Environmental opponents don’t mince words on the Keystone XL pipeline. Some call it the “fuse to the biggest carbon bomb on the planet” because of the carbon emissions from the oil it will carry.

If the pipeline is approved and the fuse lit, climate scientist James Hansen says it’s: “Game over for climate.”

Backers say the Canada-to-U.S. pipeline could lower U.S. dependence on unstable foreign sources of oil and create thousands of jobs.

This project, one of the most contentious of Barack Obama’s presidency or of any energy proposal in U.S. history, has triggered a multiyear slugfest. Critics have turned the Canada-to-U.S. pipeline into a litmus test of Obama’s commitment to fighting climate change.

Tens of thousands of protesters have circled the White House and Capitol, some dressed as polar bears and others carrying an inflatable pipeline with the words: “Climate Champion or Pipeline President.” Billionaire activist Tom Steyerhas funded prime-time TV ads against it.

Meanwhile, Obama has been lobbied to approve the project by the U.S. oil industry, Republican members of Congress and Canadian officials including Prime Minister Stephen Harper who told a business group last year that he won’t “take no for an answer.”

Rhetoric aside, what would the pipeline really do to theclimate and the economy?

Yes, it could lead to a massive spike in heat-trapping greenhouse gas emissions, but only if the oil would otherwise stay in the ground, according to a review of the State Department’s 11-volume final environmental report — released Jan. 31 — and interviews with climate scientists.

Yes, it could create jobs — but not as many as some claim. The State Department estimates that during construction, the project would create 3,900 one-year construction jobs and 38,200 indirect ones, but during operation, only about 50 jobs. Keystone’s owner, Calgary-based TransCanada, says the pipeline would generate about 9,000 construction jobs.

The State Department’s review says the 1,179-mile pipeline, which would carry heavy oil sands from Hardisty, Alberta, through Montana and South Dakota to Steele City, Neb., would do little to change U.S. gasoline prices or oil imports. The reason: Oil is traded on a global market that adjusts to shifts in supply and demand. So even if North America produces more oil, that doesn’t mean it stays here.

The biggest debate centers on another question: Would Keystone affect the development of Canada’s oil or tar sands, which sit below stretches of Alberta’s boreal forest that are about the size of New York state?

Environmentalists say it would. The State Department review — welcomed by supporters of the pipeline, including the oil industry — says it probably wouldn’t, arguing the oil would be transported by other pipelines, truck or rail.

The billion-dollar pipeline needs a permit from the State Department because it crosses an international border. Yet Obama has said he’ll make the final call. He hasn’t publicly shown his hand, but he has said Keystone’s approval depends on whether it “significantly” increases global carbon pollution.

The president was widely expected to approve or reject the project this year, following a separate review by federal agencies of whether it’s in the “national interest.” That time frame could slip, however, because a Nebraska judge invalidated a law in mid-February that allowed approval of Keystone’s route through the state.

“The pipeline is significant in size,” says Adam Brandt, a professor in Stanford University’s Department of Energy Resources Engineering. “It will carry about 1% of global crude oil output, which is a lot for a single project.”

Its controversy may be just as big as its capacity. Here are two of the key issues:

Is Keystone XL a “fuse to the largest carbon bomb on the planet”?

Potentially yes. Bill McKibben, author and environmental activist, has used this catchy phrase to mobilize grass-roots opposition. Hansen, who led NASA’s Goddard Institute for Space Studies from 1981 to 2013, agrees Keystone could unload such a bomb.

“The tar sands are about the dirtiest and most carbon-intensive of the fossil fuels, especially when you consider the damage done and energy used in getting them out of the ground,” Hansen writes in an e-mail. He says they contain twice the amount of oil burned in human history and if developed, “it is game over — we will not be able to stabilize climate.”

Alberta’s oil sands have proven reserves of 170.2 billion barrels — about 11% of global reserves. It’s the world’s third-largest source of these reserves, after Saudi Arabia and Venezuela.

Some climate scientists says Hansen has gone too far. Harvard University’s David Keith, a Canadian who opposes Keystone XL, says developing Canada’s tar sands would not necessarily emit more carbon than a large coal mine. He says environmentalists have made the pipeline “an arbitrary fight.”

“Saying that the tar sands are not necessarily worse than coal is like saying that drinking arsenic is not necessarily worse than drinking cyanide,’ says geophysicist Raymond Pierrehumbert of the University of Chicago.

He says fully developing the tar sands could by itself, “even if we suddenly stopped burning coal,” warm the planet an additional 3.6 degrees Fahrenheit by century’s end — an amount that climate scientists warn could be catastrophic.

The State Department estimates the likely carbon emissions of the 830,000 barrels per day of oil sands that Keystone could carry. It considers the life-cycle or “well to wheel” emissions” of extracting, transporting, refining and burning that amount of oil.

Those annual emissions are huge — 147 to 168 million metric tons of carbon dioxide (MMTCO2e), equivalent to running at least 30 million cars, 7.3 million homes and 42 coal-fired power plants each year. If you add emissions from operating the pipeline, its total carbon footprint over its expected 50-year life is at least 7,422 MMTCO2e — more than all U.S. carbon emissions from human activities in 2010.

Yet State’s review focuses on a much smaller number. It says tar sands would likely replace other oil in the global market, so it looks at how the switch would affect emissions. The State Department report estimates tar sands would produce 17% more carbon emissions over its life cycle than average U.S. crude. so its use could add 23.4 more MMTCO2e per year — equal to that of nearly 5 million cars.

“Oil sands are definitely worse than other types of crude,” says Sandra Yeh, research scientist at the University of California-Davis’ Institute of Transportation Studies. She says conventional mining is now used to extract about half of the tar sands, but increasingly more energy-intensive steam extraction is needed to recover deeper deposits.

Would tar sands be developed even if Keystone XL’s rejected?

Potentially yes. State’s review says the pipeline is unlikely to cause even incremental increases in emissions, because the tar sands will likely be developed anyway — as long as oil prices remain high enough. It says most of the boom in U.S. oil production in recent years is light crude, but Gulf Coast refineries also want heavy crude such as oil sands.

“Rail will likely be able to accommodate new production if new pipelines are delayed or not constructed,’ the review says. It reports the amount of Canadian oil — regular and oil sands — moved by rail has jumped from about 20,000 barrels per day in January 2012 to about 180,000 in November 2013.

State’s review says rail is a dirtier way to transport tar sands to the Gulf Coast and, compared with Keystone, could boost annual greenhouse gas emissions 28%. In an editorial last week, former head of the U.S. Geological Survey Marcia McNutt said she supported Keystone because rail and truck transports would be more environmentally damaging.

“Pipelines are the safest way … by a long shot” to move tar sands, TransCanada’s spokesman Shawn Howard says, noting the recent spate of rail accidents.

Martin Tallett, president of EnSys Energy, a Massachusetts-based consulting firm that worked on State’s report, says the costs of moving tar sands by rail rather than pipeline aren’t much different. He also says Keystone, unlike when it was first formally proposed in September 2008, is now just one of four large pending pipelines, so even if rejected, another will probably move tar sands out of Alberta — to Canada’s west or east coast.

Opponents agree the energy market has changed — so much so that Keystone is irrelevant.

“We simply don’t need this pipeline,” says Anthony Swift, attorney at the Natural Resources Defense Council, an environmental group. He says North Dakota’s existing pipelines are not even operating at full capacity, and the U.S. oil boom obviates the need for Keystone, arguing it will simply benefit Gulf Coast refineries. Citing State’s 2013 draft review, he says more than half of tar sands will be exported after being refined into products.

The vast majority of Canadian oil now being moved by rail to Gulf Coast refineries isn’t tar sands but lighter crude, according to the U.S. Energy Information Administration. A 2013 Goldman Sachs analysis said it’s costlier to move heavy crude by rail than light crude. It said rail cars have weight restrictions so they can carry more light crude at a time, and they need to be specially built to transport tar sands oil, which is so viscous that it needs to be heated in order to be unloaded.

So tar sands oil needs Keystone, Swift says, to ramp up production. Climate scientist Pierrehumbert agrees, arguing the economics of producing tar sands are now “marginal at best” and will get worse as costlier extraction is needed for deeper deposits.

“Denying Keystone will energize the Canadian opposition to the alternative routes,” he says, adding it will make it more difficult to raise capital to expand tar sands development.

Opponents also say tar sands development poses health risks. They cite a 2013 study co-led by the University of Michigan that found significantly higher levels of air pollutants and carcinogens downwind from a tar sands refinery near Edmonton, Alberta.

TransCanada’s Howard says Keystone XL provides cheaper transportation and a “security of supply for Gulf Coast refineries.” He says the United States, despite its recent production boom, will still need to import oil in coming decades, so Keystone allows it to replace some foreign sources with Canadian oil.

Christopher Field, a climate scientist at the Carnegie Institution for Science in Washington, D.C., agrees the U.S. will continue to need oil and can’t turn off that spigot overnight. Yet to avoid catastrophic global warming, he says the planet has to shift away from fossil fuels to carbon-free energy sources such as solar, wind and nuclear.

“The most troubling aspect of the Keystone pipeline is that,” he says, “it’s an encouragement to use oil longer than we should.” Field says the fight over Keystone is largely symbolic but still important, because other countries are looking to the United States to lead on climate change.