WATERLOO, Iowa — Republican presidential candidate Jeb Bush says he's "uncertain" he would have attended the global climate change summit in Paris because proposed changes in environmental policy might adversely affect U.S. economic growth.

"I worry about the economic impact for our country," he told reporters after a campaign stop here. "I haven't seen the specifics, but I worry that — put aside intentions — that these proposals could have an impact on the here and now, people who are really struggling right now."

President Obama joined leaders from 150 nations on Monday in Paris to launch a two-week gathering designed to defeat what the president considers one of humanity’s gravest threats.

In a heavily guarded conference center north of Paris, negotiators hope to forge a treaty to dramatically reduce emissions of greenhouse-gas pollution blamed for warming the planet. As part of the talks, U.S. officials announced the formation of a 20-nation project to spur funding on energy research to bolster private-sector initiatives, including one led by Microsoft co-founder Bill Gates and 27 of the world’s wealthiest private investors.

At an earlier stop in Dubuque, Iowa, Bush said he didn't know details of the Paris talks so couldn't comment specifically on the summit or its proposed outcomes. But he told voters that he thinks one of the ways the federal government should tackle climate change is by investing in research and development to spur new energy sources. But he demurred when asked by reporters later in Waterloo whether he supported the president's call to increase federal spending on research and development by tens of billions of dollars.

"I don't have a number. I just think that's the proper place for the federal government to play a role. It's not to create rules that create added costs for non-renewable sources of energy. Trying to destroy the coal industry is not an effective policy," he said.

Bush conceded that generally he supports Obama's call for greater federal investment in research.

"I've consistently said that I think it is a proper role for the federal government to invest in long-term, basic research to create an environment where the next disruptive technology comes out, or series of technologies," Bush said. "That's where the government has played a constructive role, whether it's in biomedical research or defense spending."

Bush has suggested several times over the course of the year that the federal government should increase funding for medical research, especially to tackle Alzheimer's disease.

The former Florida governor is on a two-day swing through Iowa, which included a stop Monday at a Republican Party fundraiser in rural Clinton County. He is making three stops on Tuesday in Dubuque, Waterloo and Newton, Iowa. Later this week he is poised to host major campaign donors at private meetings in his home base of Miami.