Sen. John Kerry made it clear Thursday that he will play a pivotal role in deciding the fate of the Keystone XL pipeline if he is confirmed as secretary of state.

"I'll make the appropriate judgments about it," he said, referring to the State Department's ongoing review of the 1,200-mile tar sands oil pipeline. "There are specific standards that have to be met with respect to that review, and I'm going to review those standards and make sure they're complete."

Kerry made his remarks to the Senate Foreign Relations Committee at a hearing on his nomination for the post. He was responding to a question from Sen. Barbara Boxer, a California Democrat and longtime supporter of environmental causes, who asked how Kerry would ensure that the Keystone XL decision "takes into consideration the potential impacts of the pipeline on water and air quality and mitigates any increases in the carbon pollution issue."

In his opening statement to the committee, Kerry also described climate change as one of the "life threatening issues" that defines American foreign policy.

The pipeline's opponents, who argue that building the Keystone XL would accelerate global warming, were encouraged by Kerry's words.

Kerry's remarks show that "he understands...that he's going to be relied upon to bring his perspective and his deep knowledge of the climate crisis to bear on this issue," said Peter LaFontaine, an energy policy advocate at the National Wildlife Federation in Washington, D.C.

Kerry is one of the nation's most vocal proponents of climate action. He co-authored comprehensive climate legislation that died in 2010 and has long pushed for American leadership in global climate treaty talks. Speaking about the State Department's Keystone XL review in 2011, Kerry told reporters that he would "do my best to leave no question unanswered, including every possible economic and environmental consideration, before a final decision is made."

The five-term Massachusetts Democrat has served on the foreign relations committee for 28 years and has chaired it for the last four. He is expected to breeze through the confirmation process, and a vote on his nomination could come as early as next week.

While it's President Barack Obama—not Kerry—who will have the final say on Keystone XL, "having Kerry in this role is a good sign," LaFontaine said. "Obviously there are other people in the president's ear on this. It's great to have Kerry at the table, too."

Still, LaFontaine noted that Kerry's comments didn't indicate which way the senator might go on the pipeline, and he said Kerry had offered only a "milquetoast statement" on the issue. "He was pretty careful not to say anything except that he's aware that he has a role to play in the process."

Kerry's hearing came as the pipeline's supporters are ramping up pressure on the Obama administration to approve the pipeline, which would transport heavy diluted bitumen, or dilbit, from the Canada's oil sands region to the U.S. Gulf Coast. They argue that the $5.3 billion Keystone XL project, built by TransCanada, could create thousands of U.S. and Canadian jobs and would boost the region's energy independence.

Requests for comment from the American Petroleum Institute and other pipeline supporters were not returned by deadline.

On Tuesday, Nebraska Gov. Dave Heineman, a Republican, notified the president that he had approved the pipeline's route through his state, a critical step in advancing the project. A day later, a bipartisan group of 53 senators urged Obama to approve the pipeline quickly, citing economic benefits and a minimal environmental impact.

"Canada plans to develop this oil resource, and the only question is whether we receive the oil from our friend and ally, or whether Canada is forced to look for new partners in Asia," they wrote in a letter.

The project's opponents, however, are urging Obama to connect the dots between the pipeline and global warming, pointing to the vow he made in his inaugural speech to "respond to the threat of climate change."

"You just can't square the tremendous leadership statement the president made—and that Senator Kerry has made many times—[on] climate change with the decision to move forward on Keystone," said KC Golden, policy director at Climate Solutions, a Seattle-based research and advocacy organization.

Golden recently won a Heinz Award for his work on advancing climate and clean-energy policies. Kerry's wife, Teresa Heinz, chairs the foundation that gives the awards.

Other climate advocates are less absolute about the pipeline. Rep. Henry Waxman (D-Calif.), who helped craft the failed national cap-and-trade bill in 2009, called the Keystone XL "a small issue compared to the overall objective that the president and we want to achieve" at a news conference on Thursday.

As secretary of state, Kerry would oversee the department's environmental review of the project. The State Department has authority over cross-border pipelines and is expected to release its supplemental environmental impact statement (SEIS) any day now.

The statement will help officials determine whether the pipeline is in the "national interest," a term that includes economic, energy security and climate change considerations. After it is released, the U.S. Environmental Protection Agency and other federal agencies will have a chance to weigh in on it.

The EPA criticized the State Department for failing to properly address the pipeline's climate change impacts in two previous environmental reviews of the project.

Bob Deans, a spokesman for the Natural Resources Defense Council, said it's critical that the State Department's next SEIS address the project's effect on the climate. The dilbit that the pipeline would carry emits about 20 percent more greenhouse gas emissions than conventional oil through the stages of extraction, production and usage. Earlier this week, a Greenpeace study found that Canadian tar sands production is the fifth most carbon-intensive project in the world.

"At the top of the [department's] list has to be the impact on climate," Deans said. He added that, given Kerry's leadership on climate action in the past, "we hope and expect him to recommend [the pipeline] be denied."

The Keystone XL's southern segment, called the Gulf Coast project, doesn't need the State Department's approval because it doesn't cross national borders. More than one-quarter of the southern section is already installed.