The letter to U.S. President Barack Obama from a coalition of environmental leaders, as well as liberal organizations, argues that Keystone cannot exist alongside efforts to contain climate change.
"Our rationale is simple," the letter read. "Building Keystone XL will expand production in the tarsands, and that reality is not compatible with serious efforts to battle climate change."
The dispatch follows media reports earlier this month suggesting Canada has pointedly reminded the U.S. it is setting emissions targets for the oil and gas sector — targets which should allow Obama to expedite approval of the project.
Any such assurances would be too little, too late, the environmentalists argued.
"The Harper government previously promised to take action to cut pollution across industry, but never followed through with its 2008 plan," the letter says.
"Carbon pollution from the tarsands is now projected to be twice as high in 2020 as envisioned under that plan. Simple arithmetic shows that the only way to reduce emissions from the tarsands is to cap expansion where it is now and reduce production over the coming years."
A separate letter to Obama from the Sierra Club's executive director sounded a similar tone.
"The Canadian government's promises to offset tarsands carbon pollution are nothing more than a rubber check written against an empty account," Michael Brune wrote.
"That check would bounce, just like all of the Harper government's other climate promises."
TransCanada spokesman James Millar disputed the assertions about the carbon footprint of the oilsands, pointing to an Environment Canada study that suggests they produce less than one fifth of one per cent of global greenhouse gas emissions.
"It is one thing for opponents to rail against Keystone but it's another for them to ignore the science and the facts," Millar said.
"Also, if our opponents were truly in favour of reducing GHGs, you would think they would want to reduce tanker traffic from the Middle East, Nigeria, Russia and other foreign countries in favour of a stable pipeline from Canada that runs on electric motors — not tankers that consume vast amounts of oil to power their engines. If they kill Keystone, this scenario keeps repeating itself."
The letters to Obama were sent two days after another black eye for Canadian climate efforts south of the border: A New York Times editorial accusing the Harper government of attempting to "guarantee public ignorance" by muzzling federal scientists.
The Times alleged the Tories have tried to prevent publicly financed scientists from sharing climate change data and "anything to do with Alberta's tarsands" with the public.
"This is more than an attack on academic freedom. It is an attempt to guarantee public ignorance," the editorial reads.
"The Harper policy seems designed to make sure that the tarsands project proceeds quietly, with no surprises, no bad news, no alarms from government scientists. To all the other kinds of pollution the tarsands will yield, we must now add another: the degradation of vital streams of research and information."
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Natural Resources Minister Joe Oliver has decried the Times piece, denying the government has attempted to muzzle publicly funded scientists. But last week, hundreds of scientists rallied on Parliament Hill and demanded the government cease and desist in its attempts to silence them, and urged it to stop cutting research funding.
When Obama announced his national climate plan in June, he said he'd only approve Keystone if it would not "significantly" exacerbate global greenhouse gas emissions.
In a Times interview a month later, Obama took direct aim at Canada.
"There is no doubt that Canada, at the source in those tarsands, could potentially be doing more to mitigate carbon release," he said.
In a country where a Canadian project has become a lightning rod for a long-dormant environmental movement, Obama's remarks resulted in a flurry of speculation. Some believed the president was providing some wiggle room for Canada to pull up its socks and win ultimate approval for the pipeline, while others opined he was signalling his intention to ultimately reject the project.
A national day of action to protest Keystone XL last weekend in an array of U.S. cities, however, was sparsely attended. Polls routinely suggest that most Americans back the pipeline.
The U.S. State Department has been assessing the project for years because it crosses an international border.
State Department officials are currently poring over public comments on their draft environmental analysis on Keystone XL, released earlier this year. A final decision from Obama isn't expected until early next year.
Officials at the powerful Environmental Protection Agency, meantime, are among those who sent public comments to the State Department harshly criticizing its largely positive preliminary environmental report.
The EPA raised concerns about Keystone XL’s carbon footprint, and urged the State Department to rethink its finding that the controversial pipeline would not significantly spur production of Alberta’s carbon-intensive oilsands or boost greenhouse gas emissions.
It was the second time the EPA had publicly denounced the State Department’s environmental review of the pipeline in the five years since TransCanada first applied for a permit to build Keystone XL.
After Obama rejected TransCanada’s permit application in early 2012, the energy giant revised the route around a crucial aquifer in Nebraska. But environmentalists say the new route still poses a major threat to ecologically sensitive areas along its path through six U.S. states from Alberta’s oilsands.