I'm not an expert on energy.  But my impression is that Thorium is the current front runner for a) being able to meet our energy needs and b) having enough to last hundreds or thousands of years.<p>Using the sun cannot be plan A, because as far as I've heard, solar panels barely output enough energy to pay back the energy it takes to make them.  I hope that technology will improve, but it's by no means certain that solar power's EROEI will ever match the 5X of nuclear power, or the 10X of fossil fuels.<p>Energy is by far the most limiting constraint.  With sufficient energy, you can alleviate other constraints.  For instance, if you have enough energy you can desalinize ocean water and use it to irrigate crops to feed large amounts of people.  You can always convert energy into other resources, such as food or steel.  But if you don't have energy, you get Max Max.<p>Finally, the real problem is not necessarily hitting a constraint.  If the U.S. reaches a point where we cannot continue to grow our resource usage intensity, that's not a catastrophe.  The disaster is when you hit a constraint, but your population keeps growing, because then each person will face a declining standard of living.  The other disaster is when you max out a resource and then it grows more scarce, as again, you'll face decline.