With President Obama poised to decide whether to allow construction of the Keystone XL Pipeline — based on a second, more environmentally-sensitive path — critics now appear focused on derailing the project over a climate change study.

TransCanada Corp. submitted a revised application after the president rejected the first one in January because it took the 1,700-mile-long pipeline across an aquifer in Nebraska.

However, environmental groups say producing oil from Alberta tar sands releases more carbon dioxide than conventional drilling, which would increase global warming, and that studies on the first application were inadequate.

Jeremy Symons, of the National Wildlife Federation, said Sunday the original assessment by the State Department underestimated the amount of greenhouse gas. And he called on the Environmental Protection Agency to do a more complete assessment on the second application.