfather (@hausfath): But it is clear that we will need CDR at a much larger scale than is available today, which means that its important to start investing in testing and driving costs down for these technologies today even as we keep our primary focus on reducing emissions as fast as possible 26/

Zeke Hausfather (@hausfath): Or as the report notes, "without appropriate incentive schemes and market designs, CDR implementation options could see under-investment." 27/

Zeke Hausfather (@hausfath): It also makes it clear that these technologies have risks: “the prospect of large-scale CDR could…obstruct near-term emission reduction efforts, mask insufficient policy interventions, might lead to an overreliance on technologies that are still in their infancy..." 28/

Zeke Hausfather (@hausfath): "..., could overburden future generations, might evoke new conflicts over equitable burden-sharing, [and] could impact food security, biodiversity or land rights”. 29/

Zeke Hausfather (@hausfath): For more on CDR in the report, see the final part of my @CarbonBrief article on mitigation pathways: https://www.carbonbrief.org/analysis-what-the-new-ipcc-report-says-about-how-to-limit-warming-to-1-5c-or-2c 30/

Zeke Hausfather (@hausfath): And check out report author @Oliver_Geden's epic twitter thread taking a truly deep dive: https://twitter.com/Oliver_Geden/status/1511063218415886340 31/31

Zeke Hausfather (@hausfath): Gah, carbon dioxide removal. Oh for want of a twitter edit button 😛