Deprived of sunlight, plankton that absorb carbon dioxide died off. At the same time, plankton that release carbon dioxide multiplied. In the space of a few months, thousands of lakes went from being carbon sinks to being carbon sources, according to the study, published in the Proceedings of the National Academy of Sciences.

Such a widespread transformation would typically take centuries, said lead author Jasmine Saros, of the University of Maine. “The magnitude of this and the rate of change were unprecedented,” she said. “It was such an overwhelming climate force that drove all the lakes to respond in the same way.”