IPCC Expert Meeting on Geoengineering

Annex 3: Keynote Abstracts
DAC systems is remarkable and speaks to the lack of real world experience with large DAC systems. It is clear that there
are significant differences in the way a number of key factors are parameterized in the DAC literature which helps to
explain this large disparity in the estimated cost of DAC deployment: (1) major differences in assumptions as to the
pressure drop across the entire DAC system and therefore the amount of energy needed to run the DAC system, (2) the
degree to which to which the cost of separation scales inversely with the concentration of the sought after compound is in
the original starting mixture, (i.e., it should be cheaper – potentially much cheaper—to separate concentrated CO 2 from a
biomass gasification system than from dilute CO 2 in the ambient air), and (3) the very large physical scale of DAC systems
and whether economies of scale or diseconomies of scale would dominate.
DAC systems differ from BECSS in another important respect. As carbon permit prices increase, BECSS systems produce two
very valuable commodities, carbon free electricity and certified negative emissions permits (i.e., tradeable offsets for carbon
emissions in other parts of the global energy and economic system). At some point, the relative value of these two
commodities will change and could change to the point where the more valuable commodity is the certified negative
emissions permits, but the carbon free electricity would still remain an important product from these systems. DAC systems
on the other hand produce no electricity and in fact would be net (perhaps significant) energy consumers as they would
need some form of energy to regenerate the solvent used to remove the CO 2 from the ambient air, run pumps, filters,
compress the captured CO 2 for injection into the deep subsurface as well as many other ancillary energy loads at the DAC
facility. Thus, the DAC systems produce only one product, certified negative emissions permits. The lack of a revenue
stream from being able to provide carbon free electricity could be an impediment that further complicates the early
commercial deployment of DAC systems.
CONCLUDING POINTS:
1. DAC and BECSS can be seen as “backstop” emissions mitigation technologies in that they should set a ceiling on
the cost of CO 2 emissions abatement. Developing a better understanding of the cost to deploy these systems and
the potential scale of their deployment could be a vital input into societal decisions about the relative mix of
mitigation and adaptation as response strategies to anthropogenic climate change (Socolow et al., 2011).
2. As noted by Azar et al. (2010) as well as others, the critical role that BECSS systems would play is in making it
potentially possible to stabilize atmospheric concentrations of CO 2 at low levels near 400 ppm. For stabilization
targets that are above this threshold but yet still stringent (e.g., 450-550 ppm), BECSS could be important in
significantly lowering the cost of stabilization. For example, Azar et al (2010) note that the ability to deploy BECSS
on a large scale allows atmospheric concentrations of CO 2 to be stabilized at a level 50 to 100 ppm lower than
what would be attainable without BECSS for roughly the same cost.
3. Large scale deployment of BECSS or DAC systems would likely increase demand for deep geologic CO 2 storage
reservoirs. The total demand for geologic storage reported in the literature is a small fraction of total theoretical
deep geologic storage space (Dooley and K.V. Calvin, 2010; <strong>IPCC</strong>, 2005; Krey and K. Riahi, 2009) and therefore it
seems unlikely that any increased demand for geologic storage space would be a significant deployment barrier
for BECSS and DAC.
4. For either large scale BECSS or DAC deployment to the point where there would be negative net global emissions
for perhaps decades, there would be a need to remove more than one ton of CO 2 from the atmosphere for each
ton removed by these systems as the oceans and perhaps terrestrial systems would release CO 2 stored in them
until an equilibrium is reached. The magnitude, speed, and duration of these releases from the ocean are all issues
that need to be better understood.
References
Adger W.N., J. Paavola, S. Huq, and M.J. Mace, 2006: Fairness in adaptation to climate change. MIT Press, Cambridge,
MA, USA. 312 pp., (ISBN: 9780262511933).
Azar C., K. Lindgren, M. Obersteiner, Keywan Riahi, D. van Vuuren, K. den Elzen, K. Möllersten, and E. Larson, 2010: The
feasibility of low CO 2 concentration targets and the role of bio-energy with carbon capture and storage (BECCS).
Climatic Change 100, 195–202. (DOI: 10.1007/s10584-010-9832-7).
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