- This is part two of a two-part post. Read part one here.
When Princeton University’s Syukuro Manabe first studied global warming with general circulation models (GCMs), few other researchers approved. It was the 1970s, computing power was scarce, and the GCMs had grown out of mathematical weather forecasting to become the most complex models available. “Most people thought that it was premature to use a GCM,” ‘Suki’ Manabe told interviewer Paul Edwards in 1998. But over following decades Suki would exploit GCMs widely to examine climate changes ancient and modern, helping make them the vital research tool they are today.
In the 1970s, the world’s weather and climate scientists were building international research links, meeting up to share the latest knowledge and plan their next experiments. Suki’s computer modelling work at Princeton’s Geophysical Fluid Dynamics Laboratory (GFDL) had made his mark on this community, including two notably big steps. He had used dramatically simplified GCMs to simulate the greenhouse effect for the first time, and developed the first such models linking the atmosphere and ocean. And when pioneering climate research organiser Bert Bolin invited Suki to a meeting in Stockholm, Sweden, in 1974, he had already brought these successes together.
Suki and his GFDL teammate Richard Weatherald had worked out how to push their global warming study onto whole world-scale ocean-coupled GCMs. They could now consider geographical differences and indirect effects, for example those due to changes of the distribution of snow and sea ice. Though the oceans in the world they simulated resembled a swamp, shallow and unmoving, they got a reasonably realistic picture of the difference between land and sea temperatures. Their model predicted the Earth’s surface would warm 2.9°C if the amount of CO2 in the air doubled, a figure known as climate sensitivity. That’s right in the middle of today’s very latest 1.5-4.5°C range estimate.
At the time no-one else had the computer facilities to run this GCM, and so they focussed on simpler models, and fine details within them. Scientists model climate by splitting Earth’s surface into 3D, grids reaching up into the air. They can then calculate what happens inside each cube and how it affects the surrounding cubes. But some processes are too complex or happen on scales that are too small to simulate completely, and must be replaced by ‘parameterisations’ based on measured data. To get his GCMs to work Suki had made some very simple parameterisations, and that was another worry for other scientists.
Asking the right questions
“They thought, ‘You’ve got to improve the parameterisations first’,” he recalled. “Improving parameterisations is a very important issue, and I think that we have to do everything we can to do that. But on the other hand that model was a little bit like a stereo set – when you have a $100 speaker, there’s no point using a $10,000 amplifier. Don’t spend disproportionate time on the complicated parameterisation of some process that you don’t fully understand.”
Instead, Suki stayed with his simple parameterisations for decades, applying them to many types of climate simulations. He, Richard and other scientists would look in more detail at what changing CO2 levels did in their models, and the role of natural variability and clouds. Into the 1980s they also increasingly simulated ice ages, and the role of ocean flows in climate, driven in part by Wally Broecker at Columbia University in New York. Just as Wally credited Suki’s global warming models for convincing him that human CO2 emissions were worrying, Suki now responded to a stark possibility Wally raised.
Willi Dansgaard at the University of Copenhagen, Denmark, and other scientists had found evidence of abrupt cooling in the Northern half of the planet 12,800 years ago. In this ‘Younger Dryas’ period gradual warming at the end of an ice age reversed dramatically, and then was followed by similarly rapid warming 1,300 years later. In the 1980s, Wally suggested a massive meltwater lake had suddenly emptied into the North Atlantic, shut down an ocean flow carrying warmth northwards, and flipped climate into a different state. If that were true, perhaps similar threats might come with the warming that human CO2 would bring.
Using their ocean-coupled GCM, Suki and fellow GFDL scientist Ronald Stouffer showed in 1988 that two different climate states – with and without the warming flow – could indeed be stable. By 1995, they had also shown that Wally’s melting lake idea could make the flow weaken abruptly, intensify, weaken again and then slowly recover. That wasn’t quite enough to reach a stable state without the warming flow, but was enough to cause sharp temperature rises and falls.
Emerging from the swamp
To include realistic ocean flow in these studies, Suki and Ronald had to tackle the problem of ‘flux imbalance’. Moving beyond swamp oceans produced tiny differences in movement, energy and fresh water moving between the atmosphere and ocean that could push ocean-coupled GCM simulations into unrealistic states. So they introduced ‘flux adjustments’ to stop the simulated climate drifting away from a realistic state. But these changes lacked a basis in physical theory, another factor that drew criticism. Yet because Suki and Ronald made the same adjustments both when simulating the changes they’re interested in and the baseline simulations they compare against, they felt they were justified.
“It’s a fair game in that sense,” Suki said. “And suddenly I am able to do all kinds of interesting calculations. Everybody else is complaining because when they are working so hard to improve the model, here is a guy who came up with this gimmick, and without pain gets all kinds of interesting results, which may or may not be true in their opinion. They are angry, because instead of incorporating their parameterisation, I am still using my old 30-year-old parameterisations in the atmosphere, and then using this ad hoc method I started having a great time.”
Soon other scientists started recognising the power of the approach, and using Suki’s ocean-coupled GCMs themselves. And when the UN Intergovernmental Panel on Climate Change (IPCC) began preparations for its first assessment report, that would be published in 1990, Suki couldn’t avoid it. “I avoid these committees like pests,” he admitted. “They told me I better come.” As a result the report relied heavily on a paper GFDL had published the previous year, modelling the world’s atmosphere, land and oceans with the best combination of duration and detail then available.
Though Suki has since been less involved in global climate assessments, and flux adjustment is less widely used today, he has remained active in improving climate models. In March this year he published a paper in Proceedings of the National Academy of Sciences of the USA aimed at reducing uncertainty over climate sensitivity. “The uncertainty is attributable in no small part to our inability to model reliably the feedback processes of cloud,” he told me. “Our study proposes a promising approach for evaluating modelled feedback, using the top-of-the-atmosphere fluxes of outgoing radiation observed from satellites.”
Beyond modelling, Suki today sees assessing the impact of climate change as one of the most challenging tasks ahead. “Under the current situation, it is extremely difficult, if not impossible, to mitigate emission of CO2 enough to prevent 2°C warming,” he warned. “Increasing emphasis should be placed, in my opinion, upon the adaptation to global change such as increased frequencies of drought in semiarid regions, those of floods in moist regions, and sea level rise. Also, we could increase our effort to exploit the benefit of global warming such as increased plant growth from increasing CO2, and increased availability of the Arctic Ocean due to the rapid reduction of sea ice.”
Apart from the quotes in the last two paragraphs, which are from questions I asked Suki directly, all the quotes in this blog entry are from an interview with Suki Manabe by Paul Edwards on March 14-15 1998, published by the Archives for the History of Quantum Physics Collection, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA. They are copyrighted by the AIP, and used here with its permission.
This year I’ve already written about the following pivotal climate scientists who came before Suki Manabe, or were around at the same time: Svante Arrhenius, Milutin Milanković, Guy Callendar part I, Guy Callendar part II, Hans Suess, Willi Dansgaard, Dave Keeling part I, Dave Keeling part II, Wally Broecker part I, Wally Broecker part II, Bert Bolin part I, Bert Bolin part II, Suki Manabe part I
Syukuro Manabe and Richard T. Wetherald (1975). The effects of doubling CO2 concentration on the climate of a general circulation model Journal of the Atmospheric Sciences : 10.1175/1520-0469(1975)0322.0.CO;2
Syukuro Manabe (1975). The use of comprehensive general circulation modelling for studies of the climate and climate variation The Physical Basis of Climate and Climate Modelling, Report of the International Study Conference, GARP Publications Series No. 16, World Meteorological Organization
Wallace S. Broecker, Dorothy M. Peteet & David Rind (1985). Does the ocean–atmosphere system have more than one stable mode of operation? Nature DOI: 10.1038/315021a0
Syukuro Manabe and Ronald J Stouffer (1988). Two stable equilibria of a coupled ocean-atmosphere model Journal of Climate DOI: 10.1175/1520-0442(1988)001
Syukuro Manabe and Ronald J Stouffer (1995). Simulation of abrupt climate change induced by freshwater input to the North Atlantic Nature DOI: 10.1038/378165a0
Syukuro Manabe and Ronald J Stouffer (1999). Are two modes of thermohaline circulation stable? Tellus A DOI: 10.3402/tellusa.v51i3.13461
Syukuro Manabe and Ronald J Stouffer (2000). Study of abrupt climate change by a coupled ocean-atmosphere model Quaternary Science Reviews DOI: 10.1016/S0277-3791(99)00066-9
Yoko Tsushima and Syukuro Manabe (2013). Assessment of radiative feedback in climate models using satellite observations of annual flux variation. Proceedings of the National Academy of Sciences of the United States of America, 110 (19), 7568-73 PMID: 23613585
Spencer Weart’s book, ‘The Discovery of Global Warming’ has been the starting point for this series of blog posts on scientists who played leading roles in climate science.