Capturing CO2 from ambient air - David Keith

value is based on a system built with existing technology and no optimization for the purpose of being
most defensible on feasibility terms. It is an inelegant and inefficient design by any other measure. To
illustrate this point, we applied a few obvious improvements to get a more efficient, if less defensible,
design. This reduces the total cost by 40%. We would expect that, with the decade or more of serious
technological development that would likely precede the deployment of air capture, much more drastic
improvements would be made.
But this analysis also suggests caution against exuberance for low-cost air capture. Even the improved
system, at 140 $/t-CO2 is far more costly than most conventional mitigation options. CCS from power
plants is more likely around 30 $/t-CO2, and CO2 emissions credits on the European market are currently
trading for less than that. Air capture still may be a significant savings over switching to hydrogen cars
(Keith and Farrell, 2003), but perhaps not over cellulosic ethanol (Morrow et al., 2006) as a means of
removing carbon from the transportation sector. Some early claims about the cost of calcination-based air
capture were even lower than our lower-bound estimate of 60 $/t-CO2, which is partly why we included
it. We don’t expect air capture will ever be competitive with capture from point sources. If it were, our
lower bound calculation suggests it would have to be based on a process other than calcination.
Yet the unique features of air capture – the potential for negative emissions and the backstop it provides
on the cost of carbon mitigation – mean that even at a high price like 140 $/t-CO2, it is important for climate
policy. Keith et al. (2006) demonstrated this with a long run global economic model, and the results make
sense: air capture reduces the total social cost of mitigation. It guards against high carbon costs, and to
some extent, against damage in extreme climate scenarios. By acting as a hedge against low probability,
high-damage climate scenarios, it also shifts relatively more emissions reduction to the future. In a sense,
fewer cautionary controls are needed on emissions when we know that air capture can be used to reduce
atmospheric concentrations in case of emergency. As Parson (2006) points out, it is the classic “moral
hazard” of insurance. But the danger is not so much that society, in maximizing total discounted social
welfare, chooses to emit slightly more CO2 in the near future, as that opponents of CO2 regulation will use
air capture as a political argument against action on the climate problem. If successful these arguments
could lead to near term emissions far higher than optimum from a social welfare perspective.
This is indeed a point of which air capture proponents should be wary. And the results of this analysis
should serve as a reminder that, while air capture is feasible, the cost could be very high. At 250 $/t-CO2,
air capture is best viewed as an option for use in only the direst of climate emergencies and should be little
comfort for present-day emitters.
In any case, this analysis supports the view that air capture is a serious option worthy of continued
research. We have suggested many avenues for improving the design and finding lower cost versions
of a NaOH-contactor-based air capture system. Additionally, we suggest that longer-term research into
new recovery mechanisms and new sorbents – to break the thermodynamic constraints of calcination or
hydroxide solutions, respectively – may yield the really dramatic cost reductions that would bring air
capture into widespread use.
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