Stephen Pacala transferred to emeritus status on September 1, 2023, after thirty-one years of service to Princeton University in the Department of Ecology and Evolutionary Biology. As the Frederick D. Petrie Professor of Ecology and Evolutionary Biology, Steve taught dynamic and foundational courses on ecological principles at the intersection of society’s most pressing environmental challenges. Over his time at Princeton, he became among the most influential ecologists exploring how society can address climate change, with multiple discoveries that directly informed policy. 

Steve was born in Greencastle, Indiana, in 1957, and he knew from a young age that he wanted to be a scientist. He attended Dartmouth College for his bachelor of arts degree in biology, which he earned in 1978. He then went on to Stanford University, where he studied the interactions among lizard populations on Caribbean islands and earned a Ph.D. in 1982. Thereafter, Steve was appointed assistant and then associate professor at the University of Connecticut, where he served for ten years, before moving to Princeton in 1992 as a full professor. At Princeton, Steve became an intellectual force in the growing Department of Ecology and Evolutionary Biology, and he directed the Princeton Environmental Institute from 2006 to 2014. 

An obvious question is how an ecologist who conducted Ph.D. work on lizard communities evolved over the course of his career to become a leading thinker about how society can effectively address the challenges of climate change. The answer lies in Steve’s remarkable ability to reframe topic after topic in terrestrial ecology using novel mathematical theory grounded in insightful observations of nature. 

Trained as a community ecologist, Steve transitioned early in his faculty career from studying animal systems to plant communities. Beginning in the late 1980s, he published highly influential studies examining how the fundamental observation that plants are stuck in one place and therefore only interact locally changes the rules for how species compete and coexist. A central contribution was a series of quantitative tools for predicting the dynamics of plant populations and communities when plant individuals only interact with neighbors, rather than interact globally as classically assumed. This work led to the groundbreaking studies in the 1990s and 2000s for which Steve is probably best known among ecologists: models of competition between forest trees. 

The forest dynamics literature before Steve’s contributions was dominated by heavily parameterized simulations that could only predict dynamics one generation ahead of the currently observed forest. Steve ingeniously realized that the core dynamics of the forest could be written in a simpler set of equations that not only allowed long-term trajectories to be simulated but also lent themselves to empirical parameterization. In fact, Steve field parameterized his model in eastern U.S. forests, with the resulting simulator allowing ecologists to properly understand how light competition, disturbance, and local interactions determine forest dynamics. Never comfortable modeling a system he did not fully understand, Steve also became a leading modeler of tree ecophysiological processes. Beginning in the 2010s, he contributed several key papers showing how plant hydraulic constraints influence forest mortality in response to global climate change.

These more fundamental contributions enabled Steve to meaningfully engage in the interaction between terrestrial ecology and climate change. Over the last several decades, he played central roles in efforts to determine the carbon stored in the world’s forests and forecast the fate of this carbon under climate change. To this end, he initiated the development of the land model within the NOAA Geophysical Fluid Dynamics Laboratory’s Earth System Model. Due to the contributions of Steve and his collaborators, the model is one of the most trusted means of assessing how changes to the land surface influence future climate. 

Building on his deep understanding of the earth’s carbon balance, Steve conducted impactful research analyzing how to mitigate carbon emissions. He and Princeton colleague Rob Socolow coauthored a highly influential 2004 paper identifying how existing technologies could be deployed to stop the rise of carbon emissions. He cleverly showed how changing the trajectory of carbon emissions could be visualized as a series of “stabilization wedges,” each of which corresponded to the angle of the emission trajectory reduction possible with the implementation of a given transformation of the energy system. As a master woodworker, Steve even built a physical stabilization wedges game in his shop that could be used to illustrate the messages of his paper. 

Steve has been exceptional in his ability to directly parlay his research findings and position as a preeminent scientist toward the betterment of society. At the root of his contributions is a deeply held belief in the social contract between scientists and society: public resources enable the privilege of conducting science, and thus scientists have a moral obligation to give back to the world. For Steve, this means helping society address climate change. 

Since 2005, Steve has cochaired the Science Advisory Board for the Environmental Defense Fund, working with the organization to curb methane emissions from fossil fuel operations. Since 2008, he has chaired the board of directors for Climate Central, a group devoted to publicizing the impacts of climate change and the solutions to counter those impacts. Steve initiated the massive “Net-Zero America Project” at Princeton, where a collaborative team modeled the energy system to evaluate the infrastructure required for achieving net-zero emissions in the United States by 2050. This report and its subsequent analytical tools were influential in shaping the Inflation Reduction Act, the signature climate policy of President Biden’s administration. On many occasions, the National Academy of Sciences has enlisted Steve to chair committees producing prominent reports on greenhouse gas emissions, negative emission technologies, and policy tools for accelerating the energy transition. Since 2021, Steve has served on President Biden’s Council of Advisors on Science and Technology.

The breadth and originality of the contributions highlighted here have earned Steve numerous awards, including membership in the National Academy of Sciences, fellow of the American Association for the Advancement of Science, the Presidential Award from the American Society of Naturalists, the Robert MacArthur Award and George Mercer Award from the Ecological Society of America, and the David Starr Jordan Prize. Steve’s contributions have helped train many graduate students and postdocs who are now impactful professors and researchers at leading institutions. 

In weighing the totality of Steve’s contributions, probably no other ecologist has had such massive dual impacts on how we understand the natural world and how humanity can stave off some of the worst effects of climate change. 

Written by members of the Department of Ecology and Evolutionary Biology faculty.