WASHINGTON - President Barack Obama on Tuesday put climate change at the center of the heated debate over the Keystone XL pipeline by insisting in a major policy speech that the project's fate hinges on how much carbon dioxide it contributes to the atmosphere.

"The net effects of the pipeline's impact on our climate will be absolutely critical in determining whether this project will be allowed to go forward," Obama said as he unveiled a broad plan for combatting global warming.

Obama vowed that his administration will approve the multibillion-dollar pipeline only "if this project does not significantly exacerbate the problem of carbon pollution."

By elevating climate change in the formal debate over Keystone XL, Obama suggested that the risk of environmental damage along the pipeline's path will not be central to the decision, even as a federally commissioned study released Tuesday bolstered arguments that the project itself presents no unique spill danger.

The report by the National Academy of Sciences found that the diluted bitumen that would flow through Keystone XL poses no more threat to pipelines' structural integrity than other crudes.

But for environmentalists, oil industry leaders and landowners in the proposed pipeline's path, neither Obama's remarks nor the pipeline study make a final outcome clear.

Some analysts suggested the president was paving the way for approval, based on the State Department's conclusion in March that the Keystone XL pipeline was unlikely to cause major changes in how oil companies extract dense bitumen from Canada and the carbon emissions tied to that development.

According to the State Department, oil sands development will proceed with or without Keystone XL, because other pipelines or trains can transport it to refiners.

According to the department's analysis, the amount of carbon dioxide emissions linked to oil sands production would drop just 0.83 million metric tons annually if Keystone XL were not built, in contrast to some environmentalists' warnings that new oil sands development fostered by the project could spew 27 million metric tons of greenhouse gases into the air each year.

Conservationists say the State Department's review process has been unfair and the Environmental Protection Agency has said it isn't convinced oil companies would find enough rail cars and other pipelines to sustain oil sands development without Keystone XL.

On Tuesday, environmentalists praised Obama for making climate change a decisive factor on Keystone XL but some fretted that unless the State Department alters its conclusion about the pipeline's carbon footprint, Obama's benchmark could mean the project wins approval.

'Encouraging news'

Bill McKibben, the founder of 350.org, said Obama appeared to be setting "an appropriate standard" for reviewing Keystone XL.

"The president is saying what the science has always demanded," McKibben said. "It's encouraging news."

Sen. Barbara Boxer, D-Calif., noted the tension on the issue. "The president's commitment today to only approve the tar sands pipeline if there is no net increase in carbon pollution is promising," she said, but "tar sands are one of the dirtiest fuels on the planet and the devil is in the details."

Environmentalists say that Keystone XL could contribute to climate change by expanding the marketplace for Canada's oil sands crude and providing a critical avenue for it to reach Gulf Coast refineries. Because the bitumen in Canada's oil sands is harvested through mining and energy-intensive steam-assisted techniques, critics say it has a higher carbon footprint than conventional crude.

Republicans and oil industry leaders, however, said Obama can approve the pipeline based on the criteria he set Tuesday - asserting that it won't increase emissions markedly and citing the State Department's study to support his conclusion.

"The standard the president set today should lead to speedy approval of the Keystone pipeline," said Brendan Buck, a spokesman for House Speaker John Boehner, R-Ohio. "Based on the lengthy review by the State Department, construction of the pipeline would not have a significant environmental impact."

In a research note to clients, ClearView Energy Partners noted that the State Department's estimate of incremental greenhouse gas emissions from Keystone XL is lower than projections by opponents and by the EPA.

"In our view, the president has left the door open to a final approval, and has preserved multiple avenues through it," ClearView Energy Partners said.

Under a 9-year-old executive order, the State Department is tasked with determining whether the border-crossing project is in the national interest. If any one of eight separate federal agencies - including the EPA - disagreed with State's decision, that would launch a process that would put the final verdict in Obama's hands.

Southern leg

TransCanada Corp. first sought approval to build Keystone XL in 2005. While the northern, border-crossing section is under review, the Calgary company has gone forward with construction of the southern leg in Oklahoma and Texas.

The bitumen that Keystone XL would carry is solid at 50 degrees and typically diluted with other oils for pipeline transport. According to the National Academy of Sciences report released Tuesday, the resulting mix, called diluted bitumen, isn't any more likely to cause pipelines to corrode and fail than other crudes.

The 114-page report did not examine the challenges in cleaning up spills of the dense material. The federal Pipeline and Hazardous Materials Safety Administration, which commissioned the study, asked that it focus on whether diluted bitumen poses higher pipeline corrosion and failure risks - not what happens if it does spill. Critics called that a major failing, since recent pipeline accidents proved the dense, sticky hydrocarbon can sink in water and defy typical cleanup methods.

Although diluted bitumen has been transported in the U.S. for decades, safety concerns have spiked as imports climb.

Oil industry representatives cast the National Academy of Sciences study as a vindication of Canadian crude.