KENOSHA, Wis. (AP) — The planet has faced climate change forever and humans’ pollution might not be to blame, Republican U.S. Rep. Paul Ryan said Monday during a debate against his Democratic challenger.

Ryan, favored to win re-election to his seat representing GOP-leaning southern Wisconsin, faced off against businessman Rob Zerban for an hourlong forum that touched on world events, domestic politics and the economy. One of the sharpest differences came when the moderator asked each candidate if he thought human activity is to blame for changes to the planet’s climate.

“I don’t know the answer to that question,” Ryan said. “I don’t think science does, either.”

Ryan also said efforts to combat climate change are costly and unproven, a popular position among the Republican base he will need should he seek the GOP’s presidential nomination in 2016.

Zerban said climate change is serious and man-made. He also said it’s an opportunity for Americans to invest in renewable energy that produces fewer greenhouse gas emissions, which scientists blame for climate change.

“These severe weather events have local consequences,” Zerban said, pointing to potholes on Wisconsin roads that were the result of a brutally cold winter.

Just hours before the debate began, the Pentagon released a report that rising sea levels and other effects of climate change will pose major challenges for America’s military. U.S. military officials have long warned that changes in climate patterns, resulting in increased severe weather events and coastal flooding, will have a broad and costly impact on the Defense Department’s ability to protect the nation.

Ryan has previously questioned the climate scientists’ research and data and, on Monday, said that the high costs associated with proposals to fight climate change ignore that “we’ve had climate change forever.”

“The benefits do not outweigh the costs,” Ryan said.

Zerban disagreed, saying spending on energy research would prove wise. “This is an opportunity to invest a dime to save a dollar,” he said.

Ryan and Zerban have faced each other on the ballot before but not during formal debates. When they ran against each other in 2012, Ryan was sprinting around the country as Mitt Romney’s running mate and declined to debate Zerban. Instead, Ryan faced off against Vice President Joe Biden.

Romney and Ryan failed to win the White House, but Ryan did capture an eighth congressional term with 55 percent of the vote.

With Ryan considering a White House bid in 2016, a chance to go one-on-one with a political foe was an event he agreed to take — twice. The pair meets at the University of Wisconsin-Rock County next week.

“This was my first debate. I hope I did all right,” Zerban said as the debate came to a close.

Ryan told his rival that he did fine.

“You’re much more pleasant to debate than Joe Biden,” Ryan said with a smile.

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