Keith and Farrell (2003a) discuss some of the negative issues relating to widespread use of hydrogen cars. The hydrogen fuel cell in such vehicles is said to be "zero emission," whereas in fact it does emit water vapor. Water vapor, like CO2, is a greenhouse gas but the pre-industrial atmosphere had (and still has) about 30,000 times as much water vapor as CO2, making water vapor the most abundant naturally occurring greenhouse gas. So should we worry about water vapor emissions from hydrogen cars? Pielke et al. (2003) urge that careful calculations be made to ensure we don't trade one problem for another, but Keith and Farrell (2003b) assert the effects will be insignificant, since they are not emitted in the stratospere.
References
Keith, D.W., and A.E. Farrell, 2003a: Rethinking hydrogen cars. Science, 301, 315-316.
Keith, D.W., and A.E. Farrell, 2003b: Rethinking hydrogen cars - Response. Science, 302, 1329.
Pielke, R.A., Jr., R. Klein, G. Maricle, and T. Chase, 2003: Letter to the Editor - Hydrogen cars and water vapor. Science, 302, 1329.