Sample Project: Carbon Air Capture

Carbon Air Capture (CAC) is a form of carbon capture.  Unlike CO2 “scrubbing” schemes that remove greenhouse gases from point sources such as coal stacks, air capture removes CO2 directly from the atmosphere.  It is not generally considered a practical alternative to point-source scrubbing, however a compelling case can be made for it as the “missing link” in any greenhouse gas reduction/mitigation scheme.

In their paper Climate Strategy with CO2 Capture from the Air,  David W. Keith, Minh Ha-Duong and Joshuah K. Stolaroff  studied the technology and economic feasibility of direct capture of CO2.  It is worth quoting their abstract at length here:

It is physically possible to capture CO2 directly from the air and immobilize it in geological structures. Air capture differs from conventional mitigation in three key aspects. First, it removesemissions from any part of the economy with equal ease or difficulty, so its cost provides an absolute cap on the cost of mitigation. Second, it permits reduction in concentrations faster than the natural carbon cycle: the effects of irreversibility are thus partly alleviated. Third, because it is weakly coupled to existing energy infrastructure, air capture may offer stronger economies of scale andsmaller adjustment costs than the more conventional mitigation technologies.

We assess the ultimate physical limits on the amount of energy and land required for air capture and describe two systems that might achieve air capture at prices under 200 and 500 $/tC using current technology.

… Air capture produces an environmental Kuznets curve, in which concentrations are returned to preindustrial levels.

Jonathon Resnick and I explored the logic behind this idea. We found that there are three basic options for treating climate change (whether it's caused by humans or not). They can be summed up in the following diagram:

(If you want to explore these ideas in action, you can try out our approximate-but-still-useful carbon cycle simulator, which you can download as a simple javascript html file.)

It's worth dwelling on this diagram for a bit, because it clarifies much that is opaque in the current debate about climate change mitigation. You may already know that many analysts, including David Keith, consider carbon air capture to be a form of climate engineering. After studying it we did not agree, because removing carbon emissions 'at the stack' isn't considered climate engineering, and removing it from the ambient air is simply taking the same process outside the stack; also because climate engineering treats only the symptom of global temperature, whereas air capture treats the effective (though not ultimate) cause of the problem. 

CAC is better than temperature engineering for other reasons as well. For instance, reducing the Earth's temperature has no effect on CO2's effect as a chemical pollutant. Ocean acidification, which is implicated in the ongoing deterioration of ocean ecosystems, cannot be treated by interventions that simply change the temperature. Reducing the CO2 in the atmosphere, on the other hand, reduces both global temperature and ocean acidity.

There's a further reason to separate these interventions: they have differing degrees of permanence. Keeping the Earth cool using a sunshade will only work as long as you keep the sunshade up; if it slips, you're (literally) toast. And emissions reductions of 100% will do nothing to halt the temperature rise that is already happening. Even a complete cessation of greenhouse gas production by human industry, if accomplished today, would not stop the arctic ice from melting; if the Earth has already passed any of the many tipping-points in the climate regime (such as the positive-feedback release of methane from arctic permafrost) then emissions reductions will have almost no effect on the short and medium-term outcome for the planet. Whereas, removing the CO2 that is already in the atmosphere would have an immediate and, if coupled with emissions reductions, permanent effect.

Everybody talks about emissions reductions; there's surprising support among the public for climate engineering, but few people actually know what it is; and nobody talks or thinks much about carbon air capture, despite its potential to cut the critical connection between industrial emissions and global temperature. Many factors affect the feasibility of air capture:

Our ultimate conclusion was that Carbon Air Capture is both technologically and economically feasible; where problems are likely to arise is with the complex social systems of human politics surrounding its implementation.

John Cassel, Andrew McKee and I further developed these ideas in a risk-analysis of air capture policies. This gets complicated fast, but perhaps the best way to sum up the findings of both groups is to show you a crude diagram that Jonathon and I developed early on. 

Carbon air capture is often criticized as being too expensive, regardless of its technical feasibility.  What we found supported Jonathon's and my conclusion that expense wasn't the issue; faced with the bill that serious climate change is likely to hand us, an expense on the order of $500 billion will actually save us money if it halts the process. No, the ultimate question with carbon air capture is how to turn its adoption into a virtuous circle, where CAC's use breeds more use until it becomes a self-sustaining global movement. This sort of positive-feedback loop is what has driven the adoption and now, over-use of fossil fuels; while financial instruments such as taxes can be used to reduce emissions, if they're designed to create a feedback loop they can actually  drive the reversal of atmospheric CO2 dumping:

(This is a systems diagram: s means supports, o means opposes or reduces.) This version of the diagram shows the virtuous circle being driven by a 'bounty' on carbon. What's happening here is that a tax on emissions is being used to pay for a bounty on carbon. 

The key to the success of a climate-change mitigation strategy that incorporates carbon air capture is designing a virtuous circle such as this.

References

 Grimes Associates, Scotch Plains, Los Alamos

 Robert A Freitas Jr, All Rights Reserved (2010) Water

 David Keith, Sustainable Energy (2009) (February) p. 1-41

Carbon Engineering, David Keith's company dedicated to doing CAC on a commercial scale.

Mahmoudkhani M, Heidel K R, Ferreira J C, Keith D W, Cherry R S (2009) 

 International Risk, Governance Council, International Climate, Swiss Re, Resources For, T H E Future (2008)
Environmental Science & Technology

 Jacqueline D Sharp, Mark K Jaccard, David W Keith (2009)
North 3 p. 641-651