Citation for Steven J. Davis

Steven J. Davis, 2018 Macelwane medalist
Steven J. Davis

Davis’s research combines economic and geophysical approaches that are groundbreaking. He has carved a new field relating human systems, the carbon cycle, air pollution, and climate change, thus providing solutions for some of the most significant environmental challenges of the 21st century.

Davis developed the theory for tracking carbon dioxide (CO2) emissions that are embodied in all the goods and services traded internationally. Davis learned econometric methods and applied them in an analysis that was timely and salient: International climate negotiations were mired in the debate over whether developing countries should be required to reduce their CO2 emissions. By quantifying emissions that are effectively “outsourced” from developed countries, his work provides a rigorous scientific foundation that has informed ongoing policy discussions. By recognizing the interconnectedness and universally shared responsibility for these emissions, his work contributed to the success of the 2015 Paris Climate Conference, with the subsequent agreement creating a more level playing field for all nations.

Davis made another important breakthrough in tracking future global CO2 emissions from existing energy infrastructure. Even if we never build another power plant or car, assuming that existing infrastructure continues to operate for its expected lifetime causes a certain amount of emissions to be locked in. This new approach has focused the policy community on the implications of near-term plans by many developing countries to expand the number of fossil fuel–emitting power plants, given that normal operating lifetimes for these facilities can be 30–40 years. In more recent work, Davis and his colleagues have quantified the health impacts (and human mortality) associated with global trade flows, leveraging his earlier breakthrough linking CO2 emissions and trade.

Davis’s unique background combines a legal education, a dissertation on isotope geochemistry, work with nongovernmental agencies on life cycle assessment, and a postdoctoral analysis of energy systems. This gives Davis a diverse set of research methods. As a colleague in the Earth System Science Department at the University of California, Irvine, we truly value Davis’s vision and comradery.

Steve represents a new 21st century generation of geophysicists. We are thrilled that he is this year’s Macelwane Medal winner.

—James Randerson and Ellen Druffel, University of California, Irvine


I’m honored and humbled to accept the Macelwane Medal. Thank you, Jim and Ellen, for nominating me and for your generous citation. I’m fortunate to have you both as role models and colleagues, and I can only hope to distinguish the award as you both have.

I’m also grateful to AGU and the medal committee for selecting me among so many deserving young scientists. In doing so, they highlight the breadth of modern geophysical research in a way that I hope encourages and emboldens others to color outside the lines.

As Jim and Ellen mention, my path has been winding, and though they portray my varied background in a flattering light, it was less strategic building of experience than following my interests and trusting they would lead me to somewhere I wanted to be. Luckily, I happened into some terrific mentors who were audacious enough to give me a chance. Without them, I would never have become the scientist I am today.

Still a corporate lawyer, I walked into Page Chamberlain’s Stanford office one afternoon and did my best to convince him that I wanted to be a geologist. In retrospect, I was incredibly naïve, and I can’t for the life of me understand what he saw in me that day, but he took me on as a graduate student and over the next 4 years transformed me from an attorney with a philosophy degree into an isotope geochemist.

Ken Caldeira enabled yet another leap, hiring me as a postdoc at the Carnegie Institution to work on CO2 embodied in international trade on the basis of a fun lunchtime conversation we had—again despite my knowing almost nothing about the subject. What was supposed to be a few months turned into nearly 5 years in productive pursuit of the interesting and important.

Perhaps as important as the opportunities these men afforded me, they taught by example how to balance careful execution of science with bold hypotheses, how to resist common knowledge, to be more critical of yourself than you are of others, and that data are lifeless without a good story. These lessons were bolstered by other outstanding collaborators and mentors like Rob Socolow, Jen Burney, David Lobell, Glen Peters, Dabo Guan, Qiang Zhang, Rob Jackson, and Noah Diffenbaugh. My colleagues at the University of California, Irvine, have also been constant supporters of my work. I am indebted to them all. I hope that my mentorship can inspire and enable other students and postdocs as theirs has me.

Most of all, though, I am thankful for the love and unwavering support of my wife, Kristen; my parents; and my daughters. They have given me both the confidence to chase big ideas and the reason for the chase.

—Steven J. Davis, University of California, Irvine

Citation for Walter Immerzeel

Walter Immerzeel, 2018 Macelwane medaliat
Walter Immerzeel

Since obtaining his Ph.D. in 2008, Walter Immerzeel has established himself as an influential scientist in the field of mountain hydrology and glaciology. He focuses his research on the impact of global warming on snow and glaciers in High Mountain Asia (HMA) and how it affects future water resources in the downstream, densely populated areas. His unique interdisciplinary research line combines innovative fieldwork techniques, remote sensing, and mesoscale meteorological modeling to elucidate glaciohydrological processes at the catchment scale. His most influential study to date (Science, 2010) uses the above techniques to show that contrary to what was previously believed, the contribution of HMA glacial melt to downstream runoff in most Southeast Asian rivers is smaller than a few percent, the exception being the Indus, for which up to 60% of the downstream discharge has its origin in HMA snow and glacial melt. This process understanding is used to predict the effects of climate change on the glaciohydrology and water resources of HMA as a whole. Walter and his group showed that for realistic climate scenarios maximal glacial melt is expected around 2050 in the western HMA, after which increased rainfall will compensate for the decreased meltwater runoff. In a 2017 study in Nature Walter’s group predicted that HMA glaciers could lose up to 65% of their mass by 2100.

The research of Walter and his group has been highly visible among his colleagues and in the international press. For instance, they were the first to use unmanned airborne vehicles (UAVs, or drones) in HMA glacier research, showing that locally, debris-covered glacier melt rates can even be larger than those of debris-free glaciers as a result of lakes and exposed ice cliffs. To draw attention to the detrimental changes that await the region under climate change and how scientific research can help to mitigate these changes, he has used part of his research funds to make brief, insightful documentaries. During his years working in the region, Walter has built up a strong relation with the local Nepali people and scientists. During his Ph.D. track he was employed by the International Centre for Integrated Mountain Development (ICIMOD) and lived in Kathmandu for 2 years. Currently, he still is a guest researcher at ICIMOD. Right after the April 2015 Nepal earthquake Walter joined forces with people in his remote sensing network to provide the Nepali authorities, through his link with ICIMOD, with inventories of earthquake-induced landslides as seen from space.

—Michiel R. van den Broeke, Utrecht University, Netherlands


Dear Michiel, many thanks for this wonderful citation, and I look forward to link the three poles together by working with you. I am also very grateful to AGU, the Macelwane Medal Committee, and esteemed colleagues for the nomination letters.

As someone from the Netherlands, where the highest point is 322.4 meters above sea level, it may seem strange to be awarded the Macelwane Medal on research in high-mountain Asia, and an explanation is justified. After trekking for 3 months through Nepal with my wife, Hilde, during our honeymoon in 1999, the mountains and its people were permanently imprinted in my system. This resulted in a job at the International Centre for Integrated Mountain Development (ICIMOD) in Kathmandu, where I lived and worked for 2 years. I met Dr. Roberto Quiroz there, who inspired me to pursue a Ph.D. in this field, and it turned out to be a life changer.

I am still working closely with ICIMOD with a unique group of mountain-loving people who dedicated their careers to the understanding and protection of this beautiful region. I specifically mention David Molden, Arun Shrestha, Anna Sinasalo, and Inka Koch. Without your support, many of the scientific achievements would not have been possible. The joined expeditions in the Himalayas have also sparked new friendships and long-lasting international collaborations. Working with incredible scientists, such as Joseph Shea, Patrick Wagnon, and Francesca Pellicciotti, has been a great inspiration and motivation and a lot of fun.

At Utrecht University in the Netherlands, I feel privileged to work with a great group of people. A special thanks to Marc Bierkens, who has been my mentor for years. Our brainstorm sessions formed the basis for many successful papers and proposals, and I hope we can continue like this for years! This medal is not the result of only my work, but it was true team work, and big thanks to Arthur, Emmy, Jakob, Maxime, Philip, Pleun, Remco, Sonu, and René. I also thank my family; my friends; and my wife, Hilde, in particular for making sure I realize what is really important in life and for making me realize what the red thread in my research really is.

There are huge scientific and societal challenges ahead associated with mountains and the millions of people that depend on their resources. I am proud and looking forward to keep working on them.

—Walter Immerzeel, Department of Physical Geography, Utrecht University, Netherlands

Citation for Isaac Santos

Isaac Santos, 2018 Macelwane medalist
Isaac Santos

Isaac Santos is not only an outstanding Earth scientist but also a brilliant science communicator and student mentor. With training in chemical oceanography at Florida State University under Bill Burnett, Isaac recognizes the interdisciplinary nature of our field and works collaboratively at the edge of conventional disciplinary boundaries.

Isaac uses geochemical tracers to resolve the drivers and biogeochemical implications of submarine groundwater discharge (SGD). He made elegant demonstrations that SGD is a major source of greenhouse gases and carbon to the coastal ocean and developed substantial arguments for the importance of SGD at a variety of coastal sites, including mangroves, coral reefs, estuaries, and beaches. Isaac’s innovative research relies on his vision of taking the lab to the field. He combined automated observations of radioactive and stable isotope tracers to obtain real-time and high-precision data in situ. These techniques allowed Isaac to collect unique data sets to address complicated and important interdisciplinary questions in hydrology, biogeochemistry, and oceanography. His diverse research has inspired colleagues and government agencies to consider the implications of SGD to coastal biogeochemical cycles and water quality.

Isaac’s dedication to science goes beyond the quest for new knowledge. Isaac should also be recognized for his broader contribution to our community and gregarious approach to work and student supervision. He is an unselfish colleague leading a large and productive research group. He always shares his best ideas and projects with his students. His leadership, combined with his great ability in developing collaboration and building first-class infrastructure, will help to ensure long-lasting impacts of his work. Isaac is a candid and idealistic advocate on issues of public interest. By engaging with the public and the mainstream media, his research has influenced public opinion and is helping to manage and preserve valuable coastal waterways while encouraging the fossil fuel industry to prevent greenhouse gas leakages from aging infrastructure.

With balance in research, teaching, and outreach as well as life outside academia being a desire of all, Isaac has achieved just that. Achieving balance extends his positive influence to our future generation of scientists. His outstanding track record of discovery, focus on student development, collaborative mindset, and engagement with the broader society are fully in line with AGU’s values and mission. His many and important contributions published in >100 papers, commitment to the well-being of his students, and spirit of generosity make him a well-deserved winner of the James B. Macelwane Medal.

—Ling Li, Westlake University, Hangzhou, China


Winning an AGU medal makes me feel nostalgic and grateful. It brings memories from where I come from and all the people who supported me along the way. It enlightens a path to the future.

As an undergrad in Rio Grande (Brazil), I would read AGU journals as I learned English. I would dream about publishing there. I didn’t dream about winning a medal. As a Brazilian working in regional Australia, being singled out with a medal takes me a step closer to overcoming a self-diagnosed impostor syndrome.

I am thankful for Ling’s generous nomination and honored to have worked with him. He mentions the achievement of work-life balance, but I am afraid my wife, Ana, may occasionally disagree with this. On these occasions, I often apologize and remind her that a 20-year marriage requires true love. Instead of balance, I have blended work and life as I care for my young family and students at the same time. I would achieve nothing without family encouragement and understanding.

Investigating submarine groundwater discharge requires truly interdisciplinary approaches and collaboration. My lifelong mentors Bill Burnett, Jeff Chanton, Thorsten Dittmar, Felipe Niencheski, and Emmanoel Silva-Filho have been role models inspiring me along the way. Thanks to my friends and collaborators Guilherme Lima, Rick Peterson, Christian Sanders, Damien Maher, Doug Tait, and many others whom I cannot mention in 400 words.

It is an exciting time to work on submarine groundwater discharge! I often say that submarine groundwater discharge has been put on the “to-do list” or in the “too hard basket” for long enough. Our new tools are revealing leaky shorelines worldwide, helping us to quantify the invisible, and demonstrating major implications to marine biogeochemistry. We are learning how the underground routes of biogeochemical cycles provide climatic feedbacks as groundwater releases soil carbon and nutrients to the oceans.

Macelwane was deeply interested in education. He saw himself as a servant to students, and I am proud to be associated with his vision. As researchers, we often strive for independence and can easily become too self-centered. Students unconsciously remind me of what research is all about. Their seemingly simple but penetrating questions, fresh perspectives, and full commitment to improving our world give real meaning to my research. Thanks to current and past students for raising the bar every day, keeping me grounded, and pushing me outside comfort zones—I owe this medal to you!

—Isaac Santos, Southern Cross University, Lismore, Australia

Citation for Drew Turner

Drew Turner, 2018 Macelwane medalist
Drew Turner

Drew Turner is widely recognized for his outstanding contributions to understanding energetic charged particles in the Earth’s magnetosphere. Changes in energetic particle populations illuminate how energy from the Sun couples to our local space environment. Drew’s work revealed how those processes work in systems from the Van Allen radiation belts to the boundaries of the magnetosphere and solar wind.

Drew Turner began his work at the University of Colorado under the supervision of Xinlin Li. Among many excellent students, Drew stood out for his enthusiastic questions, deep professional engagement, and creative problem solving. In those years, Drew started a string of important papers related to NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes satellite missions. In particular, Drew developed novel methods for disentangling complicated co-occurring processes that can either enhance or deplete the radiation belts. That work culminated in a 2012 Nature paper just 2 years after his Ph.D.

After the University of Colorado Drew moved to the University of California, Los Angeles, and, more recently, to the Aerospace Corporation. In his career, Drew has consistently come up with creative ways to answer some of the most important questions in our field—whether it is understanding the complicated balance of acceleration, transport, and loss processes in the radiation belts, understanding particle dynamics at the magnetopause and foreshock, or unraveling the complex processes that couple fast flows, dipolarization fronts, and energetic particle injections in storms and substorms.

Drew is able to synthesize information from different measurements, different instruments, and multiple satellite missions. One recent paper combined observations from dozens of instruments on 15 satellites making up five very different space missions. Drew also draws on his experience with spaceflight hardware to develop new data reduction and analysis routines that frequently become the basis for analysis done throughout the community. As a result, Drew’s papers are able to explore, simultaneously, the important processes that happen on local scales and the global context for those processes.

In the 8 years since his Ph.D. Drew has published over 110 papers, including 3 in Science, 4 in Nature, and 32 in Geophysical Research Letters. One factor leading to his remarkable publication record is that people seek out Drew’s insights and contributions. His body of work reflects his creativity and talent but also generosity with his ideas, his time, and his energy.

Drew Turner truly embodies values that AGU promotes and the qualities that the Macelwane Medal recognizes.

—Geoffrey D. Reeves, Los Alamos National Laboratory, N.M.


Let me put first things first: Geoff Reeves, Jacob Bortnik, Joe Fennell, and Xinlin Li, I can never thank you enough for your nomination and the ongoing tutelage and support you’ve offered to me throughout my career. I have learned so much from each of you, whether it be leadership skills and new physical perspectives from Geoff and Jacob, open-mindedness and spaceflight instrumentation from Joe, or good mentorship and the finer points of critical thinking and strategy from Xinlin. I owe much of my success in many ways to each of you, and I hope you each know how much that is appreciated.

Next, I would like to thank AGU and the Macelwane Medal Committee for this incredible honor. I will do my very best to live up to this distinction and embody the commitment to excellence in research, career development, and education that James B. Macelwane represented.

My research relies heavily on data provided by various satellite missions, and I know well that it takes teams of hundreds or more to ensure the successful development, launch, and operations of each mission. I want to say a special thank you to the teams for NASA’s THEMIS, Van Allen Probes, and Magnetospheric Multiscale (MMS) missions for providing such high-quality and scientifically rich data to the world.

Professionally, I wouldn’t be where I am today without the help, tutelage, and support of so many of my colleagues. It is impossible here to thank each personally, but I want several of you to know the especially positive impacts you’ve made on my life. Thank you, Drs. Angelopoulos, Azeem, Blake, O’Brien, Shprits, and Sivjee, for your mentorship. To Drs. Blum, Claudepierre, Cohen, Drozdov, Gabrielse, Gkioulidou, Hartinger, Hietala, Jaynes, Kellerman, Kiehas, Maget, Millan, Plaschke, Schiller, Tu, Ukhorskiy, Usanova, and Wilson, thank you so much not only for your professional advice and contributions to my research but also for your invaluable friendship.

With the few words I have left here, I must thank my family and friends for their unending and unconditional support. Mom, Dad, Shona, and my full extended family: thank you all so very much for your love and encouragement. Mom and Dad, I have pursued a career that makes me happy, just like you told us.

Finally, Krista, my love, partner, and best friend, thank you so much for the love, positivity, and kindness that you give me every day.

—Drew Turner, The Aerospace Corporation, El Segundo, Calif.

Citation for Caroline C. Ummenhofer

Caroline C. Ummenhofer, 2018 Macelwane medalist
Caroline C. Ummenhofer

Caroline Ummenhofer is at the forefront of developing a dynamic understanding of how the ocean affects the climate system. She has done particularly creative work in helping to understand the mechanisms of how ocean properties give rise to changes in large-scale atmospheric circulation that impact rainfall on land. Her core research discoveries have been in tropical climate variability, particularly in the Indian Ocean; the linkages between ocean variability and atmospheric blocking outside the tropics; and the connections between the Hadley Circulation and the ocean’s eastern boundaries. In each area, Caroline has made very substantial contributions. For example, prior to her highly cited 2009 paper in Geophysical Research Letters, it was thought that the most damaging droughts experienced in Australia were caused by Pacific Ocean conditions. However, she showed convincingly that conditions in the Indian Ocean were the real cause.

Caroline has worked with a variety of colleagues on a wide array of climate problems over millennial timescales to modern seasonal predictions. She introduces new techniques and concepts in her work and always has an eye toward the implications for society. She has taken a particular interest in showing the connections between ocean conditions and the extremes of drought and flood that influence crop yields. She has made exceptional contributions to understanding the ocean’s influence on rainfall in Australia, the United States, Southeast Asia, and Africa. She is a leading expert on the expansion of the tropics and drying of the subtropics predicted with global warming. She has taken on leadership roles in national and international projects like Climate and Ocean: Variability, Predictability and Change (CLIVAR) while maintaining a truly remarkable production rate of original scientific research. Her dynamical insights are deep, the problems she tackles are hard, and her approaches are novel and creative. She is a rising star in ocean and climate science and has already had a global impact.

On a personal level, I have to say that it has been a wonderful experience to have Caroline as a colleague for the past 6 years. I marvel at her productivity, her prodigious travel schedule, her skill at building collaborative teams, her teaching ability, and her generosity to students, colleagues, and the community at large. I am certain that you will be hearing her name in connection with important discoveries about the climate system for many years to come. Please join me in congratulating Caroline for her well-deserved honor of the James B. Macelwane Medal.

—Raymond W. Schmitt, Woods Hole Oceanographic Institution, Mass.


Thanks, Ray, for your kind words—and your relentless support and encouragement over the years. I also want to thank my nominators and the AGU Macelwane selection committee for their time and efforts.

Science, and especially climate science, has become an intrinsically collaborative effort. The research leading to this medal was only possible through contributions from many colleagues around the world. I am continuously inspired by the wealth of groundbreaking research findings by the climate and geoscience community—many of whom are at least as deserving of this medal.

The interdisciplinary nature of the field is what first drew me to ocean sciences and my degree in marine biology/oceanography at the School of Ocean Sciences, University of Wales, United Kingdom. Three individuals there in particular opened my eyes to the attraction of interdisciplinary research in biophysical interactions and paleoclimate that I enjoy at the Woods Hole Oceanographic Institution (WHOI) today, namely, Dave Bowers, David Thomas, and James Scourse.

From north Wales to New South Wales, half way around the globe, an 8-week internship in Sydney, Australia, turned unexpectedly into the best 8-year Ph.D. and postdoc experience I could have wished for. I am deeply grateful to my adviser, Matthew England, for his unstinting support and generous mentorship ever since that email about hosting my internship—and for teaching me how to be a productive, creative, and collaborative scientist. I also found a wonderful team of close colleagues-turned-friends at the University of New South Wales, particularly with Alex Sen Gupta and Andrea Taschetto.

My research on Australian rainfall and Indian Ocean dynamics was greatly enriched by interactions with colleagues at CSIRO (Peter McIntosh, Gary Meyers, Mike Pook, James Risbey). Long-standing collaborations with colleagues at the Lamont Doherty Earth Observatory (Rosanne D’Arrigo, Brendan Buckley), Claus Böning and Arne Biastoch (GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel), Jerry Meehl (National Center for Atmospheric Research), and Rhawn Denniston (Cornell College) also shaped my research.

I’ve been fortunate over the years to be able to rely on wonderful administrative staff and would like to thank them for all their hard work that supports all our scientific endeavors.

At WHOI, I found a stimulating collaborative environment ideal for exciting new interdisciplinary research, made so by the individuals working there. Too numerous to all name here, I am indebted to Ray Schmitt as my key mentor and also thank my close collaborators Young-Oh Kwon and Hyodae Seo, as well as a dynamic group of postdocs and students.

It feels like I receive this award as a representative of a larger collective—and I constantly cherish the opportunities to work with many inspiring researchers from different disciplines to tackle key questions in the field of climate science and its societal applications.

—Caroline C. Ummenhofer, Woods Hole Oceanographic Institution, Mass.


(2018), Davis, Immerzeel, Santos, Turner, and Ummenhofer receive 2018 James B. Macelwane Medals, Eos, 99, Published on 18 December 2018.

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