Susan Trumbore

Susan Trumbore

Without understanding the natural carbon cycle, we’d never know how much humans are impacting climate change. The result? It’s a lot.

If you want to know how much humans have impacted Earth’s climate, you first have to develop a baseline and understand the natural cycling of carbon in our environment. Understanding the natural carbon cycle was particularly tricky in forests and soils, until Susan Trumbore devised a way to measure it. Using the same technique that archaeologists use to carbon date objects, she used the carbon atoms created during the atomic testing of the 1940s to set a reference date to measure rates of carbon cycling.  Her contributions have helped build a solid baseline against which to measure the impact of humans on climate change, and inspire solutions that will create a better future for us all.

Find out more about Susan Trumbore, 2018 Benjamin Franklin Medalist in Earth and Environmental Science.

Earth and Environmental Science
Benjamin Franklin Medal

Max Planck Institute for Biogeochemistry │ Jena, Germany│ University of California, Irvine │ Irvine, California


For her pioneering use of radiocarbon measurements in forests and soils to assess the flow of carbon between the biosphere and atmosphere, with implications for the understanding of future climate change.

If you want to know how much humans have impacted Earth’s climate, you first have to develop a baseline and understand the natural cycling of carbon in our environment. Susan Trumbore developed a system to measure this in soils and forests, helping to establish a baseline against which we can measure the impact of humans on climate change.

Since the first atomic bomb test in 1945, hundreds of nuclear weapons have been detonated—so far, only two of them in war, but all of them contributing to a lasting legacy of fallout, reaching every part of the Earth. Even before humanity became fully aware of the effects of industrial civilization on the global climate, the byproduct of its nuclear arsenal left its mark in the form of radionuclides produced in the heart of nuclear explosions. 

We may never know the full impact of those substances on the health and longevity of the generations that follow. But in an unusual way, one byproduct of that baleful era of atomic weapons testing has proven itself a useful tool for studying the most immediate threat to our planetary environment: climate change. Just as the predictable decay of naturally occurring radiocarbon provides a means for dating organic materials, the isotopes created and disseminated in the atmosphere by nuclear tests provide a way for scientists to study the natural cycling of carbon between soil, air, and the sea. For more than three decades, Susan Trumbore has pioneered carbon cycle research, turning it into a mature discipline now indispensable to the study of climate change. 

Radiocarbon dating gets its name from radioactive carbon-14, an isotope that is produced naturally in the atmosphere and taken up by plants, in carbon dioxide during photosynthesis, and by the animals that eat them. After plants and animals die, the radiocarbon they contained steadily decays, leaving a residual concentration that scientists can measure to date the remains. Trumbore first became fascinated by the science of radiometric dating while a geology student at the University of Delaware, and carried that interest into graduate studies at Columbia University. By the time she earned her doctorate in 1989 and moved on to post-doctorate fellowships at the Swiss Federal Institute of Technology (ETH Zürich) and Lawrence Livermore National Laboratory in California, she had embraced the newly-developed technique of accelerator mass spectrometry (AMS). Unlike previous methods, AMS made possible the dating of extremely small samples with great sensitivity. She and her colleagues provided the first major demonstration of AMS's capabilities when her team at ETH, in tandem with two other independent labs, tested the famous and controversial Shroud of Turin and conclusively proved its medieval origins. This test changed AMS from an intriguing scientific possibility into a routine and proven tool—the present "gold standard" for radiocarbon measurement.

But Trumbore was interested in far more than dating religious artifacts. She wanted to study the carbon cycle: the system by which carbon moves between the Earth's atmosphere, plants and animals, and soil and oceans. The years of atomic testing had created an artificial spike above the natural levels of carbon-14 in the environment, and she realized that this excess radiocarbon could serve as a tracer to track the movement of carbon throughout the environment. To acquire the soil and plant samples she needed, she became a globetrotting scientist, roaming the planet from the Amazonian jungle to the African savanna.

Until Trumbore began her work, the factors affecting the turnover rates of carbon between living organisms and the environment were largely a matter of qualitative speculation and conceptual modeling, without a foundation of solid quantitative data to build upon. Those uncertainties not only affected science's picture of the Earth's past, but also its future: how is the carbon cycle changing over time and what role do humans play? Under AMS analysis, the samples that Trumbore gathers from all corners of the Earth are helping to answer such questions definitively, turning the entire planet into her laboratory. 

Among her findings with impact beyond her immediate field of geochemistry are her 1996 Science paper and subsequent publications in which she demonstrated that the rate of carbon turnover in topsoil is far slower than estimated in most current climate models. Her studies of the sequestration of carbon dioxide in forest systems found that the Earth's forests are beginning to release even more of the greenhouse gas as global temperatures rise. These findings have great significance to the study of anthropogenic climate change.

As director of the Department of Biogeochemical Processes at Germany's Max Planck Institute and a professor at the University of California, Irvine, Trumbore is universally recognized as the leading authority in applying radiocarbon dating to environmental science, and has mentored scores of new scientists to carry on this vital work. There is a certain irony, and yet also poetry, in how she turned the product of one of humanity's darkest eras into a means to understand and perhaps ultimately counteract the threat of climate change.

Information as of March 15, 2018