We have two messages for climate activists: Focus on replacing coal with cheap low-carbon alternatives. And push for more innovation across a suite of energy technologies - solar, wind, nuclear, batteries and biofuels - to make them more able to compete on performance and price. Coal is a dirty, expensive relic of the 20th century. Climate activists should unite behind a final drive to get the U.S. off it altogether. Moving away from coal would not only slow climate change, it would also protect the health of countless Americans. The stage is already set for this transition. With the emergence of cheap natural gas from shale, coal is no longer the most competitive energy technology in the U.S. Crippled, too, by increased production costs and more stringent federal regulations on pollution, coal is being used less and less in the U.S., even as the rest of the world uses more.