I think that moving off of a petroleum based economy is the biggest challenge in the next 50 years and will determine whether we stagnate or are prosperous. Unfortunately, I think the issue has become politicized to such an extent that many people believe we have the technology now and it’s just a matter of willpower (and money). This is actually not the case.
The New Scientist points out that renewable energy using current technology is not sustainable because it relies on rare metals or inordinate amounts of physical (land/water) resources. Even nuclear power isn’t the answer as it would run out in a mere 40-60 years if it became our primary form of energy production (although there are potentially ways to get more uranium). I have this concern about technology in general; a lot of electronics rely on rare metals that will become in increasingly short supply in the next 50 years. The key will be to move towards biological and (new) carbon based technology.
I think people are underestimating the radical advance that is required. Metals have been studied and manipulated for thousands of years, while utilization of biological components for engineering purposes is in its infancy. While there are some promising developments (e.g. algae biofuel) energy efficiency will be the dominant determinant of prosperity in the intermediate term. We have lots of technology to reduce power/fuel consumption by 50-80% that has been around for decades (geothermal heat pumps, tankless water heaters, CFLs, passive heating/cooling architectural designs, even windows and insulation design) that tend to ignored when discussing our energy problem.
Also, this. We need to do this and faster.