Overview
The leading contributor of the various greenhouse gases is CO2, of which nearly all the additional concentration arrives from the combustion of fossil fuels, and to a lesser extent the burning of tropic rain forests. 56% of all the carbon dioxide formed from the combustion of hydrocarbons still remains in the atmosphere [2], with the rest sequestered by the ocean.

Currently, approximately 22x10^9 man-made tons of CO2 are emitted per year, contributing to the 25% increase since the 18th century. Of all the fossil fuels consumed in the United States, 18% of that goes toward transportation needs, and of that 63% toward motor fuels [4] for surface travel. In 1986, 187 million automobiles [5] were in operation in the U.S., contributing a large majority of the greenhouse gases and pollutants into the atmosphere. The fuel of choice is a mixture of hydrocarbons called gasoline. Several other fuels present themselves as alternatives to gasoline including gaseous hydrocarbons derived from coal, alcohols such as methanol, gasoline blends, and hydrogen. Only one of the alternative fuels mentioned above also has the feature of producing zero greenhouse gases when combusted with air: Hydrogen.

Hydrogen (H2) as an transportation fuel has its share of problems, including storage requirements, detonability, and production. Currently, industrial H2 comes from the water-gas shift reaction with coal, but other methods include electrolysis, thermal decomposition, thermolysis, thermochemical cycles, and photolysis (photo-electrolysis, catalytic photolysis, bio-photolysis). The thermal and electrical requirements in the production of H2 gas makes a natural link with either non-greenhouse gas processes such a fission/fusion and solar photovoltaics or solar thermal systems.

Currently two gaseous fuels vie for supremacy on the ladder of alternative fuels: methane and hydrogen [15]. Carbon based methane produces greenhouse gases along with an assortment of other toxic pollutants including carbon monoxide. Both gases still produce a significant amount of nitric oxide (NO) and nitrogen dioxide (NO2), whose concentration depends on the equivalence ratio (f) and unburned gas conditions. Hydrogen is the leader in reduced tailpipe NO formation, without complex de-NOx (combined NO and NO2 concentrations) apparatus, due to its wide flammability limits.