Global Warming is high on my list. The cost for reducing CO2 in the atmosphere seems low enough that this may be the best way to solve it. There are proposals to remove CO2 from the atmosphere at ~$100/tonne. Intuitively, it seems the cheapest way to "remove" CO2 from the atmosphere is to not put more there in the first place: to stop digging up C and burning it. We could still have liquid and gaseous hydrocarbon fuel, we would just need to derive it form biomass: algae and jatropha are both probably in the less than $100 per barrel range, with, I think, algae being very scalable. I wonder if buying up all the coal and oil in the ground and keeping them there once owned wouldn't be the cheapest way to keep C from underground from making it into the air.

Human Population Growth. With less population, all other resource constraints become easier to shift towards sustainable solutions. I'd like to see fertility control tied to aid: you want welfare or international aid, you get fixed. It strikes me the absolute best place to lower population growth is among populations that can't support themselves.