There has been tantalizing, if very early, progress in the technology of capturing CO₂directly from the atmosphere. If such “air capture” could be done economically, developers of the technology imagine it could radically lower the costs and complexity of carbon capture and sequestration (CCS).

But a recent study has cast serious doubts that such an approach could ever be economically viable.

First, a reminder of how air capture could simplify the development of carbon capture infrastructure. As now envisioned, most CCS regimes would require power plants to capture emissions at or before the smokestack, then pipe the CO₂ some distance to regions with the right geology to entomb the greenhouse gas.

Building the pipeline system to transport these gases within North America, an industry insider once explained to me, could be comparable in scale and cost to the construction of the grid of natural gas pipelines that criss-cross the continent, and that have taken most of a century to build.

Air capture could do away with much of that costly network. Whether CO₂ is captured at the source, a mile away, or on the other side of the planet, it doesn’t matter to the atmosphere where the CO₂ is extracted, so long as the same amount, or more, is removed.

This would make it possible to site CO₂ injection operations directly on top of the best geological sites. It could also simplify the CCS challenge in more populated regions, where zoning complexities, property costs, and public anxieties might make local sequestration operations a headache.

A handful of companies are developing early-stage business models headed toward the goal of air capture. Here in New York City, Global Research Technologies (GRT) is working to commercialize “carbon trees” based on a proprietary material being developed at Columbia University that mops up CO₂ from ambient air. (For more on the technology, I profiled GRT’s plans over at NRDC’s OnEarth, here.)

Promising as it sounds, there’s a twist. Surveying the carbon balances behind air capture, a panel at the American Physical Society (APS) in May issued an unusually blunt rejection of the viability of such systems in a report entitled Direct Air Capture of CO₂ with Chemicals.

The report’s criticism focuses not on the technical viability of direct air capture (DAC) of CO₂, but on the peculiar catch-22 carbon balance that powering any such systems would put in motion.

In short, DAC systems would have to be powered by low-carbon energy sources such as renewables. But in any scenario where there are still higher-carbon power plants on the grid, there’s a bigger benefit to simply using the low-carbon energy sources to replace the higher-carbon power plants and hold off on the DAC systems. In other words, DAC doesn’t make sense until big CO₂ emitters are virtually eliminated from the globe’s power plant mix.

Here’s how the authors put it…

…[ DAC] is not currently an economically viable approach to mitigating climate change. Any commercially interesting DAC system would require significantly lower avoided CO₂costs…. In a world that still has centralized sources of carbon emissions, any future deployment that relies on low-carbon energy sources for powering DAC would usually be less cost-effective than simply using the low-carbon energy to displace those centralized carbon sources. Thus, coherent CO₂ mitigation postpones deployment of DAC until large, centralized CO₂ sources have been nearly eliminated on a global scale…

As I read it, the report shouldn’t derail research into DAC technologies per se. Given the alarming momentum of CO₂ growth in the atmosphere, such tools are potentially powerful aides for the long-term carbon reduction. But the findings should remove the idea that DAC offers a short cut—along with geo-engineering—that could allow humans to continue emitting, business-as-usual, in the hopes that a future, far-off technology will solve the problem of atmospheric CO₂ buildup.

“This report provides no support for arguments in favor of delay in dealing with climate change that are based on the availability of DAC as a compensating strategy,” concludes its authors, among whom is Princeton University’s Robert Socolow. Along with Stephen Pacala, Socolow co-authored the hugely influential “carbon wedges” analysis of the climate change challenge.

As Bill Sweet succinctly puts it over at IEEE Spectrum’s EnergyWise blog, the APS report “will take atmospheric capture of carbon off the policy agenda. This means, together with the collapse of an anticipated nuclear renaissance, that coming to terms with climate change will be more challenging than ever.”

Download a copy of the American Physical Society paper evaluation here, as a PDF.