Remembering the Extraordinary Scientist Paul Crutzen (1933–2021)

It may seem extraordinary that one person’s life, and, as a consequence, so many other peoples’ lives, can be so radically reshaped by a moment’s irritation. But, with Paul Crutzen, one of the greatest scientists of his time—of all time—who passed away on the 28th of January after long illness, the extraordinary had come to be habitual. Already famous for revealing the likely outcome of a nuclear winter, and a Nobel laureate for his part in deciphering the mechanisms of atmospheric ozone loss, his sudden realization that humanity had very recently stumbled into a new geological epoch of its own making, the Anthropocene, created reverberations that continue to shake not only the world of science but that of all of scholarship, now spilling into political and economic discourse worldwide.

This starburst of a scientific career could not have been predicted. As a child in wartime Holland, Crutzen survived the infamous “Hunger Winter,” in which thousands died, including some of his school friends. After the war, he continued his studies, briefly became a civil engineer, underwent military service, and met and married a Finnish girl, Terttu—a happy choice, for she was to be a mainstay thoughout his life. The chance for the academic career that he had always longed for came via a job as a computer programmer at Stockholm University’s Meteorology Institute, and that led eventually to a Ph.D., for which he chose the then-obscure topic of stratospheric ozone and had to reinvent himself as a chemist.

Ozone was soon to be a hot topic, as threats to the Earth’s protective ozone layer became apparent, Firstly, concerns about the effects of atmospheric nitrogen oxides (NOx) on ozone (which he initially studied as a postdoctoral fellow at the Clarendon Laboratory, Oxford University) through supersonic stratospheric flights crystallized an understanding that anthropogenic activities could seriously effect natural processes, the hallmark of his future career.  

Following a move to Boulder, Colo., to work at the National Oceanic and Atmospheric Administration and National Center for Atmospheric Research (at which he would eventually become a director) his attention turned to the supposedly “‘inert” chlorofluorocarbons (CFCs) used as refrigerants and in insulation. Crutzen, soon to be based mainly at the Max Planck Institute in Mainz, Germany, as director of the Atmospheric Chemistry Department, was in the thick of the work, both in deciphering the chemical processes of ozone destruction, for which he shared a Nobel Prize in 1995 with Mario Molina and F. Sherry Rowland, and in the (largely successful) global efforts via the Montreal Protocol to ban CFC use.

His readiness to face large, difficult issues head-on were also evident when, in the 1980s, with John Birks, he theorized the effects of a nuclear war, suggesting that soot and smoke injected into the stratosphere would result in winterlike conditions, with catastrophic impacts on agriculture and loss of life.

The beginning of his foray into redefining Earth’s geological history is already legendary. At a meeting in 2000 of the Scientific Committee of the International Geosphere-Biosphere Program (IGBP) in Cuernevaca, Mexico, at which one of us (WS) was present, Crutzen was listening, with increasing exasperation, to evidence of how global environmental parameters were dramatically changing in recent decades, in what was repeatedly referred to as the late Holocene Epoch (this being in formal geology the 11,700 years since the end of the last Ice Age). His exasperation spilled over into an interjection that we were no longer in the Holocene but in … (pausing to try to think of the appropriate word) … the Anthropocene.

The on-the-spot improvisation caught the attention of the audience, crystallizing the growing realization that the Earth system had recently begun to change at a much more dramatic rate and scale than through many previous millennia of slowly growing human occupation of the planet. Crutzen, characteristically, developed his idea both energetically and generously.  He discovered that the word “Anthropocene” had been independently invented a few years previously by Eugene Stoermer, a U.S.-based freshwater ecologist, who used it in discussions with his students and colleagues but not in the sense that Crutzen proposed. Nevertheless, Crutzen invited Stoermer to co-publish the term and concept, which they did the same year (although they never met).

From this beginning, the Anthropocene rapidly evolved. The IGBP/Earth system science community quickly adopted it as a central framing concept for much of their work, using the term as a de facto geological epoch succeeding the Holocene, with little understanding of the lengthy and elaborate protocols needed to formally change any part of the geologic time scale. A few years later, geologists (including two of us, JZ and CW), becoming aware of the expanding use of the term, began formally analyzing the term to see whether it really could satisfy all the geological protocols.

The process continues, with the formal outcome uncertain (the geologic time scale is designed to resist change) but it is already clear that Crutzen’s intuition was correct. The Anthropocene is real.

Over the last century, the Earth has acquired a striking and indelible geological record of human-driven perturbation, and been sharply set on a new trajectory, towards a warmer, more biologically impoverished and polluted state—one that will be more difficult for humanity to thrive in. This sobering realization quickly spread from the sciences to the humanities, provoking reimagination of their disciplines to incorporate the Earth no longer as a passive and stable backcloth for the human adventure, but as an active, hyperresponsive and dangerous actor.

Crutzen was indispensable to this scientific revolution (for such it is). The concept of a planet that is human-dominated on a geological scale had intermittently been mooted for decades, even centuries, but never been taken seriously, and certainly not by the geologists. He was, simply, the right person (of immense and deserved authority) making a conceptual leap at the right time (when sufficient evidence had built up) and in the right company (as a central figure in the highly active and international community studying contemporary global change). Moreover, this community had come to consider our planet holistically, as an integrated Earth system. His concerns over the growing realities of global warming led to a controversial foray into theoretical geoengineering, proposing injection of sulfur gases into the atmosphere to reduce insolation.

Crutzen continued to take a close interest in the Anthropocene over the following years, even as his health declined. For his 80th birthday celebratory symposium at the Max Planck Institute in Mainz, it was the Anthropocene he proposed as the theme. When Anthropocene Working Group members came for a meeting to Mainz in 2018, Crutzen, by now faring very poorly and frail, had not been to the office for three weeks—yet he turned up for two long days of detailed evidence and disputation.

He seemed not in the least possessive about his brainchild, but was unfailingly supportive and encouraging about the work being done, even when the procedural nitpicking of geological timescale work went against the grain of his own quickness and clarity. Not all great scientists are likable and good company—Crutzen was. He will be terribly missed as a person and a profoundly important presence in science, even as the revolution that he started continues.

In this tribute, we represent the Anthropocene Working Group, of which Paul Crutzen was also a member.

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