A common refrain heard from boosters of the fossil fuel industry states that converting the US economy to run on renewables would be too costly and rely too heavily on unproven technologies. Why risk tanking the nation's economy, they ask, when coal, oil, and natural gas have powered our vehicles and heated our homes for over 150 years? It's a line of argument the green movement has struggled to counter, even as solar and wind prices plummet and nations such as Germany, Denmark, and even China have steeply ramped up their renewable energy production rates.