A leading climate change activist and former NASA scientist is "crying wolf" with his "exaggerated" comments about the effects of oilsands development on the environment, Natural Resources Minister Joe Oliver charged Wednesday.

Oliver, in Washington, D.C., to shore up support for the Keystone XL pipeline, took aim at scientist James Hansen, who has been a vocal opponent of developing the oilsands.

"It does not advance the debate when people make exaggerated comments that are not rooted in the facts. And he should know that," Oliver said to reporters, following a speech to the Center for Strategic and International Studies.

In an op-ed piece for the New York Times last year, Hansen claimed that "if Canada proceeds [with oilsands development], and we do nothing, it will be game over for the climate."

Oliver countered that when a source of energy represents 1/1000th of global emissions, "to say it’s the end of the planet if that’s developed is nonsense."

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Joe Oliver speaks at Center for Strategic and International Studies in Washington, DC. Natural Resources/ Candian Press

He added that "crying wolf all the time" does not advance the serious debate.

In his speech, Oliver listed the benefits he said will come from the Keystone XL pipeline project, which would carry oil from Western Canada to the Texas Gulf Coast. He said the pipeline will create thousands of jobs and ensure the U.S. with a reliable supply of oil.

"Ultimately this comes down to a choice. The U.S. can choose Canada — a friend, neighbour and ally — as its source of oil imports," Oliver said. "Or it can choose to continue to import oil from less friendly, less stable countries with weaker — or perhaps no — environmental standards."

Oliver added that oilsands development will continue, whether the Keystone pipeline is approved or not.

But his pitch comes just days after the U.S. Environmental Protection Agency publicly rebuked the State Department over its positive environmental assessment of the proposed pipeline.

In a letter sent Monday that was widely seen as a setback for the pipeline, the EPA raised serious concerns about the project's carbon footprint and criticized the State Department's draft analysis.

The letter also urged Washington to work together with Canada to step up its efforts on climate change.

With files from The Canadian Press and The Associated Press