Except in the center of the sun (and other stars), elemental hydrogen does not exist in nature. It's attached to other elements like carbon, oxygen, and silicon. To isolate it, these chemical compounds must be pulled apart, and though hydrogen can be pulled from water, it's far more economical to pull it from methane ("natural gas").

Every molecule of methane, or CH4, has 4 atoms of hydrogen ripe for the picking. But there's a problem - what's to be done with the "C" part of the equation, that leftover atom of carbon? Without expensive carbon-capture equipment and processing, it's ejected into the atmosphere, where it quickly combines with oxygen to form greenhouse-gas CO2.

Therein lies the problem. True, when making "green" hydrogen from water there's no leftover carbon atom. But the process is more expensive, and hydrogen manufactured either way is identical - there's no reliable way to tell how it was made. Thus in practice, manufacturers of hydrogen will do what manufacturers of any commodity do best - make it the cheapest way possible, to maximize profit. Even if its impact on climate is worse than burning gasoline.

We could believe oil companies like Shell and Exxon-Mobil will do what's best for the environment and make hydrogen from water. But when there's more money to be made, when have oil companies ever done what's best for the environment?