It was in the mid-1980s, at a meeting in Switzerland, that Wally Broecker’s ears pricked up. Scientist Hans Oeschger was describing an ice core drilled into a military radar station in southern Greenland. Layer by layer, the 2-kilometer-long core revealed what the climate was like thousands of years ago. Climate changes, inferred from the amounts of carbon dioxide and one form of oxygen in the core, unfolded surprisingly quickly, in just a few decades. It seemed almost too fast to be true.
Broecker returned home to Columbia University’s Lamont-Doherty Earth Observatory and began to wonder what could cause such dramatic changes. Some of Oeschger’s data turned out to be incorrect, but the seed they planted in Broecker’s mind blossomed and ultimately changed the way scientists think about past and future climate.
Broecker, a geochemist who studied the oceans, proposed that the closure of an important ocean circulation pattern, which he called the great ocean conveyor, could cause the North Atlantic climate to change abruptly. In the past, he argued, melting ice sheets released huge pulses of water into the North Atlantic, making the water cooler and halting circulation patterns that depend on saltwater. The result: a sudden atmospheric cooling that plunged the region, including Greenland, into a great chill. (In the 2004 film Day after tomorrowan overly dramatized ocean closure covers the Statue of Liberty in ice.)
It was an unprecedented leap in knowledge for the time, when most researchers had yet to accept that the climate could change abruptly, let alone ponder what might cause such changes.
Broecker not only explained the observed changes in the Greenland ice core, but also discovered a new field. He prodded, cajoled and rallied other scientists to study the entire climate system and how it could change on a dime. “He was a great thinker,” says Dorothy Peteet, a paleoclimatologist at NASA’s Goddard Institute for Space Studies in New York City, who worked with Broecker for decades. “It was just his genuine curiosity about how the world works.”
Broecker was born in 1931 into a fundamentalist family that believed the Earth to be 6,000 years old, so he was not an obvious candidate to become an innovative geoscientist. Due to his dyslexia, he relied on conversations and visual aids to absorb information. Throughout his life, he did not use computers, a linchpin of modern science, but became an expert in radiocarbon dating. And contrary to the common silo in the sciences, he worked extensively to understand the oceans, the atmosphere, the earth, and thus the entire Earth system.
In the 1970s, scientists knew that humans were dumping excess carbon dioxide into the atmosphere, burning fossil fuels, and cutting down carbon-storing forests, and that those changes were messing with the Earth’s natural thermostat. Scientists knew that the climate had changed in the past; geological evidence over billions of years revealed hot or dry, cold or wet periods. But many scientists focused on long-term climate changes, driven by changes in the way Earth spins on its axis and revolves around the sun, both of which change the amount of sunlight the planet receives. A highly influential 1976 article he referred to these orbital changes as the “pacemaker of the ice ages”.
Antarctic ice cores and Greenland changed the game. In 1969, Willi Dansgaard of the University of Copenhagen and colleagues reported results from a Greenland ice core covering the last 100,000 years. They found large and rapid fluctuations in oxygen-18 that suggested sudden changes in temperature. The climate could oscillate rapidly, it seems, but it took another Greenland ice core and more than a decade before Broecker got the idea that the shutdown of the great ocean transportation system might be to blame.
Broecker proposed that such closure was responsible for a known cold snap that began about 12,900 years ago. As the Earth began to emerge from its orbitally influenced ice age, water melted from the northern ice sheets and washed into the North Atlantic. Ocean circulation stopped, plunging Europe into sudden cold, he said. The period, which lasted just over a millennium, is known as the Younger Dryas after an arctic flower that thrived during the cold snap. It was the last hurray of the last ice age.
Evidence that an ocean conveyor shutdown could cause dramatic climate changes soon piled up in Broecker’s favor. For example, Peteet found evidence of rapid Younger Dryas cooling in swamps near New York City, thus establishing that the cooling was not just a European phenomenon, but also spread across the Atlantic. The changes were real, widespread and rapid.
In the late 1980s and early 1990s, there was enough evidence supporting abrupt climate change that two major projects, one European and one American, began drill a couple of new cores on the Greenland ice sheet. Richard Alley, a geoscientist at Penn State, remembers working through the layers and documenting small climate changes over thousands of years. “Then we got to the end of the Younger Dryas and it was like falling off a cliff,” he says. It was “a big change after a lot of little changes,” he says. “Amazing.”
The new Greenland cores cemented scientific recognition of abrupt climate change. Although the closure of the ocean conveyor could not explain all the abrupt climate changes that have ever occurred, it showed how a single physical mechanism could trigger major disruptions across the planet. It also opened up discussions about how fast the climate could change in the future.
Broecker, who died in 2019, spent his last decades exploring the abrupt changes that are already taking place. He worked, for example, with billionaire Gary Comer, who during a yacht trip in 2001 was surprised by the reduction of Arctic sea ice, to think of new directions for climate research and climate solutions.
Broecker knew more than almost anyone about what could happen. He often described the Earth’s climate system as an angry beast that humans beat with sticks. AND one of his most famous roles was titled “Climate Change: Are We on the Brink of Pronounced Global Warming?”
It was published in 1975.