As the world warms, temperate forests could become a source of carbon dioxide emission rather than a sink according to a new study in the Proceedings of the National Academy of Sciences (PNAS). Scientists found that two forest sites in the U.S. (Wisconsin and North Carolina) emitted long-stored carbon from their soils when confronted with temperatures 10-20 degrees Fahrenheit (5.5-11.1 degrees Celsius) higher than average.