James Hansen

• 1. Introduction

  ›

  James Hansen stood before the United States Senate Committee on Energy and Natural Resources on the 23rd of June 1988, and told the assembled lawmakers that NASA was ninety-nine percent certain the Earth was warmer than at any point in the history of instrumental measurements. He said the greenhouse effect had already been detected. He said it was changing the climate now. That testimony, delivered on one of the hottest days Washington had seen that summer, would be described by scholars as the official beginning of the global warming policy debate. A physicist trained in the space science program of James Van Allen, Hansen had spent two decades building climate models and studying planetary atmospheres before arriving at that moment. How did a man who started his career mapping the clouds of Venus end up being arrested outside the White House? What does his scientific work actually say about where the planet is headed? And what happens when a government scientist decides that speaking out is more important than speaking carefully?

• 2. From Iowa to the Stars

  ›

  Hansen was born on the 29th of March 1941, in Denison, Iowa, to James Ivan Hansen and Gladys Ray Hansen. At the University of Iowa, he trained under James Van Allen, the physicist whose space science program shaped an entire generation of researchers. Hansen earned a B.A. in physics and mathematics with highest distinction in 1963, followed by an M.S. in astronomy in 1965 and a Ph.D. in physics in 1967. Between 1965 and 1966, he studied as a visiting student at both the Institute of Astrophysics at Kyoto University and the astronomy department at the University of Tokyo. He also participated in a NASA graduate traineeship from 1962 to 1966. When he arrived at the Goddard Institute for Space Studies in 1967, his first major research focus was not Earth at all. It was Venus. The work he did there, building radiative transfer models to explain an alien world, gave him tools he would later turn back on his own planet.

• 3. Clouds of Sulfuric Acid

  ›

  By 1974, the composition of Venus's clouds remained one of the most stubborn puzzles in planetary science. Many researchers had proposed candidates: liquid water, aqueous solutions of ferrous chloride, and other compounds. Hansen and his colleague Hovenier cut through the debate by analyzing the polarization of sunlight reflected from Venus. Their results showed that the clouds were spherical, with a refractive index and cloud drop effective radius that eliminated every proposed cloud type except sulfuric acid. Later work with Kiyoshi Kawabata on polarization variation revealed that the visible clouds formed a diffuse haze rather than a thick cloud deck. When the Pioneer Venus project launched in May 1978 and reached the planet later that year, Hansen collaborated with Larry Travis and other colleagues on a 1979 paper in Science that identified at least three distinct cloud materials: a thin haze layer, sulfuric acid clouds, and an unknown ultraviolet absorber below the sulfuric acid layer. The techniques Hansen developed to peer through those alien clouds became the foundation for his later work reading the atmosphere of Earth.

• 4. The Temperature Record

  ›

  The first NASA Goddard Institute for Space Studies global temperature analysis appeared in 1981. Hansen and his co-author examined surface air temperatures at meteorological stations covering the years 1880 to 1985. They demonstrated that stations closer together than 1000 kilometers showed highly correlated temperatures, particularly at mid-latitudes. That correlation allowed them to combine station data and generate reliable long-term averages even though most stations sat in the Northern Hemisphere on continental land. Their conclusion: global mean warming over the preceding century ran between 0.5 and 0.7 degrees Celsius. When the analysis was updated in 1988, the four warmest years on record were all from the 1980s. The two warmest single years were 1981 and 1987. By 1999, the data showed 1998 as the warmest year since instrumental records began in 1880, with a rate of temperature change larger than any previous period in instrument history. A 2006 update revised the cumulative estimate upward: temperatures were by then 0.8 degrees Celsius warmer than a century earlier. That same update concluded the warming was a real climate signal and not an artifact of the urban heat island effect.

• 5. Modeling a Warming World

  ›

  Hansen's first contribution to numerical climate models came with the 1974 publication of the GISS model, which he and colleagues claimed successfully simulated major features of sea-level pressure and pressure heights over North America. A 1981 paper in Science by Hansen and a Goddard team used a one-dimensional radiative-convective model to conclude that rising carbon dioxide would produce detectable warming sooner than other researchers had predicted. Hansen forecast that temperatures would rise out of the range of natural climate noise by the 1990s. He also predicted, with unusual candor, that it would be difficult to persuade politicians and the public to respond. By the early 1980s, improvements in computer speed allowed the GISS model to run for five years of simulated climate. The first climate prediction from a general circulation model that Hansen published appeared in 1988, the same year as his Senate testimony. That second-generation model estimated mean surface temperature changes under various greenhouse gas scenarios and concluded that warming would reach temperatures at least as high as during the Eemian, the last interglacial period. In 2006, Hansen and colleagues compared actual observations with the 1988 projections and found that real-world greenhouse gas forcing had tracked closest to the intermediate scenario, though they noted the agreement was partly accidental because the climate sensitivity used was higher than later estimates.

• 6. Forcings, Ice, and the Pipeline

  ›

  In 2000, Hansen published a paper titled "Global warming in the twenty-first century: an alternative scenario," arguing that non-carbon dioxide greenhouse gases and black carbon deserved more immediate attention in the short run because CO2 warming was being partly masked by cooling aerosols. He noted that non-CO2 gases were then responsible for roughly fifty percent of anthropogenic greenhouse gas warming. A 2007 paper raised a more alarming possibility. Drawing on paleoclimate data, Hansen examined what happens when ice sheets encounter warming temperatures. George Monbiot, summarizing that paper, noted that when temperatures rose 2-3 degrees Celsius above present levels roughly 3.5 million years ago, sea levels rose not by the 59 centimeters that current IPCC projections suggested but by 25 meters. Hansen himself cautioned that predicting the timing of ice sheet collapse was difficult in a nonlinear system. His words were direct: "An ice sheet response time of centuries seems probable, and we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway." In 2016, a team of 19 researchers led by Hansen published findings on how meltwater from ice sheets could slow or stop the Atlantic meridional overturning circulation, with evidence including megaboulders found on the Bahamas from Eemian-era storms. In 2023, Hansen led 18 researchers in publishing "Global Warming in the Pipeline," concluding that a doubling of atmospheric carbon dioxide would raise temperatures by 4.8 plus or minus 1.2 degrees Celsius, well above earlier estimates, and that the world would pass the 1.5 degree threshold before the end of the 2020s.

• 7. Scientist Into Activist

  ›

  Hansen retired from NASA in April 2013 after 46 years of government service, stating he intended to take a more active role in political and legal efforts to limit greenhouse gases. That role had already been taking shape for years. In 2006, he alleged that NASA public affairs staff had been ordered to review his statements after a December 2005 lecture at the American Geophysical Union in San Francisco. Two years later, the agency's inspector general confirmed that NASA's Office of Public Affairs had "reduced, marginalized or mischaracterized climate change science made available to the general public." Hansen appeared on 60 Minutes in June 2006 to describe how the George W. Bush White House had edited climate-related press releases from federal agencies. On the 23rd of June 2009, Hansen was arrested on misdemeanor charges of obstructing police during a protest against mountaintop removal mining in Raleigh County, West Virginia, alongside about 30 other protesters including actress Daryl Hannah. In August and September 2011, Hansen and 1,251 other activists were arrested during a two-week protest in front of the White House. On the 13th of February 2013, he was arrested there again, this time alongside Daryl Hannah and Robert F. Kennedy Jr., protesting the proposed Keystone pipeline extension. New York Times columnist Andrew Revkin noted after one arrest that Hansen had "pushed far beyond the boundaries of the conventional role of scientists, particularly government scientists, in the environmental policy debate."

• 8. Recognition and Controversy

  ›

  The American Meteorological Society awarded Hansen its Carl-Gustaf Rossby Research Medal in 2009, its highest honor, for contributions to climate modeling and for communicating climate science to the public. In 2001, he received the 7th Annual Heinz Award in the Environment, endowed with $250,000. Time magazine listed him among its 100 Most Influential People in 2006. In 2007, he shared the $1-million Dan David Prize. In June 2018, he shared Taiwan's Tang Prize, with a total value of NT$25 million, with Veerabhadran Ramanathan. The awards came alongside genuine criticism. In June 2009, Eileen Claussen, president of the Pew Center on Global Climate Change, said she viewed Hansen as heroic as a scientist but doubted he had a realistic sense of what was politically possible. A Washington Post writer argued the American Meteorological Society had erred in recognizing his public communication work alongside his research. Hansen was also criticized from within climate advocacy circles for attacking cap-and-trade legislation and for his co-authored 2013 paper in Environmental Science & Technology estimating that nuclear power had prevented roughly 1.8 million air pollution deaths between 1971 and 2009. Groups including the Natural Resources Defense Council, the Sierra Club, and Greenpeace pushed back against his support for nuclear energy. In the late 2010s, Hansen began touring with Michael Shellenberger to present evidence for the climatic benefits of nuclear power, citing the $2 trillion the United States had spent on new renewables without matching nuclear's annual electricity generation.