The advocates of "direct air capture," however, seemingly were dealt a blow when a two-year study released in June by a committee of the American Physical Society (APS) cast doubt on whether the technology could ever be cost-effective. Relying on data in the public domain, the APS panel expressed pessimism about the technology's prospects, at least in the short term.

Undaunted, air capture researchers are continuing their work toward demonstration of the technology and commercialization. They say that their own findings, some of which are not yet published, give them reason to be optimistic. And they say their technologies could be combined with others or used in applications that could offset costs. For example, direct air capture might prove