WASHINGTON — In a break with some conservatives, a former House speaker, Newt Gingrich, is joining those calling for “urgent” action to address global warming as he sparred yesterday with Senator Kerry on whether government or the marketplace should be the chief instrument in combating climate change.

“It is a problem. We should address it, and we should address it very actively,” Mr. Gingrich, who could enter the presidential race later this year, said at the outset of a two-hour debate with the 2004 Democratic nominee. He later called for “green conservatism,” saying he wanted “radical” changes that would combine tax incentives and even prizes to stimulate the market toward more environmentally friendly practices to reduce carbon emissions.

The statements came as welcome news to Mr. Kerry, who reminded a packed Senate caucus room that Mr. Gingrich had doubted the science behind climate change as recently as 18 months ago, when he referred to global warming as “cultural anthropology” during an appearance at the University of Vermont. Mr. Kerry said he was “excited” to hear Mr. Gingrich now speaking about the urgency in addressing climate change.

The Massachusetts senator also forced Mr. Gingrich to distance himself from a fellow Republican, Senator Inhofe of Oklahoma, who has called global warming a “hoax” and more recently berated Vice President Gore for what he deemed “misleading ” and “alarmist” warnings on climate change. Asked by Mr. Kerry what he would say to Mr. Inhofe, Mr. Gingrich responded: “My message is the evidence is sufficient that we should move toward the most efficient possible means to reduce carbon in the atmosphere.”

Standing at podiums emblazoned with the Senate seal, the two political heavyweights and one-time party leaders were meeting here in a largely free-flowing debate arranged by New York University’s John Brademas Center for the Study of Congress. The format, which sought to evoke the classic Lincoln-Douglas debates of 1858 in allowing for extended rebuttals by each participant, provided a stark and deliberate contrast to the types of political forums that have dominated modern presidential elections. Those have been criticized for yielding little more than sound bites from candidates.

Mr. Kerry was by far the feistier debater yesterday, often engaging Mr. Gingrich directly and unleashing occasional one-liners that drew laughter and applause from a sympathetic audience.

Despite a general agreement on the science behind global warming claims, the two differed strongly on how to solve the problem. Mr. Gingrich argued that a market-based approach would effect more rapid change than regulatory mandates urged by many Democrats. Mr. Kerry rejected that suggestion, saying the situation required government action and questioning whether the market would actually take heed. “That’s like saying, ‘Barry Bonds, go investigate steroids,'” he said.

Mr. Gingrich, in turn, said Mr. Kerry’s call for government-imposed caps on carbon emissions would amount to a “massive increase in the relative power of the government.” Yet he advocated tax incentives and other rewards for private innovation in environmentally friendly products, saying he was not pushing a “laissez-faire approach” to battling climate change.

Messrs. Kerry and Gingrich are each hawking books on the environment. Mr. Kerry’s “This Moment on Earth,” written with his wife, Teresa Heinz Kerry, is on sale now, while Mr. Gingrich’s “Contract With the Earth” is set to come out this fall.