In an interview with Stephen Colbert that aired last night, President Obama sounded skeptical about the Keystone XL pipeline, noting that it would create few jobs and could contribute to "disastrous" climate change.

Obama didn't say whether he ultimately planned to approve or reject the proposed pipeline, which would transport 830,000 barrels of crude oil per day, mostly from Alberta's oil sands, down to Nebraska. The administration is still awaiting the outcome of a Nebraska court case that could delay or even scuttle the project. But he cast the pipeline in a fairly negative light.

'We've got to make sure that it's not adding to the problem of carbon'

"We've got to make sure that it's not adding to the problem of carbon and climate change," Obama said. "We have to examine that, and we have to weigh that against the amount of jobs that it's actually going to create, which aren't a lot. Essentially, this is Canadian oil passing through the United States to be sold on the world market. It's not going to push down gas prices here in the United States. It's good for Canada. … But we've got to measure that against whether it's going to contribute to the overall warming of the planet, which could be disastrous."

Climate change is one of the big questions at the center of the fight over Keystone XL. President Obama has said before he will only approve the project if it does not "significantly exacerbate" carbon-dioxide emissions. Here's a guide to the dispute over that question:

How bad would Keystone XL be for carbon emissions?

A starting point here is that the oil that comes from Canada's oil sands takes a lot more energy to extract and process. That means a barrel of oil-sands crude leads to 17 percent more greenhouse-gas emissions than the average barrel of oil used in the United States.

A LOT DEPENDS ON HOW MUCH OF THE OIL WOULD BE EXTRACTED AND BURNED WITHOUT THE PIPELINE

So if you just look at the pipeline itself, it could have a real impact on emissions. According to the State Department's environmental review, the 830,000 barrels of oil that the pipeline would transport each day would add an extra 1.3 million to 27.4 million metric tons of carbon dioxide to the atmosphere each year.

How much is that? It's the equivalent of putting an extra 250,000 to 5.5 million cars on the road. (Though you could also argue that it's marginal in the grand scheme of things — humanity emitted about 37 billion metric tons of carbon-dioxide in 2013.)

But there's a twist: The State Department analysis went on to argue that most of this oil would get burned regardless. If Keystone XL gets blocked, oil companies will just ship the crude by rail or truck or alternative pipelines instead. That's why the State Department concluded that the pipeline itself ultimately wouldn't have a "significant" impact on emissions.

Other energy experts, however, have challenged the State Department's conclusion, arguing that there are all sorts of rail bottlenecks that could hamper oil-sands production if the pipeline is blocked. For instance, deadly train crashes have attracted more attention of late, and future safety regulations could make rail more expensive.

What's more, if oil prices keep plummeting — as they have done lately — then the extra cost of rail becomes a bigger burden for producers. The State Department analysis argued that if oil prices drop between $65 and $75 per barrel, then the lack of Keystone could hurt oil-sands production. (And if oil prices drop below $65 per barrel, many oil-sands projects become uneconomical with or without the pipeline.)

Ultimately, it's hard to say exactly how much extra carbon dioxide the project would lead to. What we can say is this: If Keystone XL is blocked, that will make it harder at the margins for oil-sands producers to get their oil to market. But as long as there's high demand for oil around the world, there will also be pressure to get that oil out, pipeline or no pipeline.

And what does this all mean for climate change? Even in the worst-case scenario, the amount of extra carbon dioxide coming from Keystone XL would have a fairly marginal impact on future temperature increases (as is true of any single fossil-fuel project). Ultimately, the question of global warming will be resolved by whether humanity can reduce its demand for oil, gas, and coal and scale up workable alternatives. That's a gargantuan task, and the endless argument over the pipeline is just one small piece of that.

Further reading

-- 9 questions about the Keystone XL pipeline you were too embarrassed to ask

-- Obama has vowed to cut US greenhouse-gas emissions 17 percent by 2020. Is that still possible?