Detection and attribution and climate extremes

Gabi Hegerl is famous for her work seeking to understand the processes driving climate variability, but she was initially destined to study language arts, and started off with seven years* of schooling in Latin.

Gabi Hegerl pursuing one of her passions.
Gabi Hegerl pursuing one of her passions.

Schools in Germany, at least in Gabi’s time, tended to place students into career tracks quite early, and for Gabi that meant language. But she never loved it, and soon realized “I always really enjoyed maths, even though it seemed a bit pointless”.

It turns out that her father, a lawyer, had an unfulfilled ambition to be a mathematician. But for reasons that remain unclear, he didn’t tell Gabi of his interests, and actively encouraged her to go into law. Due to the horrendous boredom of the subject, though, Gabi demured, initially pursing her interests in archaeology at the Ludwig-Maximilians University in Munich.

Describing archaeology, Gabi says “I found it really interesting but I found it just frustratingly illogical”. There was far too much fuzziness in data and interpretation, and she could never make it work entirely.

Reviewing her interests and skills, Gabi finally concluded that maths (and art!) were  immensely satisfying and continually challenging. I think the interview contains a terrific description of why people might study applied maths — a combination of a vast intellectual construct, the sense of building on prior knowledge, a simple end result, and direct applicability to real-world problems.

These interests in maths led Gabi to switch her PhD from computer logic to numerical fluid dynamics, focused on a Siemens-funded and high-risk project seeking to model fluid flow in the throat. A long way from climate science!

Juggling career options, and applying to software companies, Gabi was intrigued by the mismatch between mapped and actual glacial extents in her climbing haunts in the Alps and decided to — almost out of the blue — apply to the Max Planck Institute for Meteorology in Hamburg, working with Klaus Hasselmann and Hans von Storch.

Gabi thought of the climate work as an experiment, something to try out. But von Storch said it only made sense if an academic career was the final goal. Ultimately, Gabi agreed, even thought the salary was lower than in industry and she had to wrestle with family and relationship entanglements. “I felt putting myself to very hard work is much nicer if you feel it brings about something useful”.

Now, after a scientific career spanning the MPI, the University of Washington, Texas A&M, Duke and now the University of Edinburgh, Gabi is known for her pioneering work on the detection and attribution of climate change, climate extremes, and climate sensitivity.

Figure 3 from Gabi's most-cited senior authored paper (Nature 440, 1029-1032, 2006)
Figure 3 from Gabi’s most-cited senior authored paper, on climate sensitivity (Nature 440, 1029-1032, 2006).

We discuss the surprisingly large and long-lasting impacts of volcanoes on climate, disentangling internal vs. external controls on climate, why spectral analysis isn’t too satisfying without a mechanism,  whether or not models simulate extremes for the right reasons, how to work with Bayesian statisticians.

For me, though, the most interesting section of the interview comes in our discussion of Gabi’s work with her late husband, Tom Crowley, and the aftermath of his death in 2014.

Even after years of working with the climate community, I can think of scientists as independent constructs, visible by their papers and community activities, but not as part of a human system. The reality, of course, is intensely human.

Gabi experienced one of the most wrenching life changes one can imagine, and she tells me of the wonderful support she received from her colleagues (Simon Tett in particular), but also of the struggle to provide emotional support for her young sons and take up the family responsibilities previously handled by Tom — all while trying to keep her own career and vast network of collaborations going.

The children, naturally, emerge as the top priority, and Gabi cut back to 80% time. All of which, as she says, led to a loss of confidence in her own professional skills and knowledge. In the end, the huge challenge of working in the IPCC led to a rebuilding of faith in her abilities, showing, I think, the deep sense of personal satisfaction and confidence that can arise from contributing to a challenging scientific topic.

* In the interview, and initially on the show notes, 10 years of Latin was mentioned. It was actually seven years of Latin coursework.

Sea level rise through a geological lens

Jerry Mitrovica from Harvard University sits at the surprisingly wobbly interface between the solid Earth, oceans and ice. Trained in serious geophysics, Jerry quickly found a niche in explaining how movements of the Earth’s mantle – in three dimensions – control the apparent variation of past sea levels. In many cases, this means pointing out that many or all of our records of past sea level are fundamentally altered by processes like dynamic topography and isostatic rebound.

Credit: Jim Lor.
Credit: Jim Lor.

Jerry was born to a Greek mother and Albanian father. Their meeting in the aftermath of WW II remains fuzzy, but they soon immigrated to Melbourne, where Jerry was born as the youngest of seven siblings. Although Melbourne supported a small and vibrant Albania expat community, the move never gelled for family, and they moved to Toronto.

The shift proved difficult, as Jerry’ charismatic and intellectual father died soon after, leaving the family in dire financial straits. The local Albanian community responded by pooling together money for a down payment on a house, where Jerry’s mother lived for the rest of her life, but still … “It was my mother trying to keep the family together and survive … a widow with seven children in her late 40s”.

Jerry, however, loved Toronto, staying for all of his subsequent schooling and his initial academic career. He initially tried a PhD at Cambridge, but felt isolated and adrift. The overseas experiment lasted only a month, and Jerry returned to Toronto, where he had a strained relationship with his PhD advisor. As he says, “I wouldn’t say my PhD in Toronto was the happiest time in my life.” But the experience at Cambridge provided considerable motivation to get through, and to thrive. “I thought that leaving Cambridge after a month was a failure. I didn’t want to fail again.”

Along the way, Jerry got some key advice: “You know, you’re a theorist…you need to go to a place where data is primary”. He did, joining Irwin Shapiro at the Harvard-Smithsonian Center for Astrophysics, and it provided to be one of those fortuitous collaborations that seem to define the careers of successful scientists. “It’s just one of these wonderful things that you luck into in life, that I worked with people that were trying to build that capability, at just the time I had developed the expertise to model that.”

Jerry made predictions about rates of post-glacial isostatic rebound before the requisite GPS data existed. The predictions proved to be correct, but at the time it was a bit terrifying, and Jerry had real fears about the career consequences of being wrong “Graduate students and postdocs, even though many of them won’t admit this to you, feel extremely vulnerable. You don’t want to make a major mistake, because that’s when you’re looking for a job”.

Jerry is now famous for bringing geophysical insights to the sea level debate: gravitational attraction, sea level fingerprints, rotational shifts, isostatic rebound, and near field and far field effects have now fully oozed into the field to such an extent that it is almost impossible to imagine interpreting any sort of sea level record – be it tide gauges, salt marshes, corals, beach terraces – without considering the possible confounding effects. For many of these records, “It’s a lens alright, but it’s a contaminated, distorted lens”. Without a solid geological interpretation, some pretty big miscalculations can result.

Now, Jerry is collaborating with his students and people like Bob Kopp to bring a statistical approach to the interpretation of modern sea level rise, or sea level rise in past interglacials. As Jerry puts it, the approach is needed because “sea level records aren’t just noisy, they’re sparse”.

We spoke in aftermath of the passing of important people in Jerry life: scientists Adam Dziewonski and Rick O’Connell, as well as Jerry’s mother. Some of our conversation was, perhaps as a result, focused on the transience of academic life. As Jerry said, “That transience …takes getting used to. But in this case of course I lost colleagues that I was hoping would be around a whole lot longer”. Science and life certainly are transient, but Jerry’s work has made a major impact on geosciences that is clearly  inspiring the next generation.

Paleoceanography proxies

Tina van de Flierdt from the Department of Earth Science and Engineering at Imperial College London is an international leader in the use of geochemical proxies – particularly neodymium (Nd) – for reconstructing past ocean circulation, water masses and weathering. But her childhood and early interests pointed in a different direction.

In the lab with graduate students
Tina (right) in the lab with graduate students.

Tina grew up on a dairy farm in rural western Germany, raised by parents who were largely tied to the land but entirely supportive of her outside interests and desire to attend summer camp away from home. By her late teens, Tina knew she had a keen interest in geology, but didn’t realize that it existed as a field. It took the combination of an inspirational teacher and a sharp career counselor to set her down the path that’s led her to where she is today.

Initially, however, Tina wanted to be a hard rock geochemist, or to study volcanoes and mantle processes — passions that drew her to work in Namibia. Then, following an off-hand comment by a friend, Tina sent her CV to Alex Halliday, at the time working at ETH Zurich. Although she had no particular interest in paleoceanography, the moment she entered the lab, Tina thought “…well, screw it, I’ll do paleoceanography”. That’s all it often takes to switch gears entirely.

Tina then dove into early work on the use of Nd to trace water masses, and was a first-hand participant in the seemingly inevitable progression of a new proxy from optimism to pessimism and later reconciliation. Along the way, Tina helped to lead the GEOTRACES program, a massive international effort to disentangle the many influences on geochemical tracers. As I keep hearing on the podcast, it was a case of being in the right place at the right time, particularly if that place was Lamont.

Tina is particularly interested in the marine-terminating sector of the East Antarctic Ice Sheet during past warm periods. With her team, Tina is already producing evidence to suggest that it may be as sensitive to warming as the West Antarctic Ice Sheet. If so, the implications for sea level rise are obviously startling, and Tina is now working to move from qualitative statements to a quantitative reconstruction of past mass loss.

For a long time, the modeling community was well ahead in trying to generate these sorts of estimates, but as Tina tells it, the geochemists are rapidly catching up and the field is now in a fantastic-sounding state of collaboration and mutual stimulation of ideas.

We talk through several career topics, too: watching out for open-ended technical analysis in your early days; the unexpected rewards of teaching; the merits and problems of the UK academic assessment process; and management of peer review, particularly now that Tina is an editor at Geochimica et Cosmochimica Acta.

Climate change: it’s more than physics

Reto Knutti. Photo credit: Valérie Chételat, used by permission of ETH Zurich.

Reto Knutti and I are both interested in cake. Reto, as an analogy for the problems society faces when trying to divide up the allowable carbon emissions among historically greedy and newly desirous consumers. Me, because I love cake (ok, it’s also a great analogy).

Who wouldn't want to have as much of this as possible?
A delicious cake I made. Who wouldn’t want to have as much of this as possible? There’s caramel inside too!

Reto is a stellar climate physicist, working on many angles of climate system, including climate sensitivity, ocean circulation, aerosols, radiative forcing, changes in extremes, and allowable carbon emissions (most recently at Nature, he co-authored a paper on how regional extremes scale with global mean temperature — figure below).

Figure 3 from "Allowable CO2 emissions based on regional and impact-related climate targets" Nature doi:10.1038/nature16542.
Figure 3 from “Allowable CO2 emissions based on regional and impact-related climate targets” Nature doi:10.1038/nature16542.

Hard core IPCC Working Group I stuff, you could say.

But Reto is also interested in climate and society interactions. He’s done dozens of outreach activities, has clear ideas about how to engage with non-scientists, and thinks deeply about what models are and how to use them. So I decided to steer the conversation more towards social/policy/economics angles … fields in which neither one of us has formal training, making it all the more fun.

Reto has certainly tried hard to build scientific bridges among fields. In his experience, the theoretical and practical challenges in working across disciplines are enormous and the immediate benefits can be scant. For Reto, working with economists and social scientists: produced “…some of the hardest papers to write, ever”. And some of this work is cited hardly or not at all. If you don’t start, though, you’ll never get anywhere, and Reto doesn’t seem to regret any of the considerable effort he’s put into “non-core” activities.

In this vein, I posed a series of quasi-unanswerable questions (paraphrased):

Q: How do you get society to stop eating so much cake? A: Perhaps by massive regulation or massively reduced use, but neither seems likely. Massive investment in research and technology might be a better way forward.

Q: What’s the best way to engage with a range of civic and financial institutions on climate change? A: Keep facts and values separate: “rather than telling people what they need to do, you tell them what the facts are, what the options are, and you let them basically think on their own about what they could do or what they need to do”. Shared values definitely helps.

Q: Given that the EU and the US are at approximately similar points on the Kuznets curve, why do they have such different attitudes towards climate? A: The EU tends to think more as a society and less as an individual, and the US has fundamentally different geographical constraints and faith in technology to solve problems.

Q: How do you decide whether or not there are fundamental limitations to model projections … or put another way, is there a question for which the answer is so uncertain that modelers shouldn’t even provide an answer (inspired by Lenny Smith)? A: That’s a tough one, and actively debated in the modeling community. At the least, you can evaluate against the paleo record, check for consistency across models, and compare to modern observations. But the potential still exists that models could be missing some sort of critical physics or constraints. The potential for this kind of “miss” is probably highest for the things that are really relevant for society, like extremes and abrupt change.

Reto kindly indulged my open-ended questions about these and many other thorny issues. We did, of course, eventually get around to talking about some actual physics of the climate system!

Valérie Masson-Delmotte and the jigsaw puzzle of climate science

For Valérie Masson-Delmotte, climate science is like a jigsaw puzzle. Unlike a house of cards, where the removal of one element causes the whole thing to crash down, the central picture of a puzzle is still apparent when pieces — maybe even many pieces — are missing.

Valérie Masson-Delmotte. Rights-free image by Pierre Maraval for the “1000 Researchers project”.

Valérie works, at least in part, to fill in the missing pieces, using tools like ice cores, climate models, and statistics. I guess you could say that Valérie is an ice core scientist, but that’s like saying that Eric Ripert knows how to cook fish. Yes he does, and he’s great at it, but that kind of misses the bigger picture.

In fact I would have a hard time trying to pin down what Valérie does, because the range is so broad, spanning monsoon modeling, polar climate variability, interpretation of δ18O, age models, volcanic impacts on climate … I suppose it’s simplest to say that Valérie is trying to understand past climate variability and the underlying processes, on an astonishing variety of fronts. A great example of how Valérie and her colleagues are bringing together multiple archives and techniques is a recent Nature paper on the reconstruction of the North Atlantic Oscillation.

Figure 2 from "A model-tested North Atlantic Oscillation reconstruction for the past millennium" by Ortega et al. doi:10.1038/nature14518.
Figure 2 from “A model-tested North Atlantic Oscillation reconstruction for the past millennium” by Ortega et al. doi:10.1038/nature14518.

That’s not all, though. Valérie is the co-chair of Working Group I for the sixth IPCC assessment report, a children’s book author, and the mother of two teenage daughters.

It seems like nearly everyone gets into science by some roundabout way. In Valerie’s case, her early interest in archaeology morphed into engineering. But then, in the process of deciding whether or not to stay in science, a chance encounter with a popular science article on the Vostok ice core led to her cold-calling the French scientists who were involved in the project, and soon, the day after finishing her PhD on monsoon modeling, a postdoc at the new LSCE with the great Jean Jouzel.

The timing was perfect and, for a brief time, Valérie and her colleagues could pursue any sort of curiosity-driven science they wanted. Those days are long gone, but Valérie is still a passionate supporter of long-term, curiosity-driven science.

One theme that keeps emerging in my interviews with climate scientists is the way in which larger cultural practices inform how science gets done. For Valérie, French culture has some specific practices, particularly around raising questions. As she says, “We are not taught to ask questions … questions are supposed to reflect how smart you are … but not acknowledge your ignorance”. Yet it doesn’t seem that Valérie has any issues asking tough, reasonable questions, like when she takes me to task for not accepting important methodological advances for publication in Nature (at least in climate).

Our interview, recorded at the end of the March 2016 IPICS meeting in Hobart, Tasmania, was a great chance to hit some of the nitty-gritty of ice core science. We hash through CO2 offsets, age model synchronization with speleothems and tephra, integration of ice core records with ocean and non-polar terrestrial records, and the hunt for the oldest ice.

The many careers of Piers Sellers

I first heard of Piers Sellers some time in the mid-1990s, on a trip to the southern BOREAS field site when I was in my master’s program at the University of Montana. The talk was something on the order of “… have you heard? Piers is entering the astronaut program!” which, at the time, came as a complete non-sequitur to me. Why would someone at the peak of an influential scientific career at NASA choose to walk away?

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Lixin Wu and the rising tide of Chinese oceanography

I met Lixin Wu when I was at the Ocean University of China in Qingdao for a writing workshop (now called Nature Masterclasses). Several things impressed me about Lixin right away. First, he’s a lot of fun to be around and equally at ease in formal situations and banquets. Then, he’s clearly an inspiration to his staff and colleagues. Finally, he has big visions for his own science, the OUC, and Chinese marine science in general.

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Three decades of ice core science with Dorthe Dahl-Jensen

Dorthe Dahl-Jensen is one of the leaders of the second generation of Danish ice core scientists, following on from pioneers like Willy Dansgaard and Sigfus Johnsen. She’s began publishing in Nature and Science since 1993, and now has 16 papers between the two. Her career spans technical details, modeling, age models, abrupt change, isotopic interpretation … nearly any topic you can imagine about the Greenland ice cores.

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Into the deep ocean with Lorraine Lisiecki

Lorraine Lisiecki is in the business of understanding past variations in ocean circulation. In particular, she uses mathematical approaches to interpret observed variations in δ18O and δ13C on times scales of thousands to millions of years.

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Yusuke Yokoyama narrowly misses career in baseball, settles for stellar career in science

Nearly everyone I’ve interviewed so far has a healthy dose of “what if” in their background. But maybe no one more so than Yusuke Yokoyama, a star paleoclimatologist – geochemist – engineer – inventor at the University of Tokyo.

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