MEXICO CITY — She was an energy engineer, a quiet, driven Mexican academic who’d worked at a major U.S. government lab and investigated some of the toughest problems in climate change.

       Claudia Sheinbaum was a natural choice when the prestigious U.N. Intergovernmental Panel on Climate Change selected scientists for a landmark report in 2014. It would warn the world was hurtling toward “irreversible” damage from greenhouse gases, and call for urgent action.