In the past century, North America has shifted its main energy source first from wood to coal, then from coal to oil and natural gas  With each transition, the newly dominant fuel has had less carbon and more hydrogen than its predecessor had
It is logical to conclude that in the future the main energy source will be pure hydrogen
If each step in a series of changes involves a decrease of one attribute of the thing undergoing the change and an increase of another, the series will terminate with the first attribute eliminated and only the second attribute present.