The Trump administration is poised to radically weaken fuel efficiency and emissions standards on U.S. automobiles, in the latest blow to efforts to curb catastrophic climate change. The planned changes by Environmental Protection Agency head Scott Pruitt would roll back Obama-era rules meant to limit greenhouse gas emissions from tailpipes, including a requirement that U.S. cars average more than 50 miles per gallon by 2025. Many U.S. states, led by California, have promised to challenge the rollback.