The man behind one of the greatest geoscientific revolutions of the 20th century
There were two revolutions in Earth science during the second half of the 20th century. Everybody knows about the largely geophysics-driven Plate Tectonics revolution that transformed our understanding of the solid Earth. Fewer people seem to acknowledge the extent of the revolution in our understanding of Earth's hydrosphere and atmosphere, despite its obvious relevance to the climate and ecological predicament we face. This revolution was driven largely by isotopic geochemistry and is a story that can be told in large part through the work of Wally Broecker, who died in February.
Left, Wally Broecker (Image credit: Bruce Gilbert)
Isotopic advances
Carbon and oxygen isotopic discoveries in the 1940-50s were crucial. In addition to precision dating, radiocarbon provided a tracer for atmosphere-ocean interactions, and stable carbon isotope ratios could be used to link carbon cycling through the atmosphere, ocean and biosphere on a timescale beyond the reach of radiocarbon.
Additionally, oxygen isotope ratios in carbonate minerals were shown to give a measure of temperature, a technique successfully applied in Emiliani's 1955 Journal of Geology paper on Pleistocene temperatures. After some controversy, the paper led to proof of many glacial episodes, not just the classical four, and demonstration that their periodicity could be correlated with the solar insolation record, as proposed much earlier by Milankovitch.
Ocean circulation
Meanwhile, Broecker's early research used radiocarbon (and bomb-produced tritium) to decipher ocean circulation—a preoccupation that remained throughout his life. He was an early participant in the Deep Sea Drilling Project, joining many cruises from the Lamont-Doherty Earth Observatory where he worked throughout his career. He also used uranium-series isotopes to date coral terraces in Barbados, which led to better understanding of the ice-age record in relation to solar insolation.
Broecker's insights into ocean circulation culminated in the 1980s, with his recognition of the Great Ocean Conveyor. Warm salty water enters the North Atlantic from the south-west, then is cooled and sinks, eventually moving into the Southern Ocean and the Pacific. Critically, the North Atlantic sinking depends both on the high salinity, hence density of the input water, as well as on the cold temperature in the region. If either of these variables change, the conveyor could be cut off, with profound consequences for global climate, on a short (decadal) timescale—a switch that frequently features in current debates on climate. In 1990, Broecker received the Geological Society of London's Wollaston Medal especially for this work. He was also an Honorary Fellow.
Climate campaigner
Broecker also played a major role in debates regarding the causes of short-period oscillations in temperature and precipitation, especially at the initiation and termination of the Younger Dryas cold episode that interrupted the recovery of the North Atlantic temperature from the Last Glacial Maximum, as well as those on the Heinrich layers that record successive iceberg migrations across the Atlantic. The idea that our climate is capable of sudden shifts led to Broecker's remark that the climate is an angry beast, and we provoke it at our peril.
Broecker’s research underlies our present understanding of how the climate works. He was a scientist first and a climate campaigner second. Towards the end of his life he thought that only bold geo-engineering could save us from catastrophe.
By John Hudson
(The full version of this obituary appears below. Editor.)
Further reading
Emiliani, C. (1995) Pleistocene Temperatures. The Journal of Geology 63, 538-578; https://doi.org/10.1086/626295
Wally’s scientific autobiography 'A geochemist in his Garden of Eden' is freely available online
here
Wallace Smith Broecker (1931-2019)
The man behind one of the greatest geoscientific revolutions of the 20th century
There were two revolutions in Earth sciences during the second half of the 20th century. Everybody knows about the Plate Tectonics revolution that transformed our understanding of how the solid Earth works. It was driven largely by geophysics, especially seismology and palaeomagnetism. Fewer people seem to acknowledge the extent of the revolution in our understanding of Earth's hydrosphere and atmosphere, despite its obvious relevance to our appreciation of the climate and ecological predicament we face. This revolution was driven largely by geochemistry, especially isotope geochemistry. Geoscience is, or should be, not only the record of the rocks, as it was taught to my and subsequent generations of students, but equally a record of Earth's changing environments. (Palaeontology has always had a parallel existence as the record of life on Earth, but to many geologists, until recently it remained a handmaid of stratigraphy.)
The story can be told in large part through the work and writings of Wally Broecker, who died in February.
Isotopic advances
Discoveries about carbon and oxygen isotopes in the 1940-50s were crucial. The development of radiocarbon dating gave not only precision in chronology, but also a tracer for atmosphere-ocean interactions. Subsequently came the realization that stable carbon isotope ratios were key to understanding the carbon cycle that links the atmosphere, ocean and biosphere, on a timescale beyond the reach of radiocarbon.
A parallel discovery was that oxygen isotope ratios in carbonate minerals precipitated from water give a measure of temperature. The most successful early application of this technique, Emiliani's 1955 Journal of Geology paper on Pleistocene temperatures, which was based on deep-sea cores and their contained foraminifera, may serve as a starting point for this discussion. The paper led, after some controversy, to the proof that there were many glacial episodes, not just the classical four, and that their periodicity could be correlated with the solar insolation record, as proposed much earlier by Milankovitch.
Ocean circulation
Meanwhile, Broecker's early research used radiocarbon (and bomb-produced tritium) to decipher the circulation of the oceans, which remained a preoccupation throughout his life. He was an early participant in the Deep Sea Drilling Project, as well as on many cruises from the Lamont-Doherty Earth Observatory where he worked throughout his career. He also used uranium-series isotopes to date coral terraces in Barbados, which led to better understanding of the ice-age record in relation to solar insolation.
Broecker's insights into ocean circulation culminated in the 1980s with his recognition of the Great Ocean Conveyor. Warm salty water enters the North Atlantic from the south-west, then is cooled and sinks, eventually moving into the Southern Ocean and the Pacific. Critically, the North Atlantic sinking depends both on the high salinity, hence density of the input water, as well as on the cold temperature in the region. If either of these variables were to change, the conveyor could be cut off, with profound consequences for global climate, on a short (decadal) timescale. This provides a switch, or tipping point, a frequent topic in current debates on climate. In 1990, Broecker received the Geological Society of London's Wollaston Medal especially for this work.
At around the same time, results from Greenland ice cores showed that there were indeed short-period oscillations in temperature and precipitation, especially at the initiation and termination of the Younger Dryas cold episode that interrupted the recovery of the North Atlantic temperature from the Last Glacial Maximum. Broecker was not involved in these discoveries, but he played a major role in the debates that followed on their causes, as well as those on the Heinrich layers preserved in cores that record successive migrations of icebergs across the Atlantic. So, our climate is capable of sudden shifts, even without human intervention. This led to Broecker's remark that the climate is an angry beast, and we provoke it at our peril.
Of course, Broecker's insights are not confined to the Pleistocene. His book 'How to Build a Habitable Planet' starts with the Big Bang.
The nature of fame
In the brief comments I have seen in the press, Broecker is quoted as the first person to use the phrase 'global warning' in a scientific paper. This is rather trivial (I am told that he himself offered a reward of $200 to anybody who could find an earlier usage, as he didn't want to be remembered for that). The serious point is that the research of which he was such a distinguished exponent underlies our present understanding of how the climate works. Broecker was a scientist first and a climate campaigner second, Towards the end of his life he thought that only bold geo-engineering could save us from catastrophe.
On the day Broecker's death was announced, the Guardian newspaper had pages of adulation and a long obituary for a couturier. Wally, as he was always known, had only a short note that referred to him stiffly as Wallace Smith Broecker. Wikipedia calls him a geophysicist, which is about as accurate as calling Rostropovitch a violinist. Such is fame.
A personal note
I knew Wally from my days at Caltech in 1965, when he was a visiting Professor and I was a post-doc. I have not met him for many years, but admired his work and the clarity of his writing. I used his books in teaching and I led his nomination for the Wollaston medal.
By John Hudson
Further reading
Emiliani, C. (1995) Pleistocene Temperatures. The Journal of Geology 63, 538-578;
https://doi.org/10.1086/626295
Wally’s scientific autobiography 'A geochemist in his Garden of Eden' is freely available online
here