The National Resources Defense Council published a report this week saying Keystone XL would drive production of so-called tar sands oil and subsequently result in more carbon pollution.
NRDC Director Susan Casey-Lefkowitz said the report "clearly demonstrates" the pipeline would make climate change worse.
President Obama said he'd weigh the pipeline against its environmental footprint. A presidential permit is needed for the section of the pipeline crossing the Canadian border. A domestic leg is under construction from Oklahoma.
Shawn Howard, a spokesman for TransCanada, said the NRDC report contains nothing new.
"When one does a proper accounting of the emissions associated with this pipeline, it is a fraction of what the activists claim," he said in an emailed statement Wednesday. "If they continue to ignore even basic facts, one has to carefully examine all of their claims."
The NRDC said approving the pipeline puts a low-carbon economy in danger. TransCanada said it supports a greener future, though it could be decades away.
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