Sam started working at Lamont in 1954. One day in 1955 he got a note from Doc (Maurice) Ewing, Lamont’s director, saying to come see him. Sam didn't know whether he was going to be fired or promoted. He remembered, “When you got a call from Doc to come to his office, that was called ‘going into the lion's den.’ And so people would sort of give you their blessing, saying in a sense, we hope you come out alive and good luck. I went with great trepidation. Doc said, ‘I have a new project coming here, and it's something that if I were a young man I would be willing to push a peanut across the state of Texas with my nose to get. And do you want the job? It's to work on problems of deep circulation for the Atomic Energy Commission.’ And I said, ‘Why not? I'll be certainly happy to give it a shot.’”

In 1946, Willard Libby had proposed an innovative method for dating organic materials by measuring their content of carbon-14, a newly discovered radioactive isotope of carbon. Radiocarbon dating could provide objective age estimates for organic objects. Ewing could see that this new technique could be used for looking at the ages and circulation rates of the deep waters of the world.

The Atomic Energy Commission was rapidly accumulating wastes and had to find a place to get rid of them. One of the first suggestions was to deposit them into the deep oceans. The belief was that there wasn’t much happening down there. But by the 1950s, Ewing knew that the oceans had dynamic action going on at all levels and that one would need to examine factors of mixing and circulation before making any plans for using the oceans as a dump site. Ewing was able to get an Atomic Energy Commission contract to apply radiocarbon dating to the carbonate of sea water as a new tool that would add a new dimension to understanding of rates of circulation. He had a team effort going on. The geochemists Wallace (Wally) Broecker and J. Laurence Kulp did the radiocarbon dating and Sam developed new techniques to take the water samples. Kulp was a professor of geochemistry at Columbia from 1947 to 1965. Wallace Broecker arrived at Lamont in 1952 and stayed until he died in 2019.Their work made it very clear that it would be a terrible idea to dump atomic wastes into the ocean; the Atomic Energy Commission gave up the idea.

Broecker continued to use radiocarbon dating and other techniques to study the oceans. By the mid-1980s he had created a map of world ocean circulation, based on his and others’ studies, including those of Arnold L. Gordon, a physical oceanographer at Lamont, who in 1983 detected Indian Ocean water ‘leaking’ around the southern rim of Africa into the Atlantic Ocean, forming key part of the global inter-ocean exchange. Referred to as “The Great Ocean Conveyor,” this was a vast stream of warm, shallow water that flowed from the south Pacific into the Indian Ocean, rounded the southern tip of Africa, and then headed north through the Atlantic.

Photograph courtesy of Lamont-Doherty Earth Observatory
(Click to enlarge)

Once it hit cold water from the Arctic, the water cooled and sank near northern Europe. From there, it looped back to the Pacific where it warmed, rose and begin the cycle again. The huge flow helped to regulate global climate by moving vast amounts of heat from one location to another. The part of this flow that occupied the Atlantic Ocean was named the Atlantic Meridional Overturning Circulation (AMOC). This current carries warm surface water from the equator to the north Atlantic, where it mixes with fresh water, sinks as cold, salty water, and then travels south.

Roughly 13,000 years ago, as the world was warming after the last ice age, the AMOC slowed down enough to alter world climate. Broecker and others believed this was due to an infusion of cold, fresh water into the North Atlantic as the result of the draining of Lake Agassiz, a large Canadian fresh-water lake. Ice cores revealed that the warming process also produced abrupt melting of late Pleistocene glaciers, adding more cold, fresh water to the oceans. This large influx of fresh water may have stopped higher-density seawater in the North Atlantic from descending to lower depths, interfering with the AMOC and returning the climate of North America and northern Europe to glacial conditions for several hundred years.

Our news today is full of stories threatening disaster as a result of climate change, including several recent ones reporting the slowing down of the AMOC. Researchers wrote in a study published Sept. 25 in the journal Geophysical Research Letters that they believe the AMOC could collapse between 2025 and 2095; other scientists disagree. Although some of the conditions which may have led to the last AMOC collapse, such as the addition of cold water from melting glaciers to the underground river, are the same, the Atlantic is much warmer today and that might make a difference. One of the effects of the collapse of the AMOC today could be the cooling of northern European shores no longer bathed by the Gulf Stream. Given global warming, that might not be such a bad thing, but we really don’t know what else would happen as a result of the weakening of the AMOC. There could be world-wide ramifications, including changes in tropical rainfall, abrupt cooling across large parts of the northern hemisphere and increased sea-level rise in the North Atlantic Ocean. It’s a scary prospect.

If Wally Broecker were still alive, he would be urging us to take global warming seriously. His 1975 paper "Climatic Change: Are We on the Brink of a Pronounced Global Warming?" predicted that increased carbon dioxide levels in the atmosphere would lead to a rise in global temperature and popularized the term "global warming" to describe the phenomenon. He also warned that the climate system is “an angry beast and we are poking it with sticks.”