James E. Hansen (born March 29, 1941 in Denison, Iowa) heads the NASA Goddard Institute for Space Studies in New York City, a part of the Goddard Space Flight Center in Greenbelt, Maryland, Earth Sciences Division. He has held this position since 1981. He is also an adjunct professor in the Department of Earth and Environmental Sciences at Columbia University.

After graduate school, Hansen continued his work with radiative transfer models and attempting to understand the Venusian atmosphere. This naturally led to the same computer codes in modified form being used to understand the Earth's atmosphere. He used these codes to study the effects that aerosols and trace gases have on the climate. Hansen has also contributed to the further understanding of the Earth's climate through the development and use of global climate models.

Hansen is best known for his research in the field of climatology, his testimony on climate change to congressional committees in 1988 that helped raise broad awareness of global warming, and his advocacy of action to limit the impacts of climate change.