Some world leaders have long said it’s hard to reduce the emissions that are warming the planet because they need to use cheap, dirty coal to keep energy affordable. That argument is quickly losing its salience as the cost of renewable energy sources like wind and solar continue to fall. Last year, for the first time, renewables accounted for a majority of new electricity-generating capacity added around the world and 10.3 percent of all electricity generated globally in 2050 came from renewables. Solar panels have dropped 61 percent in price since 2009 and in some of the sunnier places in the world, like India, solar energy could be 10 percent cheaper than electricity generated by burning coal by 2020. Obstacles to the cleaner energy future remain like reducing the price of batteries capable of storing energy for use when the sun isn’t shining, but even those costs are falling. These are all signs that reducing carbon emission can be achieved more quickly and cheaply than widely believed, and that the energy revolution as arrived.