Likely leaks blot green power's perfect reputation.
A hydrogen economy could create bigger, longer-lasting ozone holes over the poles, a new study claims.
If hydrogen catches on as a 'non-polluting' fuel for energy production, leaks from its production and transport could increase the amount of the gas in the atmosphere. This change would worsen ozone depletion, calculate Yuk Yung and co-workers at the California Institute of Technology in Pasadena1.
In a world of imperfect technology, their research suggests, hydrogen is not quite the perfect green fuel it is sometimes made out to be. Although its environmental benefits would still far outweigh any drawbacks.
Devices called fuel cells convert the energy from burning hydrogen directly into electricity. Using these in place of internal combustion engines to power vehicles would drastically reduce emissions of greenhouse gases, which cause global warming, and cut pollutants such as nitrogen oxides, carbon monoxide and small carbon particles.
The world's major automobile companies are currently exploring fuel-cell technology. President George W. Bush has even pledged US$1.2 billion to develop a commercially viable hydrogen-fuelled vehicle.
The idea of a broader hydrogen economy, with the gas fuelling other power-generation demands still faces big hurdles - not least the problem of making hydrogen without relying on fossil-fuel energy - but world leaders are keen to see it happen.
Yung's team estimates that around 10% of all hydrogen manufactured will leak into the atmosphere during production, storage and transport. Current losses are already greater than this.
If so, and if all fossil-fuel energy generation were to be replaced by hydrogen fuel cells, around 60 million tonnes of human-made hydrogen would leak into the atmosphere every year: roughly four times the current amount. There are natural sources of hydrogen too, so this increase would roughly double the total hydrogen input into the atmosphere.
Being so light, hydrogen rises rapidly through the atmosphere. In the upper reaches, or stratosphere, it reacts with oxygen to form water.
A hydrogen economy, say the researchers, would make the stratosphere wetter. This would cool the lower stratosphere, particularly in the polar regions, where most hydrogen is converted to water vapour.
This, they calculate, would disrupt the ozone layer, which protects the Earth's surface from harmful ultraviolet light. It could bring about up to 8% more depletion over the North Pole, and up to 7% more over the South Pole.
Whether this will actually happen greatly depends on how quickly a hydrogen economy is introduced. The use of ozone-depleting CFC propellants and refrigerants has been largely discontinued in most developed countries, and their concentrations in the atmosphere are declining.
If it takes more than 50 years for hydrogen to become widely used as a fuel, CFCs will have largely disappeared and ozone depletion will no longer be a problem. By then, hydrogen transport and production might also be less leaky.
NaTrompme, T. K., Shia, R.-L., Allen, M., Eiler, J. M. & Yung, Y. L. Potential environmental impact of a hydrogen economy on the stratosphere. Science, 300, 1740 - 1742, (2003).