US soldiers with the NATO- led International Security Assistance Force (ISAF) are seen during a foot patrol in Kandahar, south of Kabul, Afghanistan, Jan. 7, 2012. The process of taking over security from over 130,000-strong NATO-led ISAF forces by Afghan troops would be completed by the end of 2014 when Afghanistan will take over the full leadership of its own security duties from US and NATO forces.
Recent studies suggest a divergence is occurring in average human capital levels across US cities, a path dependence story that college graduates want to live and work near others like them while also creating so-called "green" high amenity cities.
It's home-grown, plentiful, and touted as the best way to wean the US off Mideast oil. But there are limits to how far the US can tilt toward a natural gas economy.