Tens of thousands of protesters gathered in Washington Sunday to generate pressure on President Barack Obama to take concrete measures to fight global warming.

Waving banners and signs with slogans like "What will be your climate legacy?" the protesters called on Obama to reject the $7 billion Keystone XL pipeline that would bring oil from Canada to Texas, and order the Environmental Protection Agency (EPA) to set carbon standards for power plants.

"We want to challenge president Obama to be a main actor as opposed to being a puppet of the big oil companies," Canadian-born actress Evangeline Lilly, star of TV's "Lost," told AFP. "It's about telling him his speech did not fall in deaf ears."

The event at the National Mall was organized by local and national environmental groups, including the Sierra Club, under an umbrella group named Forward on Climate.

"It's time for the country to wake up, the US has been dragging its feet too long," said Mimi Brody, one of the protesters.

Among the protesters was Nolan Gould, the 14 year-old star of the TV series "Modern Family."

"I'm very hopeful things will change for kids and we'll start realizing we need to take it even further -- take further steps to save this world, because it's all we have and it's not fair to mess it up," Gould told AFP.

"It's by far, by far, the biggest rally on climate in US history," environmental activist Bill McKibben told the crowd as it assembled at the Washington Monument at the Mall. Organizers said that protesters had arrived aboard buses from 28 states.

Van Jones, a one-time Obama adviser on the environment, was strident in his demand for change.

"President Obama, all the good you've done will be wiped out by floods, by fires, by superstorms if you fail to act now!" he told the crowd, indirectly addressing the US leader.

Celebrities who have signed a petition supporting the protesters include Alec Baldwin, Susan Sarandon, Morgan Freeman, Robert Kennedy Jr and Yoko Ono.

The rally comes after the United States last year endured record high temperatures and lengthy droughts, as well as superstorm Sandy, which devastated the New York-New Jersey coastline.

The protest is "very significant," said Jeff Brown, who traveled from the northeastern state of Massachusetts for the demo. "It's demonstrating a strong environmental movement" which is "not fringe, it's mainstream."

Obama mentioned climate change during his inauguration speech in January, and in Tuesday's State of the Union he vowed to take action "for the sake of our children and our future" if Congress fails to do so.

"I will direct my cabinet to come up with executive actions we can take, now and in the future, to reduce pollution, prepare our communities for the consequences of climate change, and speed the transition to more sustainable sources of energy," Obama said in his speech.

The United States is the world's second largest CO2 emitter after China. Soon after taking office in 2009, Obama presented an ambitious measure aimed at reducing carbon dioxide emissions. But the bill ran into stiff resistance from the Republican opposition.

Arctic ice loss, weather extremes show climate risk: UN
Nairobi (AFP) Feb 18, 2013 - Last year's record shrinkage of Arctic sea ice and a spell of catastrophic droughts, floods and storms highlight the risk to the planet from climate change, the UN Environment Programme (UNEP) said on Monday.

In an annual review of the world's environment coinciding with ministerial-level talks in Nairobi, UNEP also warned of an alarming surge in elephant and rhino poaching.

In 2012, summer sea ice in the Arctic covered a record low area of 3.4 million square kilometres (1.31 million square miles), which was 18 percent below the previous recorded minimum in 2007, and 50 percent below the average in the 1980s and 1990s, UNEP said.

Land ice in Greenland also showed signs of melting and permafrost in high latitudes was in retreat, it said.

"Changing environmental conditions in the Arctic, often considered a bellwether for global climate change, have been an issue of concern for some time, but as of yet this awareness has not translated into urgent action," UNEP Executive Director Achim Steiner said.

He pointed to a rush to extract the oil and gas in the Arctic's seabed as the ice retreats and cautioned that the outcome could be even greater emissions of greenhouse gases.

"What we are seeing is that the melting of ice is prompting a rush for exactly the fossil-fuel resources that fuelled the melt in the first place," said Steiner.

"The rush to exploit these vast untapped reserves has consequences that must be carefully thought through by countries everywhere, given the global impacts and issues at stake."

The report -- the UNEP 2013 Year Book -- also noted a string of weather disasters, of which the United States bore the brunt, including its worst drought in decades as well as Hurricane Sandy, which flooded parts of New York City and the coastline of New Jersey.

"Extreme weather events... in 2012 draw attention to the need to prepare for and make efforts to prevent heavy losses" as a result of climate change, it said.

It quoted previously published estimates that Sandy will cost the insurance business between $20 and $25 billion, and building new flood defences to protect the city could cost $20 billion or more.

But these figures could rise, UNEP said.

The report also sounded the alarm over poaching.

"The number of elephants killed in 2011 likely ran into the tens of thousands, and early indications suggest that the same happened again in 2012," it said.

"A record 688 rhinos were poached in South Africa in that year, driving this species further towards extinction."

The Nairobi meeting, running until Friday, is the first since the UN's Rio+20 summit last June that called for "strengthening and upgrading" UNEP and boosting efforts in the green economy.