Pershing defended U.S. and Obama administration efforts to curb greenhouse gas emissions during a U.N. climate conference in November.
“Those who don't follow what the U.S. is doing may not be informed of the scale and extent of the effort, but it's enormous,” he said, according to The Associated Press.
Pershing announced his departure to his State Department colleagues in an email obtained Thursday by ClimateWire.
"I believe we have, collectively, made strides toward a cleaner world — and one a bit less likely to be catastrophically damaged by climate because of our efforts," Pershing said.
— This story was updated at 12:53 p.m.