In 2009, President Obama pledged to reduce America’s greenhouse gas emissions by 17 percent from 2005 levels by 2020. Thanks to several factors, the country is halfway there. On Monday, Mr. Obama announced the appointment of two seasoned officials who could fulfill that pledge — but only if the president himself helps them navigate the formidable political obstacles ahead.

Mr. Obama nominated Gina McCarthy, an experienced clean air regulator, to run the Environmental Protection Agency, and Ernest Moniz, an M.I.T. physicist and strong advocate of natural gas and nuclear power, to run the Energy Department. Both believe global warming is one of humanity’s most pressing challenges. Both have deep experience — Ms. McCarthy as an assistant administrator at the E.P.A. and an adviser to Republican governors in Connecticut and Massachusetts, Mr. Moniz as an under secretary of energy in the Clinton administration.