Hansen is a planetary scientist. He earned his doctorate from the University of Iowa’s department of physics and astronomy, when it was chaired by the legendary astrophysicist James Van Allen. For his dissertation, Hansen investigated the effect of atmospheric dust on the temperature of Venus; and it may be that this early work imparted a special knack for viewing Earth’s climate system as a whole. He joined GISS as a staff scientist in 1972 and was promoted to director in 1981. For more than 30 years, he and his dedicated research team have been producing work at the forefront of climate science.
He often employs a favorite quote from the late physicist Richard Feynman to explain his approach: “The only way to have real success in science … is to describe the evidence very carefully without regard to the way you feel it should be. If you have a theory, you must try to explain what’s good about it and what’s bad about it equally. In science you learn a kind of standard integrity and honesty.” Hansen invariably points out the shortcomings in his own arguments. When another scientist presents only the points that support his conclusion, Hansen will chide him for acting “like a lawyer.”
For about 25 years, however, the data have been telling him that Earth is getting warmer, humans are causing it, and this is bad news. In his view, moreover, the science has become so airtight in the last five years that the immense danger posed by greenhouse emissions can no longer be denied. This has placed him on a collision course with politicians and business leaders who want a different answer.
Hansen’s December talk was given in honor of greenhouse pioneer Charles David Keeling. Keeling monitored carbon dioxide on the summit of Hawaii’s Mauna Loa for almost 50 years, from 1958 until his death about six months before the meeting, and demonstrated that the concentration of atmospheric carbon dioxide had been rising the whole time.
Early in the talk, Hansen presented what may be the scariest graph in climate science, a 420,000-year record of carbon dioxide and temperature, inferred from a 3.6-kilometer ice core recovered at Russia’s Vostok station in Antarctica (see “C02 and the ‘Ornery Climate Beast,’” PDF, 631 KB). The graph puts the whole greenhouse story in a nutshell and demonstrates, as Columbia climatologist Wallace Broecker once put it, that “Earth’s climate system is an ornery beast which overreacts even to small nudges.”
Past atmospheric temperatures at Vostok may be inferred by measuring the ratio of deuterium to hydrogen, layer by layer, in the water molecules of the ice. Ancient carbon dioxide levels are recorded in the air bubbles trapped in the ice. These records show that temperature and carbon dioxide tracked each other for all but the last 200 years – both oscillating in a cycle that repeats about every 100,000 years, in step with minute changes in the shape of Earth’s orbit around the Sun. Dips in carbon dioxide and temperature correspond to ice ages, or “glacials,” and peaks to interglacials – such as the present warm period, which began about 12,000 years ago.