He asks, "What do I know that would cause me -- a reticent, midwestern scientist -- to get myself arrested in front of the White House protesting? And what would you do if you knew what I know?"
Hansen explains that his work as a climate scientist dates back to 1981 and a paper he co-authored on global warming. He and his colleagues found that "observed warming of 0.4 C in the prior century was consistent with the greenhouse effect of increasing CO2." He says that they also found, "that Earth would likely warm in the 1980s, and warming would exceed the noise level of random weather by the end of the century."
(Watch his entire TED talk, titled "Why I must speak out about climate change," in the video above.)
Most strikingly, Hansen says, the paper predicted that the 21st century "would see shifting climate zones, creation of drought-prone regions in North America and Asia, erosion of ice sheets, rising sea levels and opening of the fabled Northwest Passage."
Noting that "most of the things mentioned in our 1981 paper were facts," Hansen moves to the present and makes a case for working to decrease atmospheric carbon dioxide concentrations.
Hansen shows pictures of his young grandchildren and argues that "It would be immoral to leave these young people with a climate system spiraling out of control."
Hansen, the director of NASA's Goddard Institute for Space Studies in New York City and Adjunct Professor of Earth Sciences at Columbia University’s Earth Institute, has been outspoken on environmental issues. He was arrested last year during a protest at the White House against the Keystone XL pipeline -- the incident he mentions at the beginning of his TED talk.
Blogging for The Huffington Post last June, he urged the scientific community to step up and help "control the tar sands monster."
"The tar sands pipeline project does not serve the national interest," he wrote, "because it will result in large adverse impacts, on the public and wildlife, by contributing substantially to climate change."
While the Keystone XL pipeline was shelved in January, Hansen's message remains relevant. Polls show that the number of Americans who believe in climate change is rising, as interest groups are making concerted efforts to negatively impact public opinion and school curricula.