Using hydrogen as fuel may hurt environment. (Not So Green?).

Fossil fuels are often reviled because they produce planet-warming carbon dioxide. However, replacing them with hydrogen gas--considered to be a clean-burning source of energy--may generate a different set of environmental problems, including large and long-lasting ozone holes, according to a new analysis.

The potential problems with hydrogen don't stem from the oxidation of the gas itself, which produces only water vapor. Instead, drawbacks may arise from the almost inevitable leaks of hydrogen gas from facilities that produce it, containers that store it, and fuel cells that use it to generate electricity, says John M. Eiler, a geochemist at the California Institute of Technology in Pasadena, who has analyzed seepage rates from current hydrogen facilities.

Around 15 percent of the hydrogen gas produced to meet future fuel needs could leak into the atmosphere, Eiler and his colleagues estimate in the June 13 Science. That loss could boost the atmospheric concentration of hydrogen at Earth's surface from its natural level of 0.5 parts per million to around 2.3 ppm, they say.

According to the researchers' scenario, some of the gas leaked at ground level eventually will make its way into upper layers of the atmosphere and react with oxygen to create water. An infusion of hydrogen gas would moisturize the upper atmosphere, which is typically dry because it's so cold. The result would be cooler and cloudier upper layers, the researchers say.

That change wouldn't be good for the upper atmosphere's ozone layer because many of the chemical reactions that destroy ozone take place on ice crystals, says Eiler. If all technologies that derive their energy from fossil fuels were instead powered by hydrogen fuel cells, the ozone hole over Antarctica could get 4 percent larger, lose up to 7 percent more ozone than it currently does, and last about 1 week longer each spring. Effects on the Northern Hemisphere's ozone hole, which currently isn't as extensive as the one over Antarctica, might be stronger, the researchers hold.

A rise in atmospheric concentrations of hydrogen gas could have several other effects, says Eiler. An increase in high-altitude clouds might boost the proportion of incoming radiation reflected back into space. Also, soil microbes that use hydrogen gas as a nutrient might become more prolific, with unknown ecological results.

The atmospheric consequences described by Eiler's team "certainly are plausible," says Paul C. Novelli, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Boulder, Colo. Whether those outcomes materialize may well depend on the hydrogen-munching soil microorganisms that are a major consumer of atmospheric hydrogen, Novelli notes.

Eiler agrees. He and his colleagues' recent research suggests that as much as 90 percent of the hydrogen gas emitted from today's sources, which include vehicles and forest fires, ends up locked in soil.