by Charles F Kennel,
Scripps Institution of Oceanography, University of California San Diego
Centre for Science and Policy and Christ’s College, University of Cambridge
Walter Munk, Russ Davis, and the measurement of the ocean’s temperature
When I became SIO Director in 1997, Walter Munk was investigating a new way of measuring the increase in ocean temperature due to greenhouse warming. It is unfair to focus on one of the few areas he originated where he did not come out first, but Walter’s work on acoustic tomography presented the first deep problem I had to deal with as a new Director.
Walter Munk was a physicist’s geophysicist and an oceanographer’s oceanographer. He had the same semi-divine status in his field the founders of quantum electrodynamics had with my Physics graduate student friends at Princeton. The 2021 Nobel Laureate in Physics, Klaus Hasselmann, went so far as to publish the series of worshipful interviews he conducted with Walter in 2010. The interviews gave Klaus an excuse to visit Walter as he had done in the 1960s, when Klaus wrote some of the papers that would contribute to his Nobel prize[1]. I myself saw Klaus conducting his interviews at the round table with the spectacular view of the Pacific Ocean in Walter’s garden.
It was scandalous in an era of global warming that the temperature of the ocean below the surface was not well measured. My SIO colleague, Tim Barnett, had complained to me about the inadequacy of the observations of ocean temperature with unusual passion, even for Scripps. Theory was suggesting that up to 90% of the energy added to the earth system by anthropogenic greenhouse warming should be taken up by the oceans. Denialists and skeptics could use the apparent absence of ocean temperature increase to argue against the significance of the human role in greenhouse warming. The observational challenge was that the heat capacity of water is so large and the oceans so huge that it takes a vast amount of energy to raise the average temperature of the ocean by even 0.01oC. To see the expected increase, oceanographers had to measure tiny changes in temperature over an immense volume. Moreover, the spatial coverage of the measurements was not up to the job, because measuring instruments dropped from commercial ships are confined to commercial sea-lanes and miss important parts of the global ocean circulation. The bottom line was that oceanographers could not say definitively whether the upper layers of the ocean were warming as expected. Oceanography was failing its biggest test.
Walter Munk had pioneered an elegant way to measure ocean temperature, by generating low frequency sound waves that propagate over huge distances in a waveguide mode confined to the upper few hundred meters of the ocean. Since the sound wave speed depends on temperature, measuring sound travel times between two points gave the temperature averaged over the propagation path. Moreover, that temperature would be measured at the depths where greenhouse warming was expected to be strongest. Munk had shown that low frequency sound could propagate long distances by launching sound waves from Heard Island in the South Indian Ocean and detecting them thousands of miles away at Scripps on California’s Coast. To make enough long-distance sound observations for the climate change problem, systems of transmitters and receivers would be necessary in every ocean of the world. This project hit a snag, when the US Navy, which also wanted to use low frequency sound waves to communicate orders to launch nuclear missiles from submarines, built a much more powerful transmitter than what Munk needed. Whales, who had employed the waveguide transmission channel for thousands of years, had specialized languages to communicate with one another over long distances. They had formed cetacean tribes that communicate in distinctive languages. Scripps’ John Hildebrand had listened in on whale conversations and could hear whales discussing where their next dinner could be found. Even the engine noise of distant ships interfered with their conversations, but the Navy’s intense transmissions drove a particular species of whale, beaked whales, crazy-crazy enough to beach themselves and die.
Pictures of dead whales attract attention. The environmental community swung into opposition to Walter’s project to add to ocean sound pollution. Advocacy groups opposed Munk’s project where it hurt, in the opinion of the environmentally responsible public. Before my arrival at SIO, Walter had asked the Director’s office for help in explaining to the public that the potential value of acoustic tomography to the diagnosis of ocean warming was greater than the damage to whale hearing from his low-power transmissions. His system used much less power than the Navy needed, and its sound levels were less likely to drive beaked whales out of their minds.
Nonetheless, Acoustic Tomography was intentional creation of sound pollution, and this added to its ethical burden. I recognized that solving one environmental problem can incur other environmental costs that have to be counted as the price of progress. I certainly did not enjoy public conflict with conservation biologists but that is not the only reason why I felt ambivalent about acoustic tomography. To the physicist in me, Munk’s was an extraordinarily elegant technique, but it made average temperature measurements along the sound propagation path that would be open to multiple interpretations. In an age of climate change denialism, the deviltry in this detail would complicate the maintenance of credibility so essential to the public reception of climate change research.
Acoustic Tomography is now being used, but it has not advanced the climate change research enterprise nearly as decisively as the technique invented by Walter Munk’s inhouse rival, Russ Davis.
ARGO, POGO, and global ocean monitoring
I have long thought that Robert Hooke’s greatest tragedy was to have lived in the same scientific era as Isaac Newton, for otherwise Hooke would have been the most important English scientist of his generation. The dispute between Newton and Hooke about the nature of light is legendary in the history of physics. The conflict between Walter Munk and Russ Davis on how to measure ocean temperature brought the Hooke-Newton controversy to mind. Like Hooke, Russ Davis found himself in a more famous scientist’s shadow.
Russ Davis’ approach to measuring ocean temperature is a satellite era extension of what seagoing oceanographers have been doing since the late 19th Century. In the 1980’s, Davis found a way to fit the temperature, density, and salinity instruments ordinarily deployed from ships inside automated submersible floats. These little robots were programmed to carry instruments to depths up to 1000m, reside at depth for about two weeks traveling with the currents, and return to the surface, where their stored data would be telemetered to a spacecraft passing overhead. The paths followed by the instrumented floats gave the ocean currents as well as the record of temperature, density, and salinity measured beneath the surface. Knowing the temperature, you could calculate the heat content of the upper ocean, and with worldwide coverage, you could assess the rate that ocean heat content is changing due to greenhouse warming.
Figure 23-1 A schematic of the operation of an ARGO profiling float courtesy of the US National Oceanic and Atmospheric Administration. The little robot descends to depth, drifts with the ocean currents and dives deeper to record the temperature profile above 1000m depth as it rises to the surface. When the float arrives at the surface, it runs out an antenna and communicates its stored data to a satellite passing overhead. It will repeat this cycle until it runs out of battery power.
Russ Davis and his long-time Scripps collaborator, Dean Roemmich, proposed that national ocean agencies each deploy a share of a global system of floats and share in all the data acquired by the system. This project was given the name ARGO, after Argonautica, the ancient Greek epic poem. The elegance of ARGO was not in its basically brute force measurement technique but in the enchantment of its social design. A worldwide system of ARGO floats required international coordination and funding, exactly what would be needed if society were to make progress in fighting climate change. The US National Oceanic and Atmospheric Administration (NOAA), then led by D. James Baker, together with the UN Intergovernmental Oceanographic Commission (IOC), directed by SIO graduate Patricio Bernal, had taken up the challenge of securing and organizing international participation in ARGO.
SIO’s director of international relations, Lisa Shaffer, and I took it upon ourselves to carry the torch for ARGO. At NASA, Lisa and I had worked with the international Committee on Earth Observations, which coordinates space agency earth observation programs, and we believed the directors of ocean institutions needed a similar forum to coordinate ocean observations. We started by lining up Bob Gagosian, then Director of the Woods Hole Oceanographic Institution. With the two most famous ocean institutions in America aligned, Lisa and I went on to recruit the Director of the Southampton Oceanography Centre in the United Kingdom, at the time John Shepherd, soon after, Howard Roe. This was enough to convince Scripps graduate, Patricio Bernal, the Chilean Director of the International Oceanographic Commission (IOC) at UNESCO in Paris, to host a larger recruitment effort. IOC would sponsor an exploratory meeting of what Lisa would call the Partnership for the Observations of the Global Ocean (POGO). Later Lisa and I visited the Japanese Agency for Marine-Earth Science and Technology (JAMSTEC) in Yokohama, and China’s First and Second Institutes of Oceanography (FIO and SIO) to further promote international coordination of ocean research through POGO. Jesse Ausubel, of New York’s Rockefeller University, shared our thinking and, crucially, secured funding from the Lounsbery Foundation for the first three POGO meetings. After that, POGO would become a dues-paying organization. Scripps hosted the first official meeting, POGO-1, on its campus in 1999, and its twentieth, in 2018. POGO now has 56 institutional members in 32 countries.
ARGO is a system of almost 4000 floats today. I am not alone in believing that ARGO is the most significant observing capacity created for the diagnosis and modeling of climate change in the first two decades of the 21st Century.
[1] Klaus was, along with my Russian colleagues, Vedenov, Velikhov, and Sagdeev, an originator of the quasi-linear theory of wave turbulence that I used in my work with Harry Petschek.