Hydrogen is touted as the clean-burning fuel of the future for automobiles to replace petroleum. Its only significant combustion product is water, and so some supporters say it will help to alleviate the global climate change blamed on carbon dioxide.

A closer look reveals that hydrogen might not be as clean or as green as it seems.

Burning hydrogen produces only water vapor, but water vapor is the most abundant and most potent greenhouse gas. A recent discussion with physics buddy and fellow Honolulu Community College professor Paul Sherard about water emissions from autos piqued my curiosity and led me into the dark realm of data and calculations relating to gasoline usage and hydrogen combustion.

The Intergovernmental Panel on Climate Change and other scientific sources do not include water vapor in lists of common greenhouse gases because the amount in the atmosphere increases with temperature and so scientists consider it to be a passive rather than a dominant greenhouse gas that produces positive feedback as temperature increases.

Modeling the effect of water vapor on climate is more difficult than carbon dioxide because water can condense into clouds that reflect solar energy back into space unlike carbon dioxide, which remains gaseous under all circumstances.

Although water vapor contributes as much as 70 percent of global warming, human activities have not until recently significantly affected its atmospheric concentrations.

A 2007 report published by the National Academy of Sciences concludes that there is an emerging anthropogenic effect in the moisture content of Earth’s atmosphere and in the cycling of moisture between atmosphere, land and ocean.

A 2009 NASA report concluded "the warming of our climate produced as carbon dioxide levels rise will be greatly exacerbated — in fact, more than doubled — by water vapor."

How much water vapor would be added to the atmosphere if all gasoline burning vehicles in the U.S. were converted to burn hydrogen?

Some calculations yield the surprising and astounding result that the amount of water vapor added per year to the atmosphere would very nearly equal the amount currently present in the atmosphere. The effects of such an infusion are unknown, and I have not been able to find any reports dealing with this.

One kind of pollution is when a naturally occurring substance is either out of place or increased in quantity. Water vapor addition on this scale definitely fits that description.

Another de-greening factor not usually mentioned in hydrogen fuel discussions is the release of carbon dioxide during the production of hydrogen.

Both of the two primary ways of producing hydrogen require energy input and generate carbon as a byproduct.

One is to chemically strip the hydrogen atoms from methane (natural gas) molecules. This leaves behind the carbon from the methane to dispose of. The other is to electrolyze water with electricity, the majority of which is still produced by burning of fossil fuels.

These and other issues might not stop the hydrogen craze, but we must be aware that there is no pollution-free solution to our transportation habits and needs as they exist today.