The term “global warming” appears for the first time in print on August 8, 1975, with the publication of Wallace Smith Broecker's paper “Climatic Change: Are We on the Brink of a Pronounced Global Warming?” in the journal Science.
Five years earlier, in 1970, Broecker, a researcher at Columbia University's Lamont-Doherty Earth Observatory, published a study of ocean sediment cores that revealed the Ice Age had seen rapid transitions in its climate, with ice sheets taking tens of thousands of years to develop in freezing temperatures, followed by sudden warm periods that melted the ice.
Broecker built on this discovery in his 1975 paper, which hypothesized that the Ice Age’s rapid fluctuations had been caused by changes in "thermohaline circulation": the ocean currents and wind systems that move heat from the equator up north towards the poles and transport cold water toward the equator. Broecker later named this the "Great Ocean Conveyor." He believed that rapid changes in climate were once again possible if this conveyor belt were changed or "turned off.”
Broecker argued that there was an increasingly likely scenario for this to happen: the ongoing rise of atmospheric carbon dioxide content created by fossil fuel emissions would soon begin to warm the planet, in turn warming surface waters in the ocean and melting ice into fresh water. This would reduce the waters' density, thereby preventing cold water from sinking, altering ocean currents and effectively shutting off the conveyor belt. If that were to happen, he postulated, Europe would grow cooler as it did during the Ice Age. The more disruptive effect would come from unpredictable "on-and-off flickers” in global temperature. As Broecker put it in 1998, “the climate system is an angry beast and we are poking it with sticks."
His assessment of global warming trends remains relevant today. In 2017, a Columbia University publication found that as the planet was warming, fresh water was entering oceans at a higher rate.
Broecker died in 2019.