washington u.s. climate envoy todd stern help hammer international agreement paris last year curb carbon emissions spur development clean technologies step april state department say monday stern climate envoy seven years play enormous role achieve many climate milestones tireless work todd team many years benefit future generations every corner globe secretary state john kerry say release stern replace jonathan pershing geologist spend last three years department energy serve senior climate adviser ernest moniz department secretary fight climate change one president barack obama top priorities administration last year say still meet climate goals despite supreme court put pause clean power plan plan help prompt wide range switch coal-fired power plant natural gas-burning plant release less carbon emissions u.s. climate envoy stern step state department