Former President Jimmy Carter announced his opposition to the Keystone XL pipeline today, calling it a “linchpin for tar sands expansion and the increased pollution that will follow.”Carter joined nine other Nobel laureates, including anti-apartheid activist Archbishop Desmond Tutu, in signing a letter urging President Barack Obama and Secretary of State John Kerry to reject the $5.4 billion link between Canadian heavy crude and U.S. Gulf Coast refineries.“Not only is Carter a former president, but he is a former president whose visionary energy policies have proved prescient in recent years,” Anthony Swift, an international attorney at the Natural Resources Defense Council, said in a statement.The Nobel laureates’ open letter was published today in Politico, a Washington-based publication, as a final decision on the project that’s been under review for five years may be near. Eight federal agencies, including the Environmental Protection Agency, have until May 1 to submit comments to the U.S. State Department, which is reviewing the project because it would cross the U.S. border.There’s no timeline for a final decision which Obama, also Nobel Peace Prize winner, has said he will make after the State Department completes its review.The NRDC, the Sierra Club, the Nobel Women’s Initiative and Environmental Defence, a Canadian environmental group, sponsored the ad with the letter. Critics say the pipeline will worsen climate change by promoting development of Alberta’s oil sands, which release more carbon dioxide than other forms of crude oil. Supporters say the oil will find it’s way to market even without the pipeline, which will provide construction jobs.Susan Casey-Lefkowitz, director of the NRDC’s international program, said the letter reflects the “ethical and moral” nature of the Keystone XL debate.As the administration weighs the project, the factions fighting over it have sought to marshal current and former politicians, foreign dignitaries, scientists and celebrities to their cause.Larry Noble, a counsel at the Campaign Legal Center, a Washington-based group that monitors campaign finance and other political issues, said gaining support from well-known people can help generate public concern.It’s particularly important for Keystone opponents to do so because they can’t match the spending the oil and gas industry can make in support of the project, he said.“If you don’t do it and it’s a close vote, then you’re concerned you didn’t pull out all the stops,” Noble said in a phone interview. “Those names will get someone to read something they might not have otherwise read.”Supporters play the game too. The American Petroleum Institute, an oil and gas industry lobbying group, has said in advertisements that former presidents George W. Bush, a Republican, and Bill Clinton, a Democrat, support the piepline. R.C. Hammond, a spokesman for the Consumer Energy Alliance, an industry group whose members include Exxon Mobil Corp., said said in an e-mail that investors Warren Buffett and T. Boone Pickens also favor Keystone, as does action star Chuck Norris.Critics can point to actors Robert Redford, Daryl Hannah, and Jared Leto as allies. Last week NRDC released a list of 100 scientists and engineers urging Obama to reject the project.Swift, of the NRDC, said Carter carries special weight because of his advocacy for energy innovation during his 1977-80 presidency, which included putting solar panels on the White House roof.The State Department is reviewing whether Keystone is in the national interest, weighing factors including its environmental, economic and diplomatic impacts.