President Obama’s Inaugural Address was his boldest, clearest signal to date that he intends to take on the challenge of fighting climate change as a signature achievement of his presidency.

After barely addressing climate change during his reelection campaign, he made it a centerpiece policy issue of the soaring speech that opened his second term. “We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations,” he said to applause that rippled across the National Mall. “Some may still deny the overwhelming judgment of science, but none can avoid the devastating impact of raging fires and crippling drought and more powerful storms.”

But what, specifically, can Obama do? For now, it seems highly unlikely that he’ll be able to get a bill through Congress. After taking office in 2009 intent on pushing broad climate legislation through Congress, he hit resistance and the bill died in 2010. Prospects for comprehensive legislation seem even bleaker amid gridlock on Capitol Hill.

But the president does have in his executive arsenal one powerful–and extremely controversial–climate-change weapon he could wield without any help from Congress.

Under the terms of a Supreme Court ruling and the nation’s clean-air laws, the Environmental Protection Agency is required to issue a regulation that would force existing industrial polluters, such as coal-fired power plants and oil refineries, to slash their emissions of the greenhouse gases that cause global warming. The rule could have a profound environmental impact by reducing the nation’s global-warming pollutants by up to 20 percent.

The new limits will also ignite immediate and raging political opposition from the nation’s most powerful fossil-fuel interests and likely lead to the shutdown of many of the dirtiest coal plants, possibly costing jobs in the process. It will also be met with an avalanche of efforts by industry and lawmakers to overturn it, either in Congress or in the courts.

“In terms of actual real emissions, this is the mother of all climate rules,” said Frank O’Donnell, president of the advocacy group Clean Air Watch.

The industry agrees. “If EPA were to set a program for existing sources with fairly stringent standards and a fairly short timeline, the only way to comply is to shut down coal units, and that would be hugely disruptive,” said John McManus, vice president of environmental services for American Electric Power, a large coal-burning utility.

Last April, Obama’s EPA proposed a rule to limit carbon pollution from new, rather than existing, power plants. That rule, which is due to be finalized this April, also sparked industry grousing, but it won’t have an immediate impact on polluters. It requires power companies to build low-polluting plants in the future but doesn’t affect how polluters do business today.

The Obama administration has sat on the rule affecting existing polluters for months to avoid issuing a potential job-killer in an election year. That excuse is now gone. And advocates know it. Earlier this month, in a rare show of unity, 70 environmental organizations sent a letter to the White House, insisting that Obama has “the authority under existing law to achieve urgently needed reductions in the carbon pollution that is disrupting our climate and damaging our health.” Also this month, 150,000 small businesses sent Obama a letter urging him to use his executive authority to enact climate regulations on existing polluters.

Environmental groups hope the State of the Union address will offer a clear signal on what concrete steps Obama intends to take next. “We’re looking for the president to clearly indicate that EPA will put forward something on coal plants as the biggest polluter,” said Dan Lashof, director of the climate and clean-air program for the National Resources Defense Council.

They may have to remain patient. EPA is not likely to offer such an explosive proposal until after budget and debt-ceiling standoffs are resolved, and the White House would ideally like to see the economy in better shape before introducing the most restrictive regulation on the energy industry to date.

But the president doesn’t have the luxury of time. The rest of the world is awaiting action from Washington. At a 2009 United Nations summit, Obama signed an accord pledging that the U.S. would reduce its carbon pollution 17 percent by 2020. And in 2015, the nations of the world are scheduled to come together to sign a landmark, binding global treaty to cut carbon emissions enough to stave off the most catastrophic effects of climate change.

There is no way, experts say, that the U.S. can meet its existing climate pledge, let alone sign on with any credibility to a future global treaty, without either a new climate law from Congress or new EPA rules on existing power plants. And if the United States—historically, the largest global-warming polluter—fails to show it can meet its climate pledge, there’s no reason to expect the rest of the world’s nations to live up to their obligations.

“The new regulations on existing plants are going to be essential for the U.S. to meet its commitment internationally,” said Jennifer Morgan, director of the climate and energy program at the World Resources Institute, a think tank. “If the U.S. is seen as not acting, then other countries can hide behind the U.S. It will make it very difficult for the U.S. to have any credibility or influence.”

In other words, if Obama fails to enact a new set of climate regulations, the collapse of a global climate change treaty could be laid at his feet. But if new U.S. climate rules do act as a catalyst for a meaningful global climate treaty that would turn the tide of the worst of global warming, it would stand as a hallmark achievement of Obama’s legacy.

The urgency to solve the problem is only amping up. On Monday, the National Oceanic and Atmospheric Administration reported that 2012 was by far the hottest year on record in the contiguous U.S. It was also the nation’s second-most-extreme year for destructive weather events, with 11 disasters sustaining $1 billion each in economic losses. On Friday, a new draft report by 13 U.S. federal agencies concluded that rising temperatures caused by fossil-fuel emissions have directly contributed to increased wildfires, the spread of insect-transmitted disease, rising sea levels, melting glaciers, and decreasing water supply across the U.S. Scientists said the evidence is unequivocal that the heat and catastrophic weather are linked to climate change caused by fossil-fuel pollution—and that without governmental action to stop that pollution, worse destruction is on the way.