With all the talk of hydrogen fueled vehicles, I can’t help but wonder if millions of cars

With all the talk of hydrogen fueled vehicles, I can’t help but wonder if millions of cars driving around spewing out water vapor—a well-known “greenhouse gas” itself—is any better than the carbon dioxide emitted by traditional cars?

—Kelly Grube, Fleetwood, PA

Climate analysts do believe that water vapor in the atmosphere—mostly due to natural evaporation from bodies of water—is already contributing significantly to climate change. According to the esteemed International Panel on Climate Change, atmospheric water vapor exacerbates warming caused by the emission of fossil fuels by as much as 50 percent. However, the additional water vapor that might be created by millions of fuel-cell vehicles running on hydrogen—while it may sound like a lot—would constitute only a drop in the bucket compared to that which naturally occurs.

Water vapor is actually present in our atmosphere at much higher concentrations than carbon dioxide. According to Mississippi State University meteorologist Jeff Haby, who runs the Weather Prediction Website, the average concentration of water vapor in the atmosphere around the globe is presently between two and three percent, while carbon dioxide levels are only at about .04 percent (four one-hundredths of a percent). “That means there is more than 60 times as much water vapor in the atmosphere than carbon dioxide in average conditions,” says Haby.

However, water vapor is far less efficient at trapping heat within Earth”s atmosphere than carbon dioxide, the leading fossil-fuel-based greenhouse gas. Despite its prevalence, water vapor tends to concentrate locally and then get cycled through the meteorological system quickly (in the form of clouds and then rain). Meanwhile, carbon dioxide is an insidious greenhouse gas that lingers in the upper atmosphere for long periods of time and forms a dense barrier to the escape of heat. While water vapor can cause short-term day-to-day warming locally, carbon dioxide can actually raise the Earth”s temperature both globally and permanently.

Meanwhile, fuel cell advocates such as industrial designer Robert Q. Riley do not see the increased production of water vapor by the hoped-for hydrogen-powered vehicles of the future as a major concern. “Natural evaporation from lakes and rivers produces about 1000 times more water vapor than would come from a transportation system that was totally powered by fuel cells,” says Riley. “So the increased moisture in the air is pretty much inconsequential.”

CONTACTS: International Panel on Climate Change, www.ipcc.ch; The Weather Prediction Website, www.theweatherprediction.com; NASA”s Earth Observatory, www.earthobservatory.nasa.gov.