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.

Both will be required to use their regulatory authority creatively and aggressively. There is zero chance that Congress will enact the “bipartisan, market-based solution to climate change” that Mr. Obama called for in his State of the Union address. This means that his second-term agenda on climate change will run through Ms. McCarthy’s and Mr. Moniz’s agencies, and will depend almost entirely on executive actions that do not require Congressional approval. Here are three strategies that could make a big dent in carbon emissions.

¶Invoke the E.P.A.’s authority under the Clean Air Act to limit pollution from stationary sources, chiefly fossil-fuel power plants that account for almost 40 percent of the country’s carbon emissions. The agency has already proposed strict standards requiring new power plants to capture their emissions, an untested technology. The bigger problem is what to do with existing plants, which provide a big chunk of the nation’s electricity and which cannot be shut down quickly or by fiat. Devising a gradual phaseout will require ingenuity and persistence in the face of what are sure to be strong legal and political challenges from industry.