

Former president Jimmy Carter, shown here speaking at The Washington Post earlier this month, now opposes construction of the Keystone XL pipeline.

Former president Jimmy Carter has joined a group of Nobel laureates who oppose construction of the Keystone XL pipeline, warning President Obama and Secretary of State John F. Kerry, “You stand on the brink of making a choice that will define your legacy on one of the greatest challenges humanity has ever faced – climate change."

By announcing his opposition, Carter becomes the first former president to come out against the controversial project. Two years ago former president George W. Bush called the proposal "a no-brainer," and that same year former president Bill Clinton said that the revised route of the pipeline avoided Nebraska's Sandhills region, ""So, I think we should embrace it and develop a stakeholder-driven system of high standards for doing the work modeled on what was done with auto mileage agreement."

Prominent Democrats -- including those who have worked directly for Obama -- remain split on whether the president should grant TransCandada the right to build the nearly 1,700-mile pipeline. Americans continue to support the project by a wide margin, despite the fact that many environmentalists argue it will accelerate global warming by hastening the energy-intense extraction of heavy crude from Alberta's oil sands region.

In the letter, published as an ad in Politico Wednesday, Carter and other Nobel Prize winners such as South Africa's Archbishop Desmond Tutu argue that rejecting the presidential permit "would signal a new course for the world’s largest economy."

"You know as well as us the power of precedence that this would set,” the letter states. “This leadership by example would usher in a new era where climate change and pollution is given the urgent attention and focus it deserves in a world where the climate crisis is already a daily struggle for so many.”

Some of the Nobel laureates who signed the letter, such as Tutu and landmine activist Jody Williams, had previously expressed their opposition to the project.