Hydrogen has been heralded as the clean fuel of the future; even President George W. Bush touted it in his State of the Union speech last February. But now, scientists say that switching the world economy over to hydrogen might enlarge the ozone hole. The modeling study, published in the 13 June issue of Science, is the first to look in detail at atmospheric changes that would result from wide-scale hydrogen use.
In theory, hydrogen is the ideal fuel. Its controlled oxidation in fuel cells produces only water, instead of gases that contribute to air pollution and global climate change. One way to produce hydrogen is using fossil fuels, which would not result in great environmental benefits; instead, proponents of the technology hope to find other ways, for instance, by using solar or wind power. But few studies have looked at the effects of replacing combustion engines with fuel cells. The problem is that when hydrogen is made, transported, and stored, small quantities will inevitably leak out.
Tracey Tromp, an environmental scientist at the California Institute of Technology in Pasadena, and colleagues teamed up to model what might happen if all cars and other systems that burn fossil fuels were replaced with fuel cells. They estimate that hydrogen levels in the atmosphere could increase as much as eightfold. More hydrogen would lead to more water in the stratosphere, where the ozone layer sits, making it cooler and cloudier, and creating more ice crystals, on whose surfaces ozone-destroying reactions occur. That would make the ozone hole wider, deeper, and more persistent in the spring, the team concludes.
The researchers point out that hydrogen's impact might be lessened by a reduction in atmospheric chlorofluorocarbons, which have been dropping since they were outlawed, or if the soil absorbs more hydrogen than scientists currently think. Designing storage systems and fuel cells to minimize leakage would also help.
"The general idea--that increasing hydrogen would influence the chemistry in the stratosphere--has been in the back of people's minds, but there hasn't been a lot of experimental or even theoretical work," says biogeochemist Richard Gammon at the University of Washington, Seattle. "That's why this is such an important paper." Now, other researchers will have to weigh in with their own models, Gammon says.