Hansen 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.
After graduate school, Hansen worked with radiative transfer models, attempting to understand the Venusian atmosphere. Later he applied and refined these models to understand the Earth’s atmosphere, in particular, the effects that aerosols and trace gases have on Earth’s climate. Hansen’s development and use of global climate models has contributed to the further understanding of the Earth’s climate. In 2009 his first book, Storms of My Grandchildren, was published. In 2012 he presented a TED Talk: Why I must speak out about climate change.
As of 2014, Hansen directs the Program on Climate Science, Awareness and Solutions at Columbia University’s Earth Institute. The program is working to continue to “connect the dots” from advancing basic climate science to promoting public awareness to advocating policy actions.
Hansen is representing his granddaughter as well as “future generations” as plaintiffs in the Juliana v. US lawsuit, which is suing the United States government and some of its executive branch’s positions for not protecting a stable climate system.
In 2007, Hansen alleged that in 2005 NASA administrators had attempted to influence his public statements about the causes of climate change. Hansen said that NASA public relations staff were ordered to review his public statements and interviews after a December 2005 lecture at the American Geophysical Union in San Francisco. NASA responded that its policies are similar to those of any other federal agency in requiring employees to coordinate all statements with the public affairs office without exception. Two years after Hansen and other agency employees described a pattern of distortion and suppression of climate science by political appointees, the agency’s inspector general confirmed that such activities had taken place, with the NASA Office of Public Affairs having “reduced, marginalized or mischaracterized climate change science made available to the general public”.
In June 2006, Hansen appeared on 60 Minutes stating that the George W. Bush White House had edited climate-related press releases reported by federal agencies to make global warming seem less threatening. He also stated he was unable to speak freely without the backlash of other government officials, and that he had not experienced that level of restrictions on communicating with the public during his career.
Hansen and 1251 other activists were arrested in August and September 2011, at another demonstration in front of the White House. Hansen urged President Obama to reject the Keystone pipeline extension intended to carry more synthetic crude oil from Canada’s Athabasca Tar Sands to the Gulf of Mexico. On February 13, 2013, Hansen was again arrested at the White House, along with Daryl Hannah and Robert F. Kennedy, Jr., during a further protest against the proposed Keystone pipeline extension.