TransCanada wants to move oil from tar sands projects in Alberta, Canada, through its Keystone XL pipeline, which would extend to refineries along the southern coast of the United States.
Critics say tar sands oil development is dangerous to the environment and more corrosive than conventional crude oil. The planned route for Keystone XL runs through migratory bird pathways, water aquifers and farms in the U.S. heartland.
A group of nine recipients of the Nobel Peace prize, including the Dalai Lama and Archbishop Desmond Tutu, in a letter to Obama said Keystone XL would cause irreparable harm to the environment.
"Your rejection of the pipeline provides a tremendous opportunity to begin transition away from our dependence on oil, coal and gas and instead increase investments in renewable energies and energy efficiency," they said.
But Rolf Westgard, a lecturer at the University of Minnesota and member of the Geological Society of America, writes in the Star Tribune newspaper in Minneapolis that there are nearly 500,000 miles of pipelines in the United States and many of those are positioned on or near many of the same environments as planned for Keystone XL.
He writes it could take about 4,000 years to produce and consume all of the oil targeted for 2018 oil sands production.
"If world oil demand continues unabated for that long, we have major climate problems with or without Alberta and the Keystone XL pipeline," he said.