Subnational governments—cities, counties, and states in the U.S. and around the world—have an essential role to play in reducing greenhouse gas emissions and implementing integrative approaches to adaption. Leading up to the Paris climate talks, California Governor Jerry Brown, in partnership with the German state of Baden-Württemberg, gathered commitments from cities and regions to take specific actions toward aggressive emissions reductions, using a multitude of best practices that have been shared across the world.