Hydrogen cars and water vapor.

D.W.KEITH AND A.E.FARRELL’S POLICY FORUM “Rethinking hydrogen cars” (18 July, p. 315) draws attention to the need for broad technology assessment of a popular policy alternative. In the pursuit of this new technology, the focus on the problem to be solved can lead to insufficient attention being paid to new environmental problems that might follow from its adoption. These new problems become tomorrow’s unanticipated consequences, and the cycle begins again. This cycle could be dampened, however, with a thorough assessment of the new technology before it has completed development. This cycle is currently under way with hydrogen fuel cells. As fuel cell cars are suggested as a solution to global climate change caused by rising levels of greenhouse gas emissions, they are frequently misidentified as “zero-emissions vehicles.” Fuel cell vehicles emit water vapor. A global fleet could have the potential to emit amounts large enough to affect local or regional distribution of water vapor. Variation in water vapor affects local, regional, and global climates (1). Data on such effects are sparse because of complexities in the water vapor life cycle. However, our preliminary calculations indicate that a complete shift to fuel cell vehicles would do little to slow water vapor emissions, which presumably have increased perceptibly in some metropolitan locations through the growth in use of internal combustion engines. In some locations, changes in relative humidity related to human activity have arguably affected local and regional climate (2, 3). Depending on the fuel cell technologies actually employed, relative humidity in some locales might conceivably increase by an amount greater than with internal combustion engines. This increase could lead to shifts in local or regional precipitation or temperature patterns, with discernible effects on people and ecosystems. The broad environmental effects of fuel cell vehicles are an issue worth addressing via a technology assessment before implementing a solution (4). Not all problems can be anticipated in this manner, but if some can, then the effort will have been well spent (5). In the case of hydrogen cars, the cure may indeed be better than the disease, but we should make sure before taking our medicine. ROGER A. PIELKE JR.,* ROBERTA KLEIN, GENEVIEVE MARICLE, THOMAS CHASE Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309–0216, USA. *To whom correspondence should be addressed. E-mail: [email protected]