United States approval of TransCanada's Keystone XL pipeline will depend on its contribution to global warming, President Barack Obama said in a major climate change speech Tuesday afternoon.

"The net effects of the pipeline's impact on the climate will be absolutely critical to deciding whether this project goes forward," he reportedly said.

It was an unexpected mention of the controversial project during a speech that focused primarily on new standards for coal-fired power plants and promoting clean energy.

The White House is expected to make a final decision on Keystone XL later this fall, or perhaps even next year.

In the meantime Obama is supporting efforts to place first-ever limits on the carbon dioxide emitted by coal power plants, responsible for one-third of America's greenhouse gas emissions.

Observers speculate such sweeping measures could be part of a grand trade-off, meant to make eventual approval of Keystone XL more palatable to Obama's environmental base.

One key player in the anti-Keystone XL movement, the group 350.org, is already rejecting any such notion.

"There can't be a trade-off," spokesman Daniel Kessler told Bloomberg News. "There is not a trade that makes sense from the physics perspective."

Obama's seemingly tough words on Keystone XL actually reveal little about the White House's decision-making process.

The climate impact of the pipeline, after all, depends on how you measure it. Construction and operation of the pipeline itself would have minimal impact.

Yet the oil sands crude it carries could enable the addition of about 175 megatons of carbon to the atmosphere each year, University of Alberta business professor Andrew Leach has calculated.

That's more annual emissions than three New York cities. Still, that 175 megatons only represents about 2.6 per cent of America's annual carbon footprint (or, for comparison, about 25 percent of Canada's).*

Is that increase on its own significant enough to warrant a rejection of Keystone XL? Likely not. But there's more to Obama's decision than just the simple facts.

Obama will also be weighing the symbolic consequences –- whether approving Keystone XL will affect public perception of his environmental legacy. Or take attention away from his Tuesday pledge to "lead the world in a co-ordinated assault on a changing climate".

*correction made 9:58AM, 26 June 2013

Geoff Dembicki reports on energy and climate change for The Tyee.

Funding for this article was partially provided by the Climate Justice Project of the Canadian Centre for Policy Alternatives, with support from the Fossil Fuel Development Mitigation Fund of Tides Canada Foundation.