James Hansen, an outspoken advocate for action on climate change, announced Monday that he is retiring after nearly 50 years as a climate scientist for NASA's Goddard Institute for Space Studies.
Hansen, 72, wrote in an email that he was stepping down from the organization in order to spend more time on his campaign to cut carbon emissions. He first testified about the impending threat of climate change before Congress in 1988.
PBS NewsHour interviewed Hansen in August about how the extreme heat events of 2012 were connected to human activity and climate change.
Hansen and his team at Goddard began publishing climate models in 1981 that projected the influence of carbon dioxide on global warming. Hansen's models showed that the increase in greenhouse gases would have drastic consequences far earlier than previous studies suggested.
He has continued to publish studies and articles on the topic, including a paper in 2012 in which Hansen coined the term "climate dice" to explain how climate change increases the odds for weather disasters like droughts, hurricanes and heat waves.