Dear EarthTalk: 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 (CO2) emitted by traditional cars?--Kelly Grubby, Fleetwood, PA
Climate analysts do believe that water vapor in the atmosphere--mostly originating from natural evaporation from bodies of water--is already contributing significantly to climate change. According to the esteemed Intergovernmental 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.
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 CO2 levels are only at about .04 percent.
However, water vapor is far less efficient at trapping heat within Earth's atmosphere than CO2, 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, CO2 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, CO2 is already raising the Earth's temperature both globally and permanently.
Industrial designer Robert Q. Riley argues, "Natural evaporation from lakes and rivers produces about 1,000 times more water vapor than would come from a transportation system that was totally powered by fuel cells." CONTACT: Intergovernmental Panel on Climate Change, www.ipcc.ch; Weather Prediction Website, www.theweatherprediction.com.
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