Marching, dancing and poster-waving environmentalists chanting "Hey, Obama. We don't want no climate drama," packed several blocks on and around Washington's National Mall on Sunday, hoping to spur President Barack Obama to take strong measures against climate change.

Allison Spitz, 16, brandished a sign reading "Oil and Water Don't Mix" as she attended her first political demonstration.

"Someday, I hope to have my own kids, and I want them to live in a world that's environmentally safe and natural," Spitz said, explaining why she, her mother and her boyfriend had traveled more than 500 miles from Plymouth, Michigan.

"This is the kind of thing worth leaving home to come out and take a stand for," she said.

Throngs at the "Forward on Climate" rally spent several hours marching from the Washington Monument to the White House, then back to the Mall.

Those who arrived early, and those who didn't let the cold dissuade them from returning to the Mall, heard speakers call on the president to reject the Keystone XL pipeline and to tell the Environmental Protection Agency to set carbon standards for power plants.

During Obama's State of the Union speech on Tuesday, the president urged Congress to do more to combat climate change.

"We can choose to believe that Superstorm Sandy, and the most severe drought in decades, and the worst wildfires some states have ever seen were all just a freak coincidence. Or we can choose to believe in the overwhelming judgment of science and act before it's too late," the president said. "I urge this Congress to pursue a bipartisan, market-based solution to climate change, like the one John McCain and Joe Lieberman worked on together a few years ago. But if Congress won't act soon to protect future generations, I will."

Some activists have been urging Obama to bypass the often messy legislative process and instead focus on executive orders and regulations from the Environmental Protection Agency.

"Congress is a place where good ideas go to die," Melinda Pierce, legislative director for the environmental group the Sierra Club, said last month. "There is a tremendous amount that his administration can do without Congress. He has the authority. He doesn't have to wait for Congress."

Long-term climate change fueled by a buildup of atmospheric carbon emissions is a controversial notion politically, but it's one accepted as fact by most scientists. Atmospheric concentrations of carbon dioxide, which traps heat, have gone up sharply in recent decades, while global average temperatures are up about 1.5 degrees F since records started being kept in the late 19th century, according to NASA.

The Kyoto Protocol, the 1997 pact aimed at reining in carbon emissions, aimed to limit that increase to 3.6 degrees F (2 degrees C) by 2100 -- but a November report by the World Bank warned that trends point more toward an increase twice that big by the end of the century.

The 1,700-mile Keystone XL pipeline project -- which would extend current oil pipeline networks from Alberta, Canada's bitumen-filled ground, all the way across the United States to the Gulf of Mexico -- exposed partisan and regional divisions within Congress during Obama's first term.

Bitumen, otherwise known as tar sand, is oil-soaked dirt that needs to be cooked to extract the oil. A Congressional Research Service report released in June said oil produced from bitumen emits 14% to 20% more greenhouse gases -- the gases that can cause atmospheric hearting -- than the average transportation fuels sold or distributed in the United States.

The Alberta government website says the province is the first jurisdiction to set mandatory reduction targets for greenhouse gas emissions. However, it says while average emissions per oil barrel are declining, since the demand for oil production has increased, overall emissions have increased as well.

At Sunday's rally, Maura Cowley of the Energy Action Coalition verbalized a common theme.

"Keystone XL is a dirty and dangerous pipeline. It's literally going to cut our country in half, carrying a very dangerous fuel, and it will cause runaway climate change," Cowley said. "Young people across the country are the same generation that elected Barack Obama twice now, and we really want to see him reject the Keystone XL pipeline."

The Obama administration rejected the initial proposal from the Canadian energy company TransCanada, saying it needed more time to gauge the potential environmental impact of the project. While that decision pleased environmentalists, pipeline supporters argued that the postponement cost Americans jobs and a degree of energy independence.

The White House has probably already decided against ever approving Keystone XL, despite the pipeline's many supporters, said Patrick Michaels, the director of the Center for the Study of Science at the libertarian think tank the Cato Institute.

"The political calculus is obvious. If they come out against it, they are going to pay a very big price," Michaels said Sunday. "It's very popular around the country: the notion that you are going to be pumping more oil through this country, that there will be some jobs that will be established to put the pipeline in. There will be some jobs for maintenance, and, you know, if you want to talk about the internationalization, if you will, of oil supply, Canada is our biggest supplier and they're friendly. What's not to like?"