James Hansen

James Edward Hansen (born March 29, 1941) is an American climatologist. He is an adjunct professor directing the Program on Climate Science, Awareness and Solutions at Columbia University. He 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 global warming, on a few occasions leading to his arrest. In 2000, Hansen had proposed an alternative scenario for combating the threat of global warming by focusing on reducing non-CO2 greenhouse gases (GHGs). His estimates of global climate had found that non-CO2 GHGs (such as methane) caused more observed global warming than carbon dioxide. However, he pointed out that CO2 would become the dominant climate forcing, if global emissions continued to rise.

Early life and education Hansen was born in Denison, Iowa, to James Ivan Hansen and Gladys Ray Hansen. He was trained in physics and astronomy in the space science program of James Van Allen at the University of Iowa. He obtained a B.A. in physics and mathematics with highest distinction in 1963, an M.S. in astronomy in 1965 and a Ph.D. in physics in 1967, all three degrees from the University of Iowa. He participated in the NASA graduate traineeship from 1962 to 1966 and, at the same time, between 1965 and 1966, he was a visiting student at the Institute of Astrophysics at Kyoto University and in the department of astronomy at the University of Tokyo. He then began work at the Goddard Institute for Space Studies in 1967.

Career After graduate school, Hansen continued his work with radiative transfer models, attempting to understand the Venusian atmosphere. He later applied and refined these models to understand the Earth's atmosphere, and in particular, the effects that aerosols and trace gases have on Earth's climate. His 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 the TED Talk "Why I must speak out about climate change". From 1981 to 2013, Hansen served as director of the NASA Goddard Institute for Space Studies (GISS) in New York City, a component of the Goddard Space Flight Center. As of 2014, Hansen directed the Program on Climate Science, Awareness and Solutions at Columbia University's Earth Institute. The program seeks to connect advances in climate science with public awareness and policy advocacy through a combination of research, communication, and outreach activities. Hansen served as guardian for future generations in the climate lawsuit Juliana v. United States, in which his granddaughter Sophie Kivlehan was one of the youth plaintiffs challenging the U.S. government's climate and energy policies.

Research and publications As a college student at the University of Iowa, Hansen was attracted to science and the research done by James Van Allen's space science program in the physics and astronomy department. A decade later, his focus shifted to planetary research that involved trying to understand the climate change on earth that will result from anthropogenic changes of the atmospheric composition. Hansen has stated that one of his research interests is radiative transfer in planetary atmospheres, especially the interpretation of remote sensing of the Earth's atmosphere and surface from satellites. Because of the ability of satellites to monitor the entire globe, they may be one of the most effective ways to monitor and study global change. His other interests include the development of global circulation models to help understand the observed climate trends, and diagnosing human impacts on climate.

Studies of Venus

In the late 1960s and early 1970s, following his Ph.D. dissertation, Hansen published several papers on the planet Venus. Venus has a high brightness temperature in the radio frequencies compared to the infrared. He proposed that the hot surface was the result of aerosols trapping the internal energy of the planet. More recent studies have suggested that several billion years ago, Venus's atmosphere was much more like Earth's than it is now and that there were probably substantial quantities of liquid water on the surface, but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere. Hansen continued his study of Venus by looking at the composition of its clouds. He looked at the near-infrared reflectivity of ice clouds, compared them to observations of Venus, and found that they qualitatively agreed. He also was able to use a radiative transfer model to establish an upper limit to the size of the ice particles if the clouds were actually made of ice. By 1974, the composition of Venus' clouds had not yet been determined, with many scientists proposing a wide variety of compounds, including liquid water and aqueous solutions of ferrous chloride. Hansen and Hovenier used the polarization of sunlight reflected from the planet to establish that the clouds were spherical and had a refractive index and cloud drop effective radius which eliminated all of the proposed cloud types except sulfuric acid. Kiyoshi Kawabata and Hansen expanded upon this work by looking at the variation of polarization on Venus. They found that the visible clouds are a diffuse haze rather than a thick cloud, confirming the same results obtained from transits across the sun. The Pioneer Venus project was launched in May 1978 and reached Venus late that same year. Hansen collaborated with Larry Travis and other colleagues in a 1979 Science article that reported on the development and variability of clouds in the ultraviolet spectrum. They concluded that there are at least three different cloud materials that contribute to the images: a thin haze layer, sulfuric acid clouds and an unknown ultraviolet absorber below the sulfuric acid cloud layer. The linear polarization data obtained from the same mission confirmed that the low- and mid-level clouds were sulfuric acid with radius of about 1 micrometer. Above the cloud layer was a layer of submicrometre haze. Evidence published in the early 1980s showed that the clouds consist mainly of sulfur dioxide and sulfuric acid droplets.

Global temperature analysis

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