Hardly anyone seems to realize it, but we're on the threshold of an era of unbelievable abundance. Within a generation—sooner if we want it enough—we will be able to make a self-replicating machine, first seriously suggested by John von Neumann.
Such a machine would absorb energy through solar cells, eat rock and use the energy and minerals to make copies of itself. Numbers would grow geometrically, and if we manage to design one with a reasonably short replication time—say six months—we could have trillions working for humanity in another generation.
Regarding getting technological advances sooner: The lack of a more vigorous pursuit of the big pay-off breakthroughts has got to be the absolutely hugest opportunity cost we inflict on ourselves. We've somehow managed to allow the United States to get in a position where it is going to waste about $150 billion in Iraq this fiscal year. Yet the total budget for the US National Science Foundation for a wide range of research efforts in many areas of science is about $6 billion dollars. Granted, other research agencies get much larger chunks of money compared to the NSF. But ridiculous fiascos get far more.
The vast bulk of the wealth gained from advances in knowledge go to people other than those who generate the knowledge. So marketplaces really under-reward those who push back the edges of scientific understanding. We therefore do not get as much net benefit from science as we could. We underfund science.
Self-replicating devices (which better have great built-in controls for stopping replication) can make solar photovoltaics cheap.
Right now the human race uses about 13 trillion watts: the solar cells required to produce that much power would take up less than a fifth of one percent of the Earth's land surface—remember that the Earth intercepts more solar energy in an hour than the human race uses in a year. That's a lot of solar cell acreage, but it's affordable as long as they make themselves. We could put them in deserts—in fact, they'd all fit inside the Rub' al Khali, the Empty Quarter of Saudi Arabia. As I understand it, we like depending on the Saudis for energy.
But there are better ways. Solar energy works better in space—sure, the weather is better, but also consider that the vast majority of the Sun's energy misses the Earth. In fact only about one part in two billion warms this planet. Space-based self-replicating systems could harvest some of that lost sunlight—enough to make possible a lot of energy-expensive projects that are currently impractical.
Greg also expects the incredibly greater ability to harness energy and matter to make interstellar probes possible within the lifetimes of some people who are still alive.
If Greg is correct then projections that we'll be burning huge amounts of fossil fuels in the latter part of the 21st century are entirely wrong. We could bring the age of fossil fuels to an end much sooner by pushing much harder to accelerate the rate of advance of nanotechnology. That'd pay orders of magnitude greater dividends than attempts to make everyone think warm fuzzy thoughts about hybrids and hostile thoughts about SUVs.