Sen. Mark Kirk (Photo by Brian Kersey/Getty Images) Sen. Mark Kirk (Brian Kersey/Getty Images)

This story has been updated.

Four and-a-half years ago, Illinois Republican Mark Kirk voted for a sweeping House bill that would have imposed nationwide restrictions on greenhouse gas emissions linked to climate change.

What a difference a couple of elections can make.

Kirk, who was serving in the House at the time but is now running for reelection in the Senate, described global warming Wednesday as a largely-natural phenomenon.

"We had the previous warming period, which was called the global optimum, and the best way to talk about that is when Leif Erickson went west from his home, he discovered a landmass that he called Greenland, because it was," Kirk told a reporter from E&E News. "And that was called the global optimum, because the planet was much warmer. By calling Greenland 'green land,' we know that the climate has been changing pretty regularly within recorded memory."

Long considered a moderate, Kirk had received favorable ratings from environmentalists early in his career. In 2010, Kirk received a 70 percent score from the League of Conservation Voters for his House record, one of the highest percentages given to a Republican. Kirk was one of just eight Republicans who voted in favor of the 2009 climate bill authored by then-Reps. Henry Waxman (D-Calif.) and Edward J. Markey (D-Mass.).

In a statement Thursday, Kirk said he believes human activity plays a role in climate change but policymakers need to be measured in how they address it.

"Climate change is real and human beings definitely play a role. As I have said since 2010, I will not support a carbon tax or similar attempts which hurt the Illinois or American economy. The vote this week is another desperate attempt to derail Keystone," he said.

Kirk, who beat his Democratic opponent in 2010 by less than 60,000 votes, has since questioned his previous climate stance. In an August 2009 radio interview with WIND's "Big John & Cisco In The Morning," Kirk said that while he believed the House climate bill would stall in the Senate — which is what happened — if he had to revisit the issue, "I will be going through every detail and thinking about all of my constituents who got a hold of me on this issue. Because there has been an issue that I've heard nothing else about in the last couple of weeks."

That same month, he told "Fox News Sunday" that he had gotten significant feedback from Illinois voters on his Waxman-Markey vote, and when he considered the issue in the future he would keep in mind that "The energy interests of Illinois are far broader and deeper than my North Shore district."

Now facing reelection in a Democratic-leaning state, Kirk must weigh the views of conservative Republican primary voters against a more liberal general electorate. His recent climate comments have infuriated some national environmental groups.

“We are baffled by Senator Kirk’s comments," said Environmental Defense Fund spokesman Keith Gaby. "This sounds more like something Senator Inhofe or Rush Limbaugh might say. His supporters in Illinois must be scratching their heads. Unfortunately, his record in the Senate has also been disappointing, including co-sponsorship of Senator McConnell’s resolution to block" an Environmental Protection Agency proposed rule on existing power plants.

"When he was in the House, he was a forward-thinking moderate on climate and energy issues – our hope is that he can return to that approach and be the kind of leader Illinois sent to the Senate," Gaby added. "His reversal on environmental protection has been very disappointing."

Howard A. Learner, executive director of the Midwest-based Environmental Law & Policy Center, said in a statement his advocacy group plans to work with Illinois scientists and business leaders "to brief Senator Kirk on the most recent compelling scientific data on the realities of climate change."

"Most Midwesterners understand why it’s important to reduce carbon pollution from coal plants to help protect public health and our Great Lakes from climate change impacts,” Learner said.

In other words, climate change policy is sure to be an issue in Kirk's reelection bid--regardless of where he comes down on the issue.