In the 1990s, Ellen Thomas and James Zachos uncovered an anomalous episode in the planet’s history in which massive quantities of CO2 and methane were released into the atmosphere and global temperatures rose by between 5 and 6ºC. The event, whose presumed origin was volcanic activity, turned the oceans more acidic and unleashed one of the biggest extinctions of deep-sea fauna in the whole history of Earth.

This interval, which occurred 56 million years ago, is now known as the Paleo-Eocene Thermal Maximum (PETM), and its associated greenhouse gas emissions, and their consequences, are comparable to today’s climate change caused by fossil fuel combustion, lending weight to the numerical models currently in use to predict future climate evolution.

“Zachos and Thomas’ research has laid the foundation for the climate change prediction models we are using today,” explains the nominator of the two new laureates, Laia Alegret, Professor of Paleontology at the University of Zaragoza and a member of the Spanish Royal Academy of Exact, Physical and Natural Sciences.

The PETM is considered the best historical analog for modern climate change. The similarities in carbon emissions, rising temperatures and ocean acidification, along with the detailed extent to which the event is known, make it an ideal testing ground for climate predictions derived from numerical models, to see if they hold up. It is in this sense a kind of “natural experiment” that has served to validate and bound the predictive models currently in use, remarked Professor Zachos in an interview shortly after hearing of the award.

A “history book” on the planet’s climate

Interview with Ellen Thomas

The discovery of PETM had its beginnings in a 1987 drilling expedition to the Antarctic Ocean, with Thomas among its crew. And it happened, the awardee relates, out of sheer “serendipity.” As an expert in microscopic fossils, it was her job to analyze the sediment samples obtained through ocean floor drilling in search of benthic foraminifera, microscopic organisms that live on the ocean floor.

Because of how they are deposited, sediments are a kind of planetary history book whose deepening layers represent ever more distant epochs. On coming to the layer marking the boundary between the Paleocene and Eocene, Thomas observed a series of very large changes in these deep-sea organisms: “It was not at all what I had expected,” she recalls. “I was looking for relatively minor changes, because the deep ocean is the planet’s largest habitat and unlikely to change over short time scales.”

But what she saw in the sample was so huge an extinction in such a stable environment that it had to be caused by a change operating at the global scale. Immediately the whole expedition crew got down to analyzing the shells’ chemical composition to ascertain what conditions they had been formed in, essentially water temperature and acidity.

Thomas observed that the mass extinction had occurred at a time of pronounced global warming. “It was the largest extinction of deep-sea fauna within the last 90 million years,” Laia Alegret explains.

Although the extinction event had been documented in earlier scientific papers, Thomas was the first to study it in detail and, crucially, to locate its origin in a global change at the boundary of the Paleocene and Eocene.

The biggest global warming episode in the last 65 million years

Interview with James Zachos

Not long after came the final confirmation of this momentous upheaval, thanks to the work of James Zachos. When analyzing terrestrial sediments obtained in Wyoming (United States), he observed certain changes in the nature of the carbon they contained occurring just at the Paleocene-Eocene boundary. One was a marked perturbation in levels of the carbon-13 isotope, suggesting that large quantities of this element had been released into the atmosphere over extremely short geological time scales.

“Suddenly all the pieces of the jigsaw started to fall into place, and they were also consistent with the greenhouse effect theory,” Zachos relates. “What Thomas had found in the deep sea, Zachos was now observing on land at thousands of kilometers distance,” adds Laia Alegret. “This was proof that what they were looking at was a major global event, affecting not just terrestrial systems but also the ocean floor and surface.”

Since then, Zachos and Thomas have collaborated in an effort to unravel Earth’s climatic variations throughout its geological history. In 2001 they published a paper in Science that featured the most complete temperature curve for the last 65 million years, known as the “Zachos curve.” In all this time, the PETM stands out at the single biggest warming event on record. And other similar though less intense warming events also emerged, for which researchers coined the term “hyperthermals.”

A new research avenue to study the impacts of climate change

The above Science paper currently ranks among the most highly cited in geosciences. The identification of other hyperthermal intervals helped round out the natural experiment facilitated by the PETM, enabling the identification of warming events of differing magnitude, greenhouse gas emissions at different rates and the multiple impacts ensuing from each episode. “It opened a new line of research that is now being pursued by hundreds of scientists and has won a prominent place in the best scientific journals,” remarks nominator Alegret.

The historical knowledge provided by Zachos and Thomas has been fed back into predictive models for the impact of present-day climate change to test the soundness of their forecasts. “We have been able to confirm that the greenhouse effect theory is essentially correct,” says Zachos, “and this has made us more confident about our ability to predict future climate.”

The droughts and severe rainfall episodes that are part of climate change as we are currently experiencing it reflect changes in the hydrologic cycle which have also been documented during the PETM. Not only that, PETM studies have confirmed that it takes tens of thousands of years for excess carbon in the atmosphere to be sequestered by natural processes. For the awardee researchers, this finding proves that we cannot rely solely on forests to absorb the carbon emitted by burning fossil fuels as a solution to the current crisis.

Zachos, in fact, would like to see the process speeded up by recourse to technologies that capture atmospheric CO2 so it can be buried where it will not produce any greenhouse effect. Already carbon is being injected into the Earth’s crust and some propose doing the same in the ocean, while plans have been mooted to accelerate the decomposition of rocks or even to pulverize them for worldwide use as a fertilizing agent. “All these actions, taken together, could start removing CO2 from the atmosphere at a relatively fast rate,” he considers.

A warning for the future

The mass extinction brought about by the PETM is a warning to us all. And among its most devastating consequences is biodiversity decline. As Thomas observes: “The PETM records show us that in basins like the Mediterranean oxygen levels fell so sharply that many organisms could not survive, and this of course led to a loss of marine biodiversity.”

Although there is no scientific consensus on what caused this rapid and massive release of greenhouse gases, recent research suggests that volcanic activity in the North Atlantic may have triggered a chain reaction. As well as the greenhouse effect, the gases released altered the oceanic currents, which in turn produced a small warming of the oceans and destabilized the permafrost (frozen ground) and the methane hydrates located below the seabed.

The collapsing of these gas reservoirs would have released yet more carbon into the atmosphere, ramping up the both the greenhouse effect and the disruption of ocean currents. Estimates point to an emissions total of between 2,000 and 5.000 gigatonnes of carbon (one gigatonne being a billion tonnes). And yet the current rate of emissions driven by human industrial activity over the course of two centuries is, says Zachos, around ten times higher.

“If we compare this with the time scale of today’s anthropogenic warming, which represents the tiniest fraction of Earth’s history, no more than two centuries since the Industrial Revolution, it is clear that our emission levels are already sky high. Past analogs, like the PETM, warn us that we are set on what threatens to be a very dangerous path,” says Miquel Canals, Director of the Department of Earth and Ocean Dynamics at the University of Barcelona, and a member of the award committee.

The urgency of acting now to avoid worst-case scenarios

Asked whether today’s human-induced greenhouse effect could end up triggering a warming event as extreme as that suffered during the PETM, Zachos believes that “it could easily happen” if we go on burning fossil fuels and sticking to the familiar but pernicious routine of “business as usual.” However, the laureate is convinced we still have time to remedy the situation, or at least to avert its worst consequences: “We could prevent greater warming by reducing carbon emissions, switching to renewable energies as we are now doing in a big way. But the longer we take, the more difficult it will be to keep carbon dioxide levels below those that would give us 4 or 5ºC of warming.”

Zachos warns that certain impacts are “probably unavoidable.” He believes, for instance, that “we are already committed to a meter or two of sea-level rise even if we could cut carbon emissions immediately.” Nonetheless, he continues, “we still have the opportunity to prevent the worst-case scenarios if we can reverse or reduce carbon emissions. Otherwise we may be talking about sea-level rise on the order of 10 to 15 meters.”

Thomas, meantime, admits to feeling “fairly pessimistic” about stopping global warming from causing grave harm to the human population: “Regarding sea level, for example, I am pretty much convinced that we have underestimated the rate of increase, and will see serious effects in populated areas, like my own country of birth, the Netherlands, where large expanses may end up under water, along with large parts of New York and Florida. People will have no choice but to emigrate.”

She is especially concerned about the impacts of warming on the hydrologic cycle, as shown in records for the PETM, and how they may affect agriculture: “Many zones will dry up meaning they will no longer be suitable for major food crops.”

“Global warming is here,” affirms Thomas. “I live in Connecticut, and at this time of year, early January, it usually snows and we have freezing every night, but that is not happening this year, it has hardly snowed at all. There was a day not long ago when it was actually warmer in Alaska than in Texas, and that is absolutely abnormal, but it is exactly what our models are predicting from the effects of global warming in the Arctic, that you get these huge swings between hot and cold.”

“The reality,” she concludes, “is that the anti-climate change measures taken to date are wholly insufficient. I don’t want to sound alarmist, but I do think we are in serious trouble, not for the planet – which will go on without us – but for ourselves. And we are going to have to change things very, very rapidly.”

Laureate bio notes

James Zachos (California, United States, 1959) holds BS degrees in Geology and Economics from the State University of New York (1981), an MS in Geology from the University of South Carolina (1983) and a PhD in Geological Oceanography from the University of Rhode Island (1988). He worked on the Ocean Drilling Program (ODP) Leg 120 as an organic geochemist before moving to the University of Michigan, where he conducted research until 1992. That same year he joined the faculty at the University of California, Santa Cruz (UCSC), participating simultaneously in ODP legs 198 (as a sedimentologist) and 208 (as chief scientist). At UCSC he has served as a professor and chair of the Department of Earth & Planetary Sciences and is currently a Distinguished Professor and holder of the Ida Benson Lynn Chair of Ocean Health. Zachos is the author of more than 170 publications that have more than 47,000 citations on Google Scholar with an h-index of 95.

Ellen Thomas (Hengelo, Netherlands, 1950) holds a BSc in Earth Sciences from the University of Utrecht (Netherlands), where she went on to earn an MSc followed by a PhD in 1979. Except for brief stays in the UK and Italy, since 1979 she has spent her academic life in the United States, holding simultaneous appointments from the 1990s onwards at two universities in New Haven (Connecticut): Yale, where she was a senior research scientist in the Department of Earth and Planetary Sciences from 2005 to 2021; and Wesleyan, where she is currently Harold T. Stearns Professor of Integrated Sciences, Emerita. The impact of her research is evidenced by 34 grants, almost 180 papers published in international journals – with collectively over 28,400 citations on Google Scholar and an h-index of 72 – ­and upwards of 440 conference presentations. She has been editor-in-chief of Paleoceanography and Paleoclimatology and editor of Geology, among other editorial positions, and has acted as an advisor to institutions like the U.S. National Science Foundation, the Ocean Drilling Program and the Royal Society of New Zealand.

Climate Change committee and evaluation support panel

The committee in this category was chaired by Bjorn Stevens, Director of the Max Planck Institute for Meteorology (Hamburg, Germany), with Carlos Duarte, holder of the Tarek Ahmed Juffali Research Chair in Red Sea Ecology at King Abdullah University of Science and Technology (Thuwal, Saudi Arabia), acting as secretary. Remaining members were Sandrine Bony, Director of Research in the Laboratoire de Météorologie Dynamique (LMD) at Sorbonne University (Paris, France); Miquel Canals, Director of the Department of Earth and Ocean Dynamics at the University of Barcelona (Spain); José Manuel Gutiérrez, Research Professor and Director of the Institute of Physics of Cantabria (IFCA) and Coordinating Lead Author of the Atlas chapter in the IPCC’s Sixth Assessment Report; Martin Heimann, Director Emeritus in the Department of Biogeochemical Systems at the Max Planck Institute for Biogeochemistry (Jena, Germany); Edward Rubin, Alumni Chair Professor of Environmental Engineering and Science Emeritus at Carnegie Mellon University (Pittsburgh, United States); Paul Wassmann, Professor Emeritus in the Department of Arctic and Marine Biology at UiT The Arctic University of Norway; and Julie Winkler, Professor of Geography in the Department of Geography, Environment and Spatial Sciences of Michigan State University (United States).

The evaluation support panel of the Spanish National Research Council (CSIC) was coordinated by Blas Valero Garcés, Deputy Coordinator of the Global Life Area and research professor at the Pyrenean Institute of Ecology (IPE, CSIC), and formed by Santiago Beguería Portugués, tenured scientist at the Aula Dei Experimental School (EEAD); Francisca Martínez Ruiz, research scientist at the Andalusian Earth Sciences Institute (IACT, CSIC-UGR); and Ángel Ruíz Mantecón, Deputy Coordinator of the Global Life Area and research professor at the Mountain Stockbreeding Institute (IGM, CSIC-UNLE).

About the BBVA Foundation Frontiers of Knowledge Awards

The BBVA Foundation centers its activity on the promotion of world-class scientific research and cultural creation, and the recognition of talent.

The BBVA Foundation Frontiers of Knowledge Awards, funded with 400,000 euros in each of their eight categories, recognize and reward contributions of singular impact in science, technology, the humanities and music, privileging those that significantly enlarge the stock of knowledge in a discipline, open up new fields, or build bridges between disciplinary areas. The goal of the awards, established in 2008, is to celebrate and promote the value of knowledge as a public good without frontiers, the best instrument at our command to take on the great global challenges of our time and expand the worldviews of individuals for the benefit of all humanity. Their eight categories address the knowledge map of the 21st century, from basic knowledge to fields devoted to understanding and interrelating the natural environment by way of closely connected domains such as biology and medicine or economics, information technologies, social sciences and the humanities, and the universal art of music.

The BBVA Foundation has been aided in the evaluation of nominees for the Frontiers Award in Climate Change by the Spanish National Research Council (CSIC), the country’s premier public research organization. CSIC appoints evaluation support panels made up of leading experts in the corresponding knowledge area, who are charged with undertaking an initial assessment of the candidates proposed by numerous institutions across the world, and drawing up a reasoned shortlist for the consideration of the award committees. CSIC is also responsible for designating each committee’s chair and participates in the selection of its members, thus helping to ensure objectivity in the recognition of innovation and scientific excellence.