In 1975 the planet had been cooling gently for more than three decades, and media speculation about climate focused on the prospect of a coming ice age. Then Wallace Broecker, a professor of geochemistry at Columbia University in New York, published a paper in the journal Science, arguing that the world was “on the brink of a pronounced global warming” as humans pumped carbon dioxide into the atmosphere.
Sure enough, the world began to heat up before the 1970s were out and Broecker’s prediction that temperatures in the 21st century would rise “beyond the limits experienced during the last thousand years” came true.
Broecker, who has died aged 87, introduced the term “global warming” to scientific literature. His pioneering work over 67 years as a student and researcher at Columbia led many climate scientists to regard him as the grandfather or dean of their field. Michael Oppenheimer, professor of geosciences at Princeton University, said after his death: “He was not fooled by the cooling of the 1970s. He saw clearly the unprecedented warming now playing out and made his views clear, even when few were willing to listen.”
Scientists had known since the 19th century that carbon dioxide was a greenhouse gas that traps solar heat in the atmosphere, but the likely impact of rising CO2 levels was not clear because many different factors affect climate. These range from other man-made pollutants, to volcanic activity and astronomical cycles resulting from changes in the relative position of Earth and sun.
Geochemical evidence, gathered by Broecker during decades of field trips around the world, convinced him that increasing CO2 would override the other factors. Episodes of warming and cooling in the distant past were associated strongly with natural changes in atmospheric composition, he found.
His ideas quickly won acceptance in the scientific community and, as global temperatures rose, from a wider public. Broecker, who enjoyed engaging with people outside his field of research, eagerly briefed politicians on the need to restrain the burning of fossil fuels. He testified at the first congressional hearings on climate change led by Al Gore in 1984.
Wally Broecker, as he was known to friends and colleagues, was born in November 1931 in Chicago and grew up in the prosperous suburb of Oak Park, where his father ran a petrol station. His fundamentalist Christian family enforced a ban on drinking, dancing and movies, and believed Earth was created just a few thousand years ago. He encountered similar beliefs at nearby Wheaton College, Illinois, the evangelical institution where he studied as an undergraduate and contemplated a career as an insurance actuary.
Broecker broke into science in 1952, when an older friend from Wheaton got him a summer internship at Columbia University’s Lamont Geological Observatory. He adored the lab work on radiocarbon dating and showed real aptitude for it. At first some of the Columbia geochemists made fun of his background, calling him a “theo-chemist”, but Broecker shed his Christian beliefs.
He put his greatest research effort into understanding how global circulation in the oceans affects climate as the waters absorb CO2 from the atmosphere and transfer heat around the world. In the 1980s he pulled the evidence together into a synthesis that he called “the great ocean conveyor”.
Geological data showed that climate change could suddenly switch important parts of the conveyor on or off, he warned, leading to very rapid regional warming or cooling.
“The climate system is an angry beast,” he used to say, “and we are poking it with sticks.” In particular, he warned that warming currents in the north Atlantic might shut down within decades, leading to severe local cooling while the world as a whole continued to heat up. An exaggerated version of this idea inspired the 2004 disaster movie The Day After Tomorrow.
Broecker’s first wife, Grace, died in 2007 after the couple had been married 55 years. He is survived by his second wife, Elizabeth Clark, a colleague whom he married in 2009, and by seven children and numerous grandchildren and great-grandchildren.
According to Columbia University, Broecker wrote as many as 500 scientific papers and 17 books. He received innumerable awards and many millions of dollars in research grants. Although his friendly manner was occasionally punctuated with outbursts of volcanic temper, he aroused much affection as well as admiration among climate scientists.
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