Senator John Kerry ducked when he was asked directly at his confirmation hearing whether he would block or back the controversial Keystone XL pipeline, but President Barack Obama's pick for secretary of state delivered a passionate pitch on the virtues of clean energy.

When Wyoming Republican Senator John Barrasso, who supports TransCanada Corp.'s multibillion-dollar pipeline to carry Canadian oil sands crude to U.S. refineries, asked whether he agrees with the 53 senators who favour the project, Mr. Kerry pointedly declined and then launched into a lengthy championing of clean energy.

"The solution to climate change is energy policy," said Mr. Kerry, 69, who has long supported the need for sweeping efforts to mitigate man-made global warming.

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In a forceful argument, the Massachusetts Democrat argued the clean-energy sector offers a huge boon to U.S. economic growth. "The opportunity of a new energy policy so vastly outweighs the downsides that you are expressing concern about," he told Mr. Barrasso, who had said that failing to approve Keystone would harm the U.S. economy.

On a day when most of Mr. Kerry's carefully prepared answers were predictable and few questions were hostile, the long-serving legislator (who formerly chaired the Senate foreign relations committee that is now reviewing his fitness to serve as secretary of state) was clearly energized about climate change.

"We've got to get into the clean energy race – in Massachusetts one of the fastest growing parts of the economy is clean energy, and it's the same in California," Mr. Kerry said. He argued that leading the transformation to non-fossil fuel energy sources would prove as big an economic boost to the United States as the technology revolution of the 1990s.

"It's a job creator – I cannot emphasize this enough," he added. "The energy market is a $6-trillion market with four billion to five billion users, going up to nine billion in the next 20 to 30 years."

As for the pending decision on Keystone XL – which is being reviewed again by the State Department because the pipeline crosses the Canadian-U.S. border – Mr. Kerry was not about to get ahead of the timeline.

"There's a statutory process that's currently ongoing, it's under way … and it will not be long [before it is complete]," he said referring to the State Department's environmental assessment.

"And at that time I will make an appropriate judgment."

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Environmentalists who oppose Keystone XL will have been buoyed by Mr. Kerry's championing of clean energy at the confirmation hearing. In his opening remarks, he made a point of restating his long-held view that climate change poses a grave security risk to the United States. U.S. foreign policy, he said, must be shaped "by life-threatening issues like climate change."

Big oil, which along with the Canadian government supports the Keystone project, has lobbied hard to win approval for the pipeline, which would send 830,000 barrels of Alberta oil sands crude to the U.S. Gulf Coast.

"We urge you to choose jobs, economic development and American energy security," said a letter signed earlier this week by 44 Republicans and nine Democrats out of the 100 U.S. senators.

Keystone XL now stands as a litmus test of whether Mr. Obama's second-term administration will lay down an early marker on its commitment to curbing carbon emissions. On Monday, in his inaugural speech, the President renewed his vow to grapple with the threats posed by man-made climate change.

Mr. Kerry made it clear that he wasn't buying the claims that pursuing clean energy is bad for the U.S. economy or its relations with the rest of the world.

"I will be a passionate advocate on this, but not based on ideology but based on facts, based on science," he said, referring to the job-creation potential of new energy sources.

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Mr. Kerry also said he believes American leadership in foreign affairs is directly tied to its ability to revitalize its economy. "Foreign policy is economic policy," he said. "It is urgent that we show people in the rest of the world that we can get our business done in an effective and timely way."

On the looming confrontation with Iran over its controversial nuclear program, Mr. Kerry echoed Mr. Obama's long-standing promise to keep Tehran's ruling mullahs from getting nuclear warheads. "Our policy is not containment … it is prevention, and the clock is ticking," Mr. Kerry said.

Vexed relations with Iran produced the day's only outburst, when a female protester interrupted the hearing to call for peace with the Islamic republic. Unfazed, Mr. Kerry recalled that his own first appearance before the Senate foreign relations committee was 42 years ago, when, as a young, long-haired, anti-war protester and former naval officer, he testified against the U.S. conflict in Vietnam.