Simply put, climate change is the biggest environmental problem the U.S. has ever faced. The nation’s existing fleet of power plants that burn fossil fuels will need to be replaced by nuclear reactors, photovoltaics or power plants that employ the sun’s heat, massive wind farms or tapping into the energy from hot rocks deep beneath our feet. As electricity is cleaned up, hybrids, electric cars or other cleaner vehicles will be needed to eliminate the globe-warming pollution emanating from the more than 100 million tailpipes in the U.S.