According to the Intergovernmental Panel on Climate Change (IPCC), primary energy demands are likely to experience a 40 to 150% increase by 2050, with emissions rising to between 48 and 55 Gt CO2/yr.  Predictive models project atmospheric CO2 concentrations ranging from 535 to 983 parts per million (ppm) by 2100, roughly double the present value of 390ppm, resulting in a global mean temperature change from 1990 to 2100 of between 1.4°C to 6.1°C. [2,3] This future scenario brings with it the risk of dangerous climate change, and steps should be taken not only to reduce CO2 emission levels, but to also reduce the amount of CO2 in the atmosphere.
Direct air capture is a geoengineering technique which removes carbon dioxide (CO2) from the atmosphere. This is a variant of post-combustion CO2 scrubbing used for large point sources such as fossil fuel power plants. Invoking the minimum energy of separation required by the second law of thermodynamics, a drawback of direct air capture is that more energy is required to separate the CO2 from air than from a power plant because it is more dilute in CO2 (390ppm in air and about 12 mole% in coal power plant flue gas).
A technique proposed by Lackner utilizes an absorption column whereby a sodium hydroxide solution absorbs CO2 from incoming air, forming sodium carbonate. Calcium hydroxide is then added to cause calcium carbonate to precipitate out of solution. Lastly, the calcium carbonate is heated or calcined at high temperatures to form calcium oxide and CO2, which is then compressed for storage. Baciocchi et al. proposed a design based on this idea and evaluated the economics and energetic of such a scheme.  It concludes that the energy requirement of the calcination step is prohibitively large and this is due to the highly endothermic nature of the reaction. The real energy demand the overall process is nearly double the heat of combustion of coal (17GJ/ton CO2 and 9GJ/ton CO2, respectively), meaning that twice as much energy is required to remove the CO2 emitted from a given amount of coal compared to the energy content of the coal.
Researchers like Keith have improved upon the chemistry of the process by avoiding the calcination process, reducing energy requirements to levels comparable to conventional amine-based sorbent systems used in power plants (150kJ/mol CO2 and 130kJ/mol CO2, respectively). 
A technique that avoids sodium hydroxide absorption is one proposed by Lackner.  Lackner presents a novel solid sorbent technology in the form of an anionic exchange resin that absorbs carbon dioxide when dry and releases it when exposed to moisture. Due to modest temperatures of 45°C required to drive CO2 off the sorbent, it has an energy consumption of about 50kJ/mol CO2 without the need for fossil-derived heating. However, costs associated with the sorbent and sorbent recycling are high.
A recent report by the American Physical Society evaluated the feasibility of a direct air capture system replying on sodium hydroxide absorption as outlined by Baciocchi.  One of its key findings is the issue of "net carbon." Since all capture technologies require energy inputs, and US electricity supply is carbon intensive, it is crucial that the amount of CO2 captured is significantly greater than the CO2 emitted in the capture process. This highlights the importance of adopting a technology that has small energy requirements. This certainly makes Lackner's anionic resin technology appealing compared to the energy intensive calcination step associated with sodium hydroxide absorption. The cost of the air contactor is also too high presently (capture costs are seven to nine times higher than the estimated cost of a reference post-combustion-capture system) and the report advocates the use of passive contact systems such as cooling towers, thereby avoiding the energy costs of operating an absorption-stripping system. Lackner proposes a filter-like air collector design for deployment in dry desert-like climates with low wind speeds not suitable for windmills.
Air capture is and will remain more expensive than capture from large point sources due to the low concentration of CO2 in air compared to flue gases. Large throughput and low concentration also results in the size of air capture devices being much larger than those designed for flue gas scrubbers.
There are several advantages for direct air capture. Perhaps most importantly, large point sources contribute no more than 50% of the overall carbon dioxide emissions and this is the only technology that is able to tackle distributed emissions from transportation and heating. Furthermore, it is the only technology that allows for removal of CO2 already in the atmosphere. Fossil-derived fuels have an unmatched energy density and it is hard to imagine a transportation sector without them in the short run. Air capture can compensate for any emitted CO2 by capturing an equal amount of CO2 at a different location and time. This provides a viable remediation option for industries with high levels of emissions should carbon taxes become significant. In addition, the atmosphere transports CO2 from source to sink at no additional cost, and capture facilities do not need to be located near sources.
While currently uneconomical, it is my opinion that in the long run technologies along Lackner's theme of solid sorbents are best suited for direct air capture and are the only viable solution to mitigate CO2 that has already been emitted into the atmosphere. Sorbent costs will be lowered as air capture becomes adopted on a large scale. Combined with possible carbon taxes on CO2 emitting industries, direct air capture could become part and parcel of industry of the future.
© Alexander McCurdy. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R.E.H. Sims et al, "2007: Energy Supply" in Climate Change 2007: Mitigation, ed. by B. Metz et al. (Cambridge U. Press, 2007). [Available online from the Intergovernmental Panel on Climate Change.]
 J. T. Houghton et al., eds., Climate Change 2001: The Scientific Basis (Cambridge U. Press, 2001). [Available online from the Intergovernmental Panel on Climate Change].
 S. Solomon et al., eds., Climate Change 2007: The Physical Science Basis (Cambridge U. Press, 2007). [Available online from the Intergovernmental Panel on Climate Change.]
 R. Baciocchi, G. Storti and M. Mazzotti, "Process Design and Energy Requirements for the Capture of Carbon Dioxide from Air," Chemical Engineering and Processing 45, 1047 (2006).
 M. Mahmoudkhani and D. W. Keith, "Low-energy Sodium Hydroxide Recovery for CO2 Capture from Atmospheric Air - Thermodynamics Analysis," Int. J. Greenhouse Gas Control 3, 376 (2009).
 K.S. Lackner, "Capture of Carbon Dioxide from Ambient Air," Eur. Phys. J. Special Topics 176, 93 (2009).
 "Direct Air Capture of CO2 with Chemicals", American Physical Society, 1 Jun 11.