At some point there's a threshold beyond which alternative enegry sources become more profitable than fossil fuels, and at that point zillions of dollars will be spent on R&D. We're getting there, albeit perhaps a little more slowly than environmentalists would prefer.

One way to ensure that doesn't happen is to put a tax on carbon emissions. Governments then become dependent on fossil fuel usage for revenue and will enact policies that ensure that revenue stream never gets cut off.