PORTLAND — Environmental activists called on Bowdoin College on Thursday to stop investing endowment money in companies they say contribute to global warming.

Environmental writer and activist Bill McKibben of Vermont, joined by representatives from Unity College, Sierra Club Maine, and a student from Bowdoin College, said the liberal arts school in Brunswick can help combat global warming by stopping investments in oil, gas and coal companies and other businesses connected to fossil fuels.

Students at 234 American colleges and universities have launched petition drives or taken other steps urging school presidents and trustees to stop investing in the fossil fuel industry, McKibben said in a conference call with reporters.

At Bowdoin, about 500 students have signed petitions, said Hugh Ratcliffe, a sophomore from Austin, Texas. Even more signatures have been collected from alumni and townspeople in Brunswick, he said.

Students have met with Bowdoin President Barry Mills to talk about the school’s investments, Ratcliffe said. Mills was sympathetic, but told them that adopting a no-investment policy in an industry could hurt the school’s endowment and, in turn, diminish the amount of scholarship money available to students, Ratcliffe said.

Bowdoin spokesman Doug Cook said Mills has met with students several times, but the school has no plans to divest its investments that have holdings in fossil fuel industry.

But McKibben said schools typically have only a small percentage of their investments, perhaps 1 or 2 percent, in publicly traded companies in the fossil fuel industry, so shifting those investments elsewhere shouldn’t be a huge undertaking.

Most colleges are already committed to sustainability and having green campuses, so divesting themselves of stocks in companies that add to the world’s carbon footprint shouldn’t be too big of a leap, he said.

Three schools – Unity College in Maine, Hampshire College in Massachusetts, and Sterling College in Vermont – have adopted policies to lead them away from investments in fossil fuel companies.

McKibben said petitions have been circulating at five schools in Maine – Bowdoin, Bates and Colby colleges, College of the Atlantic and the University of Southern Maine.