The simple reason why we need to put a price on carbonSolar and wind are growing fast - by 42% and 19% respectively during the past year - but coal has a much larger installed base, so even much lower growth represents a huge number of new dirty power plants that will pollute for decades to come. And since the cost of running coal plants doesn't include all the environmental damage caused (aka externalities), they appears cheap. So while coal's growth was only 6% in 2012, it's still a huge problem:
From 2001 to 2010, the amount of electricity generated with coal increased by 2,700 terawatt hours. Over the same period, electricity from non-fossil sources—including wind, solar, biomass, hydropower, and nuclear—increased by less than half that amount: or 1,300 terawatt hours. (source)
As long as there's no price on pollution - which a price on carbon would be a good proxy for, though it would be great to also tax the emission of mercury and other toxins - renewables will keep growing fast, but not as fast as they should considering all their benefits which aren't reflected in the price system. Basically, clean energy sources are fighting with a hand tied behind their back.
“In 2011, the last year data has been published, China built as many coal plants as there are in Texas and Ohio combined, even as it led the world in wind deployment. Even China, with its seemingly endless government budgets is still implementing fossil fuels because it’s cheap and high-performing.” --Matthew Stepp, a senior analyst at the Information Technology and Innovation Foundation.