President Trump is making good on Candidate Trump’s promise to undo Barack Obama’s important, albeit insufficient, climate legacy. He has already been able to rescind some regulations with just the stroke of a pen, such as Obama’s directive to federal agencies to reduce greenhouse gas emissions. However, his attempts to roll back the hallmarks of Obama’s climate legacy—including emission standards for new and existing power plants and regulations on methane emissions from oil and gas drilling—will need to go through the cumbersome federal rule-making process and will likely get bogged down in court challenges from states and environmentalists. Trump may not succeed on all these fronts, but his reversal of emissions-reduction momentum, as feeble as it was, carries profound dangers for the future stability of the climate system.