The difference is that every part of the plateau has the same energy potential due to gravity, while every part of the slope does not have the same potential, and the boulder rolling down the slope can cash in some of the difference in potential to keep itself moving. The greater the difference in potential, the greater the payoff in terms of energy released. Notice, though, what happens when the boulder on the slope finally lurches to a stop at the bottom of the valley below: it stops, and another push won’t get it going again. It still has a lot of potential energy in that position – it has, in theory, 4500 miles to fall until it reaches the center of the earth – but there’s nowhere it can go to release any of that energy. Without a difference in potential, how much energy you’ve got is a meaningless statistic. (This is, incidentally, why the quest for zero point energy is an exercise in absurdity; by definition, zero point energy is at the lowest possible potential state, and therefore cannot be made to do any work at all.)
The same rule applies to every energy resource: there has to be a difference in potential that allows energy to be released, and the bigger the difference, the bigger the benefit. With petroleum, the difference is in chemical energy. Those long chains of carbon and hydrogen atoms have a lot of energy to release when they come apart and combine with highly reactive oxygen instead; the short chains that form natural gas have less, and the carbon in coal has less still, though it’s still a lot by the standards of other energy sources. All the extraordinary things our species has done with fossil fuels over the last three hundred years are functions, in effect, of the difference in chemical potential energy between a barrel of oil and a cloud of smoke.
Why are these reflections as welcome in the collective conversation of our time as a slug in a fresh green salad? Because they point up the profoundly shortsighted nature of the decisions that made the world in which all of us now live. The immense potential energy locked up in fossil fuels was put there by millions of years of photosynthesis. It’s as though, to return to our metaphor, living things down through the ages rolled boulders uphill and perched them high above the valley floor. After a half billion years or so, our species came along, and figured out how to roll those boulders downhill. As long as there are still plenty of boulders in place, we can continue using them, but when the rate at which we want to send boulders rolling downhill outstrips the boulder supply, it’s a waste of breath to insist that we can get the same results by bouncing pebbles across the valley floor.
The problem here is that very few people want to deal with that reality. The great majority will make themselves believe in zero point energy and evil space lizards and any other absurdity you care to name, rather than gulp and take a deep breath and admit that the prosperity we’ve enjoyed for the last three centuries was bought at our grandchildren’s expense. I sometimes suspect that one of the reasons so many people like to imagine an apocalyptic end to the industrial age is that sudden extinction is easier to contemplate than the experience of slowly waking up to the full extent of our own collective stupidity.