Speaking at Washington's Georgetown University, the president said science "accumulated and reviewed over decades tells us that our planet is changing in ways that will have profound impacts on all of humankind."
"The 12 warmest years in recorded history have all come in the last 15 years," he said. "Last year, temperatures in some areas of the ocean reached record highs. And ice in the Arctic shrank to its smallest size on record, faster than most models had predicted it would."
The president said droughts, fires and floods -- while not solely caused by climate change -- are nevertheless are "affect[ed] by a warming planet."
"And we know that the costs of these events can be measured in lost lives and lost livelihoods, lost homes, lost businesses, hundreds of billions of dollars in emergency services and disaster relief," Obama said.
"In fact, those who are already feeling the effects of climate change don't have time to deny it," he said. "Firefighters are braving longer wildfire seasons, and states and federal governments have to figure out how to budget for that."
The president said "the question now is whether we will have the courage to act before it's too late."
"In my State of the Union address, I urged Congress to come up with a bipartisan market-based solution to climate change, like the one that Republican and Democratic senators worked on together a few years ago," Obama said. "And I still want to see that happen. I'm willing to work with anyone to make that happen."
However, he said climate change "demands our attention now" and told his audience his plan would cut carbon pollution, "protect our country from the impacts of climate change," and make the United States a world leader "in a coordinated assault on a changing climate."
The president called for using more clean sources of energy and imposing federal limits on the level of greenhouse gases power plants can emit. He said he has directed the Environmental Protection Agency "to put an end to the limitless dumping of carbon pollution from our power plants and complete new pollution standards for both new and existing power plants."
A 2007 Supreme Court decision affirmed the EPA's mandate to regulate greenhouse gases.
Obama called for upgrading the nation's infrastructure to prepare it for severe weather.
He said the United States will not "suddenly stop producing fossil fuels," because that would hurt the economy, and "transitioning to a clean energy economy takes time."
Ahead of the speech, senior administration officials said the State Department was independently evaluating whether to grant a permit on the proposed Keystone XL pipeline that would carry asphalt-like petroleum extracted from tar sands in western Canada to refineries along the Texas Gulf Coast.
Environmentalists have turned the Keystone XL permitting decision into a litmus test of the administration's stance on global warming.
Obama Tuesday said allowing the pipeline to be built will require "a finding that doing so would be in our nation's interest. And our national interest will be served, only if this project does not significantly exacerbate the problem of carbon pollution.
"The net effects of the pipeline's impact on our climate will be absolutely critical to determining whether this project is allowed to go forward."
Obama again called for an end to federal tax breaks for big oil companies and more investment "in the clean energy companies that will fuel our future."
A White House release said the administration will make as much as $8 billion in loan guarantees available to support investments in innovative technologies, including those supporting fossil-fuel energy.
Obama said the Interior Department -- which manages and conserves most federal lands and natural resources -- will permit enough renewable-energy projects such as wind and solar on public lands to power more than 6 million homes by 2020.
Renewable energy comes from resources that are continually replenished and also includes rain, tides, waves and thermal energy generated and stored beneath Earth's surface.
While seeking to reduce carbon dioxide fumes, Obama's orders will simultaneously seek to prepare the country for climate change's effects, White House officials said.
Moving forward, the administration will help state and local governments strengthen roads, bridges and shorelines "so we can better protect people's homes, businesses and way of life from severe weather," officials said.
The administration wants to "make sure new roads ... are built to withstand extreme weather," an official said.
The administration will also strengthen areas ravaged by Sandy last October "against future extreme weather" and "create sustainable and resilient hospitals in the face of climate change through a public-private partnership with the healthcare industry," the White House said in a statement.
Thousands of patients, including those in intensive care, were evacuated from hospitals crippled by the storm.
A new program will also help farmers, ranchers and landowners maintain agricultural productivity in the face of growing droughts and wildfires, the statement said.
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