In 1988, Dr. James Hansen, director of the NASA Goddard Institute for Space Studies, brought national attention to global warming when he said in Congressional testimony: “The greenhouse effect has been detected and is changing our climate now.” Watch an interview with Dr. Hansen:

This interview originally aired 8/12/2008.

You can also read an interview Dr. Hansen gave FRONTLINE in January 2007 after NASA press officials attempted to curb his public comments on climate change.