Remembering Wally Broecker

Dr. Wallace Broecker, a Columbia University geophysicist and climate scientist, passed away this week at the age of 87.  He served as a professor at Columbia for over half a century and published over five-hundred papers in the field.  Broecker is best known, though, for the 1975 paper he published in Science called “Climate Change: Are We on the Brink of a Pronounced Global Warming?”  This paper is significant not simply for the fact that it inserted the term “global warming” into the middle of discussions of the Earth’s climate, first with scientists and then eventually with the public at large, but also because it largely resolved the debate going on in the climate science community about what was happening to the Earth’s climate and what the ultimate cause of that was.

Scientists had been aware, at least in an abstract sense, that carbon dioxide was capable of warming the atmosphere and that levels of it in the atmosphere were increasing  since the early part of the twentieth century.  However, it wasn't until 1958 that amounts of carbon dioxide in the atmosphere were first quantified.  The primary instrument to do this was set up on the summit of Mauna Loa in Hawaii by a scientist named Charles Keeling.  It became clear from Keeling’s data by the mid-1960s that CO2 levels were rising sharply, and that the rate of increase was accelerating.   Global temperature records were sufficiently well-established at that point in time for scientists to know that the planet on a whole had been cooling for the previous three decades, however.  Reconciling the rising CO2 levels and the greenhouse effect with cooling temperatures proved to be a challenge.  In fact, a paper published by two scientists from NASA/GISS in 1971 suggested that the cooling effect of aerosols was overwhelming the warming effect of carbon dioxide, and that humanity was running the risk of plunging the planet into a new Ice Age.  That paper turned out to be flawed, but a new scientific technique revealed that the cooling at the time was primarily the result not of human activity but a natural cycle. Ice and sediment cores were being used to estimate global temperatures in the recent and distant geologic past.  An eighty-year cycle of global warming and cooling found in this data could explain the cooling between the 1930s and the 1960s better than increases in global aerosol amounts could.

This is where Broecker’s famous 1975 paper comes in.  Using a computer model, Broecker was able to show that the natural cycle observed in the sediment cores, superimposed with the warming influence of carbon dioxide, could explain what had been observed in the temperature record up to that point.  But the model did not stop in 1975.  It also predicted that, as the cooling cycle was about to reverse itself and the warming effect of CO2 was strengthening steadily, a sharp and clearly discernible global temperature increase was imminent.  “Once this happens,” Broecker wrote in the abstract, the exponential rise in the atmospheric carbon dioxide content will tend to become a significant factor and by early in the next century will have driven the mean planetary temperature beyond the limits experienced during the last 1000 years.”  That actually wound up happening in the late 1990s, but it had become evident by the early 1980s that Broecker’s predictions were coming true.  As a result, the consequences of human activity on the Earth's climate became very clear to nearly the entire climate science community.

I only met Wally Broecker once myself.  He worked at Columbia, but he was mainly based at the Lamont-Doherty Earth Observatory in Palisades, NY and didn't come down to the Manhattan campus that often.  As my major was applied physics rather than earth science, I didn't go up to Lamont very often, either.  When I did meet him, at a talk he gave at GISS in 1996, I found him to be a very warm, friendly, and outgoing person with an accessible and approachable personality.  I was new to GISS at the time, but it was clear that everybody in the room looked up to him, and that was good enough for me.  He had a remarkable life and career, and while he may no longer be with us, his work remains an essential part of a critically important science.

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