WASHINGTON: President Barack Obama, who rarely uttered the words "climate change" or "global warming" during the second half of his first term and during his re-election campaign, has re-inserted it boldly back into his lexicon.

In his fifth State of the Union address before Congress, Obama sounded like he did in his first, urging lawmakers to limit gases blamed for global warming "for the sake of our children and our future." Those words followed his inaugural address, in which he said: "We will respond to the threat of climate change."

The difference between then and now is that Obama knows Congress is unlikely to agree. He said that if Congress won't act, he will through executive action. The question is, What will he do?

In his toolbox are things as small as requiring appliances to be more efficient and as big as controlling the largest single source of heat-trapping emissions: the carbon pollution from the nation's coal-fired power plants. How boldly will he act in the face of inevitable pushback from industries and the costs of any new regulations to the fragile economy?

Environmentalists already are pressing Obama to kill the proposed Keystone XL oil pipeline from western Canada to refineries on the Texas Gulf Coast as a first public test of his commitment to climate change.

"It's like trying to get to Rome, and there are three or four different roads that get you there," said David Doniger, a senior attorney at the Natural Resources Defense Council, an advocacy group.

One of the more expensive options available to Obama is regulating greenhouse gases from the oldest, dirtiest coal plants, which are already struggling to compete with cheap natural gas. Whether operators choose to shut them down or invest in pollution controls, the cost is likely to raise electricity rates. By contrast, moves to boost energy efficiency in buildings and appliances wouldn't cost as much.

David Victor, a professor at the University of California, San Diego, who has written books on climate change policy, said: "We are close to the point where we have done what we can with regulation without exposing the economy to a lot of extra cost."

Also looming is a pledge made by Obama in December 2009 during international climate negotiations in Copenhagen, Denmark, to reduce U.S. emissions of greenhouse gases by 17 percent below 2005 levels by 2020. Many experts say that target is feasible without Congress passing new legislation to do things such as capping carbon pollution or taxing it, but it would require a combination of other bold and costly moves.

They include: curbing pollution from existing coal-fired power plants and refineries; phasing out hydrofluorocarbons - refrigeration compounds that have helped repair the hole in the Earth's ozone layer caused by chlorofluorocarbons but which are more powerful than carbon dioxide when it comes to global warming; slashing the leaks of methane gas from natural gas production and transmission; and boosting energy efficiency, something Obama has already done for vehicles.

Of those options, the Obama administration has revealed his plans for only one: energy efficiency. The president will request $200 million in his 2014 budget to reward states for saving energy and has pledged to double by 2030 how much the United States produces for every unit of energy.

"That is the first thing you do before you do renewable energy" because it would cut overall demand for energy, said William S. Becker, the executive director of the Presidential Climate Action Project, which since 2007 has been developing policy recommendations on climate and security for the president. Among the 10 ideas the project laid out in a 2012 report was to make America the most energy-efficient nation in the world.

Another was to have the president work with Congress to put a price on carbon. Obama's 2009 call for legislation died in Congress in 2010 despite Democrats being in charge of the House of Representatives and Senate. After Republicans took control of the House after midterm elections in 2010, Obama abandoned legislation and said it was only "one way of skinning the cat" on climate.

His administration has already found other means - by doubling fuel-economy standards for cars and proposing the first-ever greenhouse gas standards for new power plants.

Obama could go further by tightening fuel-economy standards for trucks, requiring climate change to be a factor in environmental reviews of projects such as bridges, oil and gas wells, and industrial facilities, and enforcing U.S. Securities and Exchange Commission guidelines that require companies to disclose climate change impacts.

But many experts agree that those actions, and new ones, will fall short of what is needed to halt the buildup of heat-trapping gases in the atmosphere.

"Certainly if Congress is able to act, we would have much more comprehensive plans and get larger greenhouse gas reductions faster," said Dina Kruger, the former director of the U.S. Environmental Protection Agency's climate change division. "We can't wait for the perfect solution. We need to start now."