How do we know that human activities—namely the emissions from our tailpipes and smokestacks—are responsible for warming the planet? To Benjamin Santer, a climate scientist at the Lawrence Livermore National Laboratory in California, the answer lies clearly within the data. It turns out that, just as perpetrators leave hard evidence like fingerprints and DNA samples at the scene of a crime, the various causes of climate change leave distinct signatures or patterns that climate scientists can identify if they look carefully enough. And hardly anyone has been looking longer or more closely for climate-change fingerprints than Santer.
More than 23 years ago, just out of graduate school, Santer joined the Max Planck Institute for Meteorology in Germany and, working with geophysicist Klaus Hasselmann, became one of the world's first scientists actively trying to tease out the signs of human influence on global climate from what he calls "the background noise of climate variability." Their research took advantage of early computer climate models that were generating a wealth of information about human "fingerprints" – the patterns of climate change produced by human influences. Santer was fascinated by the problem and well positioned by training and aptitude to tackle it. "It was an exciting time," he recalls. Scientists had documented for years that the average surface temperature of the planet was rising. But this new work took a different approach by looking more closely at geographical and altitudinal patterns of warming.
The key insight of the research is straightforward: the factors that might account for global warming—what climate scientists call "forcings"—operate in different ways. For instance, Santer explains, if the earth's warming were caused by an increase in the sun's energy output, "you would expect to see warming from the top of the atmospheric column straight down to the surface." But if massive volcanic eruptions, say, were a significant factor, their influence would show up with a distinctly different profile. When such eruptions occur, the dust they produce can reach upper portions of Earth's atmosphere, and remain there for several years. Because volcanic dust absorbs incoming sunlight, preventing it from penetrating to the earth's surface, the data would show cooling in the troposphere (the atmospheric layer closest to the surface) and heating in the stratosphere (the layer above the troposphere).
But, Santer points out, those two profiles are "not at all what the data show." His research, now replicated by many others, instead documents a telltale warming of the troposphere and cooling of the stratosphere—the precise fingerprint that scientists since the 1960s had predicted would occur from the intensified "greenhouse effect" as increasing amounts of heat-trapping carbon dioxide from fossil-fuel emissions built up in the atmosphere.
Because of his groundbreaking work, Santer was selected as the lead author on a chapter of the 1995 report issued by the Intergovernmental Panel on Climate Change (IPCC). That year, for the first time, the report said that "the balance of evidence suggests a discernible human influence on global climate." That measured statement has, of course, been dramatically strengthened in the latest IPCC report, which concludes that there is a greater than 90 percent likelihood that human activities have been the main cause of warming since the middle of the twentieth century.
Santer's cutting-edge research led to widespread acclaim from his colleagues and earned him many accolades, including a MacArthur "genius grant," but his high-profile role in the 1995 IPCC report made him a target of those trying to stir up controversy and confuse the public about global warming. For instance, after the 1995 report was issued, an industry–funded group led an effort to discredit Santer personally by spuriously claiming that he had altered the IPCC's findings. He had not.
"Nothing in my university training prepared me for what I faced in the aftermath of that report," Santer says of the vicious personal attacks by fossil-fuel interests. "You are prepared as a scientist to defend your research. But I was not prepared to defend my personal integrity. I never imagined I'd have to do that."
Fifteen years later, the evidence that human activity is causing global warming is stronger than ever and accepted by the overwhelming majority of scientists. Our understanding of climate fingerprinting has also become far more sophisticated and now shows human causation in the measured changes in ocean temperatures, Arctic sea ice, precipitation, atmospheric moisture, and many other aspects of climate change.
Some of Santer's more recent work, for instance, addresses changes in the height of the tropopause—the boundary between the troposphere, the more turbulent lower layer, and the more stable stratosphere above. (Between 5 and 10 miles above the earth's surface, a marker of the tropopause can be seen in the flat, anvil-like top of a thundercloud.) Measurements over the course of several recent decades have shown that the tropopause has risen markedly. By studying tropopause changes in computer climate models, and comparing model output with observations, Santer was able to show that both the warming of the lower atmosphere and cooling of the stratosphere led to a rise in the height of the tropopause—and that the observed rise in the tropopause matched the fingerprint of an increase in heat-trapping gases. "Nobody had looked at it before," Santer says, "but the data showed clearly that natural causes alone simply could not provide a convincing explanation for the observed change."
All the climate fingerprinting research to date, Santer explains, has arrived at the same conclusion, namely that "natural causes cannot provide a convincing explanation for the particular patterns of climate change we see." That, he says, is why scientists "have come to have such confidence in our understanding of what is happening—not because of the claims of any one individual, but because of the breadth of scientific work and reproducibility of the results."
Despite the confidence of the scientific community in this conclusion, however, Santer is still regularly singled out for the vitriol of climate deniers. Soft-spoken, meticulous, and cautious by nature, he is an unlikely lightning rod. And yet he often receives hate mail — and worse. Late one evening several years ago, he answered the doorbell to find a dead rat on the doorstep and heard a driver shouting curses from a Hummer (fittingly) zooming away down the street. Little wonder that Santer, in private e-mails, vented some frustration to his former colleagues in the Climate Research Unit at the University of East Anglia in Britain. His e-mails, along with those of other scientists, became part of a cache that was stolen and misrepresented in an effort to distract and mislead the public just before the 2009 climate summit in Copenhagen.
Santer tries to rise above the worst of the attacks. He says he learned a thing or two about being in tough spots growing up. When he was eleven, his father's work for an international food service company moved the family from suburban Maryland to Germany. At the time, he didn't speak a word of German. His family enrolled him in a British army school, where he was the only kid who didn't live on the military base, didn't play soccer, and had never heard of cricket. He still remembers his teacher whacking him on the head with a rolled-up lesson book for writing in pencil instead of using the pen and inkwell on his desk as the European kids had been taught to do. "Let's put it this way," he says; "it was character-building."
Today Santer perseveres with much the same attitude. Unbowed by what he politely calls the “forces of unreason,” he continues to turn out world-class research and makes himself available to speak about the scientific data on global warming. As he puts it: “I continue to believe that if the science is credible, people will do something about it. Since my research is funded by the federal government, I’ve learned that my job is not just to do the best scientific research I can. I also have a responsibility to try to explain to people what it means, so an informed electorate can make good choices.”