President Trump plans this week to unveil a proposal that would empower states to establish emission standards for coal-fired power plants rather than speeding their retirement — a major overhaul of the Obama administration’s signature climate policy. The plan, which is projected to release at least 12 times the amount of carbon dioxide into the atmosphere compared with the Obama rule over the next decade, comes as scientists have warned that the world will experience increasingly dire climate effects absent a major cut in carbon emissions.