Scientists from some of the most prominent institutions in the U.S. have written a letter to president Barack Obama urging him not to approve a pipeline that would deliver bitumen from Alberta's oilsands.
The letter is signed by 19 researchers from Princeton, Harvard and Columbia universities, as well as facilities such as the Woods Hole Research Centre and the Scripps Institute of Oceanography.
It was organized by environmentalist and author Bill McKibbon.
The United States government is debating whether to approve the Keystone XL pipeline.
Advocates say it would give that country a safe and secure energy supply and give Canada an assured market.
But some say the pipeline would be unsafe and would lock the U.S. into high-carbon oil when the country is trying to reduce greenhouse gas emissions.