If applied consistently, the logic that treats carbon dioxide as air pollution would end whatever hope remains for hydrogen-powered vehicles.
Last month the US Environmental Protection Agency proposed to find that CO2 and five other greenhouse gases contribute to air pollution and might endanger public health or welfare.
The agency acted under a 2007 Supreme Court ruling that greenhouse gases fit the Clean Air Act's definition of "pollutant." It didn't matter that CO2 governs breathing of animals and sustains plants, among other things.
The ruling addressed legal wording, not scientific questions about human influence on global average temperature. But it means that if EPA makes an endangerment finding, the agency has to regulate emissions of them from motor vehicles.
So a gas essential to life plus methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are "pollutants" by court order because they are greenhouse gases, regardless of the degree to which human emissions of them may have contributed to the uneven warming observed over the past 150 years.
By extension, this contortion of the meaning of "pollutant" bodes ill for hydrogen as a vehicle fuel.
The administration of former President George W. Bush feverishly promoted hydrogen as a vehicle fuel in direct combustion or through use as the energy carrier in onboard fuel cells.
Since then, hydrogen seems to have fallen from favor, partly for technical reasons overlooked in the initial hubbub but also because biofuels have broader political support.
But the initial appeal was strong. Hydrogen was said to be a zero-pollution vehicle fuel. Its combustion emits only water.
But wait: Water vapor's greenhouse influence is far stronger than CO2's. Especially if the rise in global average temperature resumes after an apparent 10-year pause, more water in the environment should mean more water vapor in the atmosphere and hence more warming.
By the legal reasoning of the day, blind as it is to technicalities such as how much extra water vapor is really at stake here, that's pollution. If CO2 endangers life, water vapor must do so many times over.
A ban on hydrogen-powered vehicles thus seems only logical.
(Online May 1, 2009; author's e-mail: email@example.com)