Four years ago, in sharp contrast to the torpor and denial of the George W. Bush years, President Obama described climate change as one of humanity's most pressing challenges and pledged an all-out effort to pass a cap-and-trade bill limiting greenhouse gas emissions.
Then came one roadblock after another. Congress did not pass a climate bill, cap-and-trade became a dirty word, and, with the 2012 elections approaching, climate change disappeared from the president's vocabulary. He spoke about green jobs and clean energy but not about why these were necessary. In the immediate aftermath of Hurricane Sandy, he spoke only obliquely about the threat of rising seas and extreme weather events, both of which scientists have linked to a warming climate.
Since his re-election, Mr. Obama has agreed to foster a "conversation" on climate change and an "education process" about long-term steps to address it. He needs to do a good deal more than that. Intellectually, Mr. Obama grasps the problem as well as anyone. The question is whether he will bring the powers of the presidency to bear on the problem.
Enlisting market forces in the fight against global warming by putting a price on carbon - through cap-and-trade or a direct tax - seems out of the question for this Congress. But there are weapons at Mr. Obama's disposal that do not require Congressional approval and could go a long way to reducing emissions and reasserting America's global leadership.
One imperative is to make sure that natural gas - which this nation has in abundance and which emits only half the carbon as coal - can be extracted without risk to drinking water or the atmosphere. This may require national legislation to replace the often porous state regulations. Another imperative is to invest not only in familiar alternative energy sources like wind and solar power, but also in basic research, next-generation nuclear plants and experimental technologies that could smooth the path to a low-carbon economy.
Mr. Obama's most promising near-term strategy may be to invoke the Environmental Protection Agency's authority under the Clean Air Act to limit emissions from stationary sources, chiefly power plants.
The agency has already taken a step in that direction by proposing strict emission standards for new power plants that virtually ensure that no new coal-fired plants will be built unless they capture their carbon emissions, which would require employing new technologies that have not been proved on a commercial scale. But that leaves the bigger problem of what to do with existing coal-fired power plants, which still generate roughly 40 percent of the nation's power and obviously cannot be shut down quickly or by fiat.
The Natural Resources Defense Council recently proposed an innovative scheme that would set overall emissions targets but let the individual states - and the utilities that operate in them - figure out how to meet them by making their boilers more efficient, switching to cleaner fuels or by subsidizing energy efficiency and encouraging reduced consumption by individuals and businesses.
Any such regulations are likely to be strongly opposed by industry and will require real persistence on the administration's part. If Mr. Obama takes this approach, he will certainly need a determined leader at E.P.A. to devise and carry out the rules. Lisa Jackson, the E.P.A. administrator who on Thursday announced her resignation after four productive years in one of the federal government's most thankless jobs, was just such a leader.
She suffered setbacks - most notably the White House's regrettable decision to overrule her science-based proposal to update national health standards for ozone, or smog. But she accomplished much, including tougher standards for power plant emissions of mercury and other air toxics, new health standards for soot, and, most important, her agency's finding that carbon dioxide and five other gases that contribute to global warming constituted a danger to public health and could thus be regulated under the Clean Air Act.
That ruling, known as the endangerment finding, made possible the administration's historic new emissions standards for cars and light trucks. It also provided the basis for the first steps toward regulating emissions from new power plants, and, possibly, further steps requiring existing plants to reduce global warming pollution.
In 2009, at the climate summit meeting in Copenhagen, Mr. Obama pledged to reduce this country's greenhouse gas emissions by 17 percent below 2005 levels by 2020. This seemed an impossible goal once Congress rejected the cap-and-trade bill. But the increased use of cheap natural gas, the new fuel standards, the mercury rules and other factors have already put this country on track for a 10 percent reduction by 2020.
By some estimates, reaching the 17 percent goal is well within Mr. Obama's grasp. He has the means at hand to seize it.