There won’t be one.
Just a consolidation.

It is happening already. Oil is getting more expensive. When it reaches $200–300 per barrel and transportation fuel gets to $12 per gallon, there will be rapid change to alternatives.
Unfortunately, it is human nature to be shortsighted and only concern ourselves with the price of gasoline today rather than planning for our future.
When home heating oil reaches $12 per gallon, the price of 3300 sq ft McMansions will fall faster than donuts made with transfats. The new energy source revolution will begin.

Water and oxygen. Slowly, the depletion of one then the other – especially as we are tearing down the rain forests faster then we are replanting trees.

When oil gets absurdly expensive and people are forced to develop alternative fuels.

I agree with those who say that it will be an evolution into new energy sources. And when I say “new”, I don’t necessarily mean “novel”, as in “never seen before”, but “newly economical” forms of energy.

Solar photovoltaics are improving direct conversion of sunlight to electricity for all kinds of uses, from domestic (household) energy use to light industry and even (potentially) automotive uses. (But the conversion rates have to be orders of magnitude better than they are now, because right now using photovoltaics for automotive uses is in absurdly small and lightweight vehicles traveling in places such as the Australian Outback where there is little traffic and endless sunlight.)

Wind-powered electricity generation is not going to provide more than about 5–7% of most energy needs; unless we find dramatic new improvements in the generators, we’ve pretty much tapped our potential there, I think, except for the ever-more-marginal places we attempt to build them.

Tide-powered electricity has some promise, but the engineering obstacles are enormous, in terms of reliability, maintainability and initial installation.

We’re not likely to improve nuclear fission much, or to accept the long-term transportation and storage issues of spent fuel, politically. But fusion still shows promise for implementation within the next 20 years or so.

We haven’t figured a way to tap any usable fraction of the Earth’s own electro-magnetic field. We’ve been using the magnetic poles for navigation for several hundred years, but the Earth itself is a huge generator—we just don’t know how to plug in to it. Ditto thunderstorms, which produce more electrical power in individual lightning strikes than whole cities use in a week. And there are estimates that at any given second on this planet there are approximately 1500 lightning strikes. We haven’t tapped the first one yet.

There’s also a possibility—far out on the horizon—of some combination of robotic and biological energy production and use. What if we could grow artificial muscle mass that could be grafted onto robots to perform simple repetitive tasks (even turning a generator, for example, to produce usable electric power), or if we find a way to generate electric power from some kind of artificial photosynthesis, as plants have been doing for millions hundreds of millions of years.

Once the energy problem is solved, then distillation of water becomes much less of an issue, and air pollution would be (one hopes) nothing more than an historical curiosity.

But even if we had those things today—right now—they wouldn’t be used by anyone rational, not with “actual costs” for fossil fuels being what they are. The oil has to run out first, and maybe the coal, too.

I see two biggies on the horizon. As @worriedguy, @xxii and @CyanoticWasp mentioned, cheap renewable, sustainable energy. Developing that technology will give a massive boost to the world economy. Lord hope it comes soon enough to save us from polluting our planet back into the stone age and having to start all over again. Assuming we are quick enough to solve the global warming problem, then my answer here covers the second HUGE change that is coming.

Massive improvements in efficiency. Not too long from now our phones will be able to do everything we now do with all our media and computing equipment. No more radios. No more TVs. No more computers. No camcorders or cameras or telescopes or microscopes. And they all will have access to 96% of the information there is in the world.

They will drive our cars and run other equipment. They will control every appliance in our kitchens and our houses. They will respond to our voices—there will be no need for keyboards, except in situations where you need to be silent, and even then they might respond to subvocalized commands or requests.

Because these machines will be so important, the need for batteries and more and more powerful and more and more small batteries.

There will be different ramifications in all different industries and sectors of the economy. No more books in schools. No more large pieces of equipment in hospitals—at least as far as vital signs and such are concerned. Automation will be rampant, and that will free people to do more intellectual work. What physical work there is to be done will be done by machines controlled by remote operators.

I don’t even think there will be ship dismemberment on the shores of the Indian ocean. Even that will be done by automated, semi-autonomous machinery.

The key thing will be resources. We will need more and more of the stuff of which things are made. Metals, energy sources, and brain power.