It's too expensive.
It would cost too many jobs.
We don't have time.
There isn't enough land on the planet.
It's not reliable enough.
The density (units of energy per unit of weight or volume) isn't high enough for transportation purposes.
All of the above have been put forth as ostensibly rational reasons that the world can't convert to clean, renewable energy.
"It would kill jobs" is a current favorite among politicians who want to pose as representatives of working Americans, all the while pushing policies which have been shown around the world to enrich investors, not workers.
"It's not reliable enough" is popular with folks who want to appear sympathetic to the environment, while preparing to keep a lookout for the cops while the actual raping is going on.
"There's just not enough time" is a favorite of the nuclear industry. Or, at least, it was before Americans heard the words "Fukushima Daichi". It may still be.
But the good news is that none of these objections has any empirical basis. As a recent study published in Energy Policy (39, pp. 1165-1190) shows, there are no technological or economic reasons not to convert to a world powered by renewable energy. The authors (Mark Jacobson of Stanford and Mark Delucchi of UC-Davis) look at costs, materials, technologies and sheer land space. They clearly demonstrate that, using only technologies which already exist, the world could meet all of its new energy needs from renewable sources by 2030, and could supplant all fossil fuel combustion and all nuclear generation by 2050. All this could happen at energy costs comparable to what society is now paying.
Jacobson and Delucchi posit an energy economy based roughly 90% on wind and solar generation, with the other 10% coming from hydro, geothermal, and tidal sources. Their numbers for electrical generation include society's transportation needs, since vehicles would be powered by hydrogen fuel cells, with the hydrogen supply itself being electrically powered. And, since fuel cell vehicles (like other electric cars and trucks) are inherently more efficient than internal-combustion vehicles, up to 30% of transportation energy needs are eliminated by way of increased efficiency.
If (in some alternate political universe) this scenario were converted into a project equivalent to Apollo, Manhattan and the Tennessee Valley Authority all wrapped into one, the specifics delivered in 20 and 40 years would differ in some regards from what these guys envision. But their study needn't be entirely prescient to be incredibly important. If nothing else, it demonstrates that what are now put forth as impassable obstacles are really just implacable excuses. That the problems aren't technological or economic, but political. That the end that threatens us comes not in the form of a bang or even a whimper, but as a campaign contribution and a media blitz.