Dr. J. William Hirzy, a chemistry professor at American Universiy, rests outside the rally route with a graph he uses to teach his students about the relationship between atmospheric carbon dioxide and global temperature.

Reverand Lennox Yearwood, Jr., addresses a crowd of up to 40,000 people at the Forward on Climate Rally in Washington, D.C. on February 17, 2013.

Protestors chant "Shift the Power" during the Forward on Climate Rally

Actress Rosario Dawson addresses the crowd at the Forward on Climate Rally, before marching in solidarity with activists from the Indigenous Environmental Network

The march started on Constituion Ave, south of the White House, and circled around the north side before returning to the starting point, with a crowd filled the streets and stretched over a mile long.

Marchers filled Pennsylvania Avenue directly in front of the White House, with many participants stopping to chant slogans and take pictures beside the fence.

Carl Gibson, from Madison, WI, joined the protest in support of friends who had previously been arrested at anti-Keystone demonstrations in Texas. He values how the Climate movement unites a diverse group of political, environmental and social groups, saying, "It's the thing that ties everyone together...we have a common enemy."

The Grim Reaper was a common theme, ranging from a costumed group to a cutout on a sign that read, "The only steady job on a dying planet will be mine"

Marchers brought a variety of musical instruments to the Forward on Climate Rally, from harmonicas, trumpets and guitars to a communal drum pushed on a homemade dolly

Protestors chant "Shift the Power" during the Forward on Climate Rally

Tens of thousands of protesters turned out on the National Mall Sunday to encourage President Obama to make good on his commitment to act on climate change.

In his Inaugural address from outside the U.S. Capitol, the president said: "We will respond to the threat of climate change knowing that the failure to do so would betray our children and future generations."

Just a few weeks later, next to the Washington Monument, Paul Birkeland was one of a couple dozen people holding a long white tube above their heads.

"It's a backbone. It's a spine. The idea is to ask the president to have some spine and stand up to oil companies. And reject the Keystone Pipeline," Birkeland says.

The activists are focusing on the Keystone XL pipeline because it would carry tar sands oil from Alberta, Canada, to refineries on the coast of the Gulf of Mexico. To make this oil, companies use complex extraction and processing techniques that use a lot of energy. So it has a larger greenhouse gas footprint than conventional crude.

Democratic Senator Sheldon Whitehouse from Rhode Island told the crowd that Congress is sleepwalking through the crisis on climate change. But he said protesters have an important ally.

"There's a man over there in the White House, he has found his voice on climate change. Are we going to have his back," Whitehouse asked.

Other speakers sounded less sure of the president's intentions.

Van Jones, a former adviser to President Obama, says that it would be disastrous if the project gets a green light.

"It would be like lighting a fuse on a carbon bomb — that's what it would be like Mr. President," Jones says.

The Obama administration already let the southern portion of the Keystone XL pipeline go ahead. The State Department is expected to decide soon on the part that would cross the border from Canada and stretch to Oklahoma.

Organizers say it was the biggest climate rally ever in the United States. They claim about 35,000 people participated — although there was no independent crowd count.

The crowd did stretch for several blocks as it made its way around the White House. Despite a cold wind and snow flurries, parents brought along young children.

Heather Clark wrapped her two toddlers in a sleeping bag and put them in their stroller.

"Events like Hurricane Sandy and Hurricane Katrina and everything that I've been reading lately says it's happening. And if we don't do something really, really soon we're all going to be in a state where we don't recognize the planet where we live," Clark says.

Buses brought college students from many states to the National Mall.

Will Jones, was one of them, traveling overnight from Eastern Michigan University. He thinks the president is under a lot of pressure from oil companies.

"Now is the time for him to man up a little bit and make a decision. That's what he's in office to do," Jones says.

But some energy experts say environmentalists are focusing too much on Keystone. They argue that even if that pipeline isn't built, Canadian Tar sands oil will find another way to flow.

Retired Army Col. Dan Nolan represents a group of national security experts called Operation Free. He says there are clear national security benefits to getting more oil from Canada.

"We're putting America's sons and daughters at risk by having to protect oil production capability in the Middle East," Nolan says.

The real challenge he says is to shift the world off oil and other fossil fuels.

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