Former Vice President Al Gore on Thursday implored President Obama to kill the Keystone XL pipeline.

"This should be vetoed. It is an atrocity. It is a threat to our future," Gore said at a Washington conference hosted by left-leaning think tank the Center for American Progress.

Gore praised Obama's recent efforts on reducing greenhouse gas emissions, a strategy that includes new carbon emissions rules for power plants. Gore said he believed climate change was a "legacy issue" for the president.

But if Obama is to cement that legacy, he needs to axe the proposed pipeline that would bring dense, carbon-rich oil from the tar sands of Canada to Gulf Coast oil refineries.

"We are at the point now where we are going after these ridiculously dirty and dangerous fossil fuels," said Gore, who later chastised lawmakers who have created an "alternate version of reality" by denying the impact of human activity on climate change.

The State Department is currently reviewing Keystone. It's assessing comments on a preliminary environmental report before moving onto the final version, which will be used to determine whether the project is in the national interest.

The administration has not set a timeline for a final decision on Keystone, which has been in limbo for several years.

Canada's government, business groups, the oil industry and some labor unions have spearheaded lobbying blitzes to get the pipeline approved. Congressional Republicans and centrist Democrats are also on board, saying Keystone would provide jobs and strengthen energy security.

But Keystone's opponents, including Gore, want Obama to scrap the pipeline because they say it would ramp up greenhouse gas emissions.

The State Department's preliminary environmental report, conducted by London-based contractor Environmental Resources Management, said that wouldn't happen.

That analysis said rail and other pipelines would bring oil sands to market without Keystone, adding that the pipeline also would displace heavy crude imports from Venezuela.

But the pipeline's detractors argue the report was flawed, charging that ERM had previously done work for Keystone builder TransCanada Corp. The State Department's Office of Inspector General is looking into whether the agency did enough to gauge potential conflict of interest by ERM.