"Direct Air Capture" (DAC) is a term that has been used to describe a technology that seeks to suck carbon dioxide out of the ambient air. That contrasts with "carbon capture and storage," (CCS) which seeks to either suck carbon dioxide out of the flue stream of a fossil fuel combustion process, or to de-carbonize coal before combustion.
DAC has never been a mainstream technology, in large part because so much has been invested politically and economically in CCS technology. Senator Lamar Alexander once said "w[e] should launch another mini-Manhattan Project and reserve a Nobel Prize for the scientist who can get rid of the carbon from existing coal plants, because coal provides half our energy." But it would appear to deserve some attention, as an effective post-combustion technology could buy some time. David Keith, who moved from the University of Calgary to Harvard a few years ago, has been the primary agitator for this technology, and even founded his own startup company, Carbon Engineering, which counts Bill Gates as one of its investors. It is a Carbon Engineering pilot project that will begin operations in Squamish, British Columbia, next year. Keith claimed in 2009 that carbon dioxide could be captured at a cost of "closer to $100 per ton than $500 per ton." The American Physical Society came out with a withering criticism, offering its own less sanguine estimate of $600 per ton, at the very least. The physical problem with direct air capture is that carbon dioxide comprises such a small fraction of our ambient air -- 0.04% -- so that capture from ambient air is inherently less efficient than, say, capture near the source (a coal-fired power plant). Nevertheless, I'm going to borrow my Governor's go-to line when faced with uncomfortable scientific facts: "I'm not a scientist." Let's see how this pilot plant in British Columbia does.
The politics of direct air capture are simply that no politician has heard of it, and that David Keith or Carbon Engineering is not yet a major political or campaign contributing source. It has to get picked up by an AEP or a Duke. But given the century-long atmospheric life of carbon dioxide, this technology has to be seriously considered.
The one downside of this technology is highlighted in an article I wrote in 2011: that success would ultimately decrease the pressure to reach an international agreement to reduce emissions. Research on direct air capture could allow say, China and India, to simply free-ride off of the research efforts of Canada and the United States. We could build the massive carbon dioxide collector arrays, and China and India would then go on building coal-fired power plants and emitting. That might be politically unappealing.