WASHINGTON — President Barack Obama declared the debate over climate change and its causes obsolete Tuesday as he announced a wide-ranging plan to tackle pollution and prepare communities for global warming.

In a major speech at Georgetown University, Obama warned Americans of the deep and disastrous effects of climate change, urging them to take action before it’s too late.

“As a president, as a father and as an American, I’m here to say we need to act,” Obama said.

Obama announced he was directing his administration to launch the first-ever federal regulations on heat-trapping gases emitted by new and existing power plants — “to put an end to the limitless dumping of carbon pollution.”

Other aspects of the plan will boost renewable energy production on federal lands, increase efficiency standards and prepare communities to deal with higher temperatures.

Even before Obama unveiled his plan Tuesday, Republican critics in Congress were lambasting it as a job-killer that would threaten the economic recovery. Obama dismissed those critics, noting the same arguments have been used in the past when the U.S. has taken other steps to protect the environment.

“That’s what they said every time,” Obama said. “And every time, they’ve been wrong.”

Obama touted America’s strengths — research, technology and innovation — as factors that make the U.S. uniquely poised to take on the challenges of global warming. He mocked those who deny that humans are contributing to the warming of the planet, adding that he “doesn’t have much patience” for anybody who refuses to acknowledge the problem.

“We don’t have time for a meeting of the flat-earth society,” Obama said.

Obama also offered a rare insight into his administration’s deliberations on Keystone XL, an oil pipeline whose potential approval has sparked an intense fight between environmental activists and energy producers.

The White House has insisted the State Department is making the decision independently, but Obama said Tuesday he’s instructing the department to approve it only if the project won’t increase overall, net emissions of greenhouse gases.

“Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation’s interests,” Obama said. “Our national interest would be served only if this project does not significantly exacerbate the problem of carbon pollution.”

Obama’s far-reaching plan marks Obama’s most prominent effort yet to deliver on a major priority he laid out in his first presidential campaign and recommitted to at the start of his second term: to fight climate change in the U.S. and abroad and prepare American communities for its effects. Environmental activists have been irked that Obama’s high-minded goals never materialized into a comprehensive plan.

But the proposals will face stiff opposition from some members of Congress, and the controls on power plants are likely to be challenged in court. Even if political and legal challenges are overcome, the proposals will take years to implement. And by using executive action instead of seeking legislative fixes, Obama will be hard-pressed to provide the federal funding that community leaders and environmental activists say are needed to prepare states and towns for climate change.

By expanding permitting on public lands, Obama hopes to generate enough electricity from renewable energy projects such as wind and solar to power the equivalent of 6 million homes by 2020, effectively doubling the electric capacity federal lands now produce. He also set a goal to install 100 megawatts of energy-producing capacity at federal housing projects by the end of the decade.

Obama also announced $8 billion in federal loan guarantees to spur investment in technologies that can keep carbon dioxide produced by power plants from being released into the atmosphere.

But the linchpin of Obama’s plan is the controls on new and existing power plants. Forty percent of U.S. carbon dioxide emissions, and one-third of greenhouse gases overall, come from electric power plants, according to the federal Energy Information Administration. The Obama administration already has proposed controls on new plants, but those controls have been delayed and not yet finalized.

Tuesday’s announcement came just weeks after Obama’s nominee to head the Environmental Protection Agency, Gina McCarthy, assured senators during her confirmation process that the EPA was “not currently” developing any regulations on existing sources of greenhouse gases. McCarthy said if EPA were to look at such regulations, it would allow states, the public and others to “offer meaningful input on potential approaches.”

Republicans quickly dismissed Obama’s plan, calling it a “war on coal” and a “war on jobs,” reflecting the opposition to climate legislation on Capitol Hill that prompted a frustrated Obama to sidestep lawmakers and take action himself.

“It’s tantamount to kicking the ladder out from beneath the feet of many Americans struggling in today’s economy,” Senate Minority Leader Mitch McConnell, R-Ky., said on the floor of the Senate.

Environmental groups offered a mix of praise and wariness that Obama would follow through on the ambitious goals he laid out. Bill Snape of the Center for Biological Diversity described it as too little, too late.

“What he’s proposing isn’t big enough, doesn’t move fast enough, to match the terrifying magnitude of the climate crisis,” Snape said.

Others hailed the plan, galvanized by the fact that Obama was taking action on his own after the reluctance in Congress to tackle the issue using legislation.

“The president nailed it: this can’t wait,” said Frances Beinecke, president of the Natural Resources Defense Council. “We will cut this carbon pollution today so our children don’t inherit climate chaos tomorrow.”