Obama lays out climate action plan
US President Barack Obama has laid out a package of measures aimed at curbing climate change, including limits on emissions from power plants.
He also unveiled plans for an expansion of renewable energy projects, improved flood resilience and calls for an international climate deal.
Administration officials had earlier rejected the idea of a "carbon tax".
President Obama pledged in his inaugural address in January to act on climate change in his second term.'Moral obligation'
The activists who gathered at Georgetown University saw a president finally living up to campaign promises first made years ago.
For Barack Obama, pausing frequently to wipe the sweat from his brow - surely the White House didn't choose one of the hottest days of the year to make a point? - this was a chance to lay down a gauntlet.
It's certainly his boldest statement of intent yet on the difficult ground of climate change.
But it's been abundantly clear throughout his more than four years in office that Congress is not simply going to do his bidding. The president may think the debate over climate science is over, but there are still plenty of sceptics ready and willing to say otherwise.
Bypassing Congress may help the president to realise some of his proposals, but Mr Obama knows that tough political and legal battles lie ahead.
Speaking at Georgetown University in Washington DC, President Obama said: "As a president, as a father and as an American, I am here to say we need to act."
President Obama mocked critics who contend climate change is not a threat.
"I don't have much patience for anyone who denies that this challenge is real," he said. "We don't have time for a meeting of the Flat Earth Society."
The president said climate change posed an immediate threat, with the 12 hottest years on record all occurring in the past 15 years.
He added: "While we may not live to see the full realisation of our ambition, we will have the satisfaction of knowing that the world we leave to our children will be better off for what we did."
Most of the president's agenda can be executed without congressional approval, but some issues are likely to face opposition.
The top Republican in the House of Representatives, House Speaker John Boehner, has called the plans "absolutely crazy".
Finally, 16 years after the global agreement to tackle climate change in the Kyoto Protocol, the world can see how the US intends to play its part. It may be cutting CO2 only 4% on 1990 levels by 2020 - less than a fifth of the amount achieved in the EU - but this is at least a plan, and some of the US green think-tanks are grateful for it.
But this is part of what the White House calls an "all of the above" strategy which includes new efficiency standards on trucks, electrical appliances and government buildings - a change that will lift the US out of the 1950s design age; a reduction in short-lived greenhouse gases like methane and soot; a further doubling of wind power, especially on public land; future-proofing infrastructure against climate damages and more.
There are things to upset environmentalists, like the absence of any commitment to drop Keystone XL and the continuing support for biofuels. Nor is the plan as precisely quantified as the UK's climate policy, for instance, which commits to methodically cutting emissions through to 2050. But if the president has the stomach for a legal fight over bypassing Congress on coal, if he's willing to impose extra measures in a few years and if his policies don't get overturned, today's announcement could help the US achieve its international carbon pledges up to 2020. That would be a start.
On Tuesday, the president reaffirmed his 2009 commitment to reducing greenhouse gas emissions by 17% below 2005 levels by the end of the decade.
Critics say these reductions are too modest, and less aggressive than European Union targets.
The plan includes the first-ever limits on carbon emissions from new and existing power plants. These are the single biggest source of carbon pollution, accounting for a third of US greenhouse gas emissions and 40% of its carbon output.
But it remains unclear how strict these limits will be.
Last year, the Environmental Protection Agency proposed regulating emissions from new power plants, but that plan was delayed.
Seven US governors have asked President Obama to abandon this proposal, which they say would "effectively shutter" coal-fired power plants and prevent the construction of new ones.
Senate Minority Leader Mitch McConnell, a Republican, said imposing carbon rules on power plants amounted to a "war on coal".
"This is a huge step in the wrong direction, particularly in the middle of the most tepid recovery after a deep recession in anyone's memory," Mr McConnell said.
President Obama also called for the US to stop supporting new coal-fired plants abroad.
His plan would exempt plants in the poorest nations if the cleanest technology available in those countries is being used.Pipeline challenge
President Obama called for more solar and wind energy projects on public lands, with the aim of powering the equivalent of six million homes by 2020. He also set higher goals for renewable energy at federal housing projects.
In addition, he announced $8bn (£5bn) in federal loan guarantees to spur investment in green technologies.
President Obama also broached the subject of the $7bn, 1,700 mile (2,700km) Keystone XL pipeline, meant to bring heavy crude from the tar sands of Alberta, Canada to the refineries of Texas, saying it should only proceed if it was in the nation's interest.
"The net effects of the pipeline's impact on the climate will be absolutely critical to deciding whether this project goes forward," he said.
Backed by industry and labour unions but staunchly opposed by green campaigners, Keystone XL has turned into one of the biggest environmental challenges of the president's time in office.
Saleemul Huq, senior fellow at the International Institute for Environment and Development, said the plan was "too little too late".
"While it is good to see a leader of the world's richest country and biggest cumulative polluter finally promise to take actions," he said, "after over a decade of refusal to do so, the problem has become much bigger while the US was ignoring it."