IPCC Expert Meeting on Geoengineering

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Annex 3: Keynote Abstracts Nordhaus W.D., 1975: Can we control carbon dioxide? IIASA Technical Papers, 63–75. Rau G.H., and K. Caldeira, 1999: Enhanced carbonate dissolution: a means of sequestering waste CO2 as ocean bicarbonate. Energy Conversion and Management 40, 1803–1813. (DOI: 10.1016/S0196-8904(99)00071-0). Sabine C.L., R.A. Feely, N. Gruber, R.M. Key, K. Lee, J.L. Bullister, R. Wanninkhof, C.S. Wong, D.W.R. Wallace, B. Tilbrook, F.J. Millero, T.-H. Peng, A. Kozyr, T. Ono, and A.F. Rios, 2004: The Oceanic Sink for Anthropogenic CO 2. Science 305, 367–371. (DOI: 10.1126/science.1097403). The Royal Society, 2009: Geoengineering the climate: Science, governance and uncertainty. Royal Society, London, UK. 82 pp., (ISBN: 9780854037735). <strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 27
Annex 3: Keynote Abstracts Keynote II-2: Carbon Dioxide Removal on Land: Biological and Chemical Peter Cox University of Exeter, UK This talk will review land-based Carbon Dioxide Removal (CDR) techniques based on chemical and biological approaches. Free-air capture of CO 2 through physical processes will be covered in a subsequent talk. The Royal Society (2009) considered CDR approaches to be relatively low risk compared to SRM geoengineering, as CDR deals with the primary cause of anthropogenic climate change and ocean acidification. Nevertheless, many of the proposed land-based CDR techniques imply land-use changes that could have consequences for regional climates and ecosystem services. Such issues cut across all three of the Working Groups of the <strong>IPCC</strong> AR5. Chemical approaches to CDR rely on accelerating the natural processes of rock weathering, which removes CO 2 from the atmosphere on multi-millenial timescales. One land-based approach would involve spreading Olivine (Mg2SiO4) over agricultural fields to enhance weathering by one or two orders of magnitude (Schuiling and Krijgsman, 2006). Some studies suggest that the carbon sink produced would be relatively small: 0.1 to 1% of current global CO 2 emissions (Hartmann and Kempe, 2008; Hangx and Spiers, 2009). In general, enhanced weathering methods typically involve mining and moving a larger mass of minerals than the CO 2 captured, so they may also be expensive (The Royal Society, 2009). Land-based biological CDR involves diverting carbon captured by plants to long-lived reservoirs. The different approaches are typically distinguished by the nature of the long-lived carbon store: Afforestation involves planting new forests to accumulate and store carbon. Although relatively benign, in the absence of harvesting, the ultimate accumulated sink is limited by the additional carbon “carrying capacity” of the land, which is probably of the order of 150-200 GtC. Biochar (or charcoal) can be created by thermally-decomposing biomass in a low oxygen environment. Most proposals involve adding biochar to soil to increase carbon storage and improve agricultural productivity. The potential sink by 2100 has been estimated to be significant at 5.5-9.5 GtC/yr (Lehmann et al., 2006). However, the long-term effects on land ecosystems are not well known, and there are few estimates of economic costs. Biomass Burial is an alternative which involves burying wood in anoxic environments (e.g. deep in the soil, Zeng, 2008) where decomposition would be much slower. Estimates of the size of the sink are disputed but are likely to be limited by the cost of burial and competition for biomass with other approaches (e.g. Biochar and BECCS). Lenton (2010) estimates a potential carbon sink of less than 1 GtC/yr, and warns of the possibility of counter-productive emissions of methane from anaerobic decomposition. Bioenergy with Carbon Sequestration (BECS) is a hybrid approach in which bioenergy crops are grown and used as fuel, and the CO 2 emissions are captured and stored. BECS could yield a large potential carbon sink (3- 10.5 GtC/yr; Lenton, 2010), and might even be used to lower atmospheric CO 2. However, it has the same issues as biofuels and conventional CCS, namely potential conflicts for food production and the requirement for safe geological CO 2 storage. BECS will be considered in more detail in a subsequent talk, but is included here for comparison. The Royal Society (2009) evaluated each of the CDR approaches based on their climate effectiveness, affordability, and safety, concluding that most were likely to be significantly more expensive than conventional CCS. A recent review by Lenton (2010) attempted to quantify the potential contribution of the various biological CDR approaches to stabilization of atmospheric CO 2 (see Figure A.3.1, which is based on the upper limits given by Lenton, 2010). These figures suggest that biochar and BECS could together contribute a carbon sink of 14 GtC/yr by 2100. <strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 28
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