Hydrogen can be manufactured by using solar energy and water. It can also be produced from wind energy or hydro-electricity and water. Hydrogen burned as a fuel produces only water and traces of oxides of nitrogen. Water and oxides of nitrogen are natural in our atmosphere. Rain storms produce both.
Solar engine gensets hold the world efficiency record for converting solar energy to electricity.
Using the electricity to make hydrogen is 30 times more efficient than the best green plant's photosynthesis process. We can do better. The Hydrogen left over could be used in cars to clean the air. We should not burn petrocarbons and deny future generations of opportunities to use fossil reserves to make diamonds, carbon-fiber products that are stronger than steel and countless other new products. Burning Hydrogen does not contribute to the Greenhouse Effect.