The State Department has just released its 2,000-page draft environmental review of the controversial Keystone XL pipeline, which would carry oil from the tar sands of Alberta, Canada, down to Steele City, Nebraska and on to the Gulf of Mexico.

(The Washington Post)

The big takeaway: The State Department concluded that either blocking or approving the Keystone XL pipeline would have a small impact on overall greenhouse-gas emissions and future tar-sands expansions. That's because, in its view, most of Alberta's oil will find a way to get to the market anyway — if not by pipeline, then by rail.

At the high end, the review concludes, blocking all proposed future pipelines out of Alberta would cut tar sands production 2 to 4 percent and have the same climate impact as taking one million cars off the road by 2030. That's the equivalent of about 0.07 percent of current U.S. emissions. (Other environmental groups, however, have argued that the climate impacts could be six times as large as this estimate.)

Now, this report is not a final approval of the pipeline. Next up, there's a 45-day public comment period for the environmental review. Then the State Department will have to determine whether the pipeline is in America's national interest. Finally, President Obama has to decide whether to approve it or not.

Still, this is a key step in the process. Here are five key points from today's report:

1) Oil from the tar sands produces 17 percent more greenhouse-gas emissions over its life-cycle than regular oil. As such, the State Department points out that the pipeline could increase overall carbon pollution — if it leads to a major expansion of Canada's tar-sands production. That, in turn, would worsen global warming. So that brings us to the key question...

2) The State Department thinks blocking the Keystone pipeline would have only a small impact on tar-sands production and climate change. If Canada can't find an outlet for its oil, there will be less production. So what happens if Keystone XL gets blocked? A lot depends on whether other pipelines get built, such as a proposed project through British Columbia — as well as how much oil can be shipped profitably by rail instead.

The State Department considered two scenarios. First, if the Keystone pipeline is blocked, and other proposed pipelines go forward, tar sands production in Alberta "could decrease by approximately 0.4 to 0.6 percent" by 2030. A fairly small drop.

Alternatively, if Keystone XL was blocked and all of the proposed pipeline capacity out of Alberta were restricted, then tar sands production would drop by 2 to 4 percent by 2030.

That's a real dip, albeit a small one. In that second scenario, we would see a decrease of somewhere between 0.07 to 5.3 million metric tons of carbon dioxide per year by 2030. At the high end, that would be like taking one million cars off the road. But that's also equivalent to just 0.07 percent of current U.S. emissions.

Environmentalists, for their part, are hotly disputing this section of the review. The Natural Resources Defense Council, for one, has argued (pdf) the climate impact of Keystone could be at least six times as large as the State Department's estimate.

Other groups are skeptical that the oil will flow no matter what if Keystone is blocked. “Canadian tar sands exports are blocked to the west by tribes that won’t sell out their natural resources to Big Oil, and blocked to the east by the European Union’s declaration that it won’t buy dirty tar sands oil," said Jim Lyon of the National Wildlife Federation. "Without access to major U.S. export terminals from Keystone XL and other routes, tar sands production will be substantially slowed."

3) If the Keystone XL pipeline is blocked, rail transport could make up most of the difference. "Rail and supporting non-pipeline modes should be capable... of providing the capacity needed to transport all incremental Western Canadian and Bakken crude oil production to markets if there were no additional pipeline projects approved." Elsewhere, Robert Bryce has explained how rail companies will deliver oil if the pipeline gets blocked.

4) A pipeline spill is "very unlikely" to affect the key Ogallala Aquifer. The Ogallala Aquifer in the Midwest is one of the key freshwater sources for the Great Plains. So there's plenty of justifiable worry about running the Keystone pipeline through this area, given the potential for leaks.

But the State Department's not overly concerned about this: "Overall, it is very unlikely that the proposed pipeline area would affect water quality in the [Great Plains Area] due to weak downward gradients (downward groundwater flows) in the aquifers overlying the [Great Plains Area]."

5) The Keystone XL project, if built, would support 42,000 jobs over its two-year construction period. "Including direct, indirect, and induced effects, the proposed Project would potentially support approximately 42,100 average annual jobs across the United States over a 1-to 2-year construction period (of which, approximately 3,900 would be directly employed in construction activities)." As with all construction projects, these jobs would be temporary.

Related: Just how dirty are Canada's oil sands, anyway?