Several automakers have announced plans to bring hydrogen fuel cell vehicles to market in the US and abroad within the next year or two. There has been lots of chatter about whether this is a good thing or not, including whether the cars will ultimately reduce pollution on a “well to wheels” basis. They emit only water, but most hydrogen fuel is currently produced from natural gas, which creates emissions of carbon and methane. Eventually, some of the emerging technologies seeking to produce hydrogen fuel from renewable sources such as biogas and solar may become commercially attractive, but for the medium term, one must assume most hydrogen fuel will be produced from natural gas. So how much carbon emissions would be saved? This chart from the Department of Energy lays out the carbon emissions for transportation according to the fuel source. It estimates that using hydrogen produced from natural gas would emit half the carbon currently emitted by internal combustion engines, and almost one-third less carbon than would be emitted by vehicles burning natural gas.