James Edward Hansen (born 29 March 1941) is an American adjunct professor in the Department of Earth and Environmental Sciences at Columbia University. He is best known for his research in climatology, his 1988 Congressional testimony on climate change that helped raise broad awareness of global warming, and his advocacy of action to avoid dangerous climate change. In recent years he has become a climate activist to mitigate the effects of climate change, on a few occasions leading to his arrest.