PORTLAND, Maine — Environmental activists called on Maine’s Bowdoin College Thursday to stop investing its endowment 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.