Paul J. Crutzen (1933-2021).

Paul J. Crutzen, renowned atmospheric chemist and meteorologist, died on 28 January after a long illness. He was 87. During his inspired scientific career, he made breakthroughs that shed light on the ozone layer, air pollution, greenhouse gases, nuclear winter, and the effect of human activities on climate. Crutzen's towering academic achievements influenced environmental policies worldwide. Through it all, he maintained an open, joyful, and graceful disposition that endeared him to colleagues and students alike. Born on 3 December 1933 to a working-class family in Amsterdam, Crutzen survived the Dutch famine of 1944–1945 (dubbed the “hunger winter”). In 1954, he earned a technical degree in civil engineering. Before leaving the Netherlands, he worked briefly as a bridge construction engineer and married the love of his life, Terttu Soininen. In 1959, an opportunity arose to move to Stockholm University and work as a computer programmer, giving Crutzen a chance to realize his long-held dream of an academic life. It did not take long for his talents to shine through, and he transitioned to the Department of Meteorology, where he earned his M.Sc. (filosofie kandidat degree) in 1963, his Ph.D. (filosofie licentiat degree) in meteorology in 1968, and his D.Sc. (filosofie doctor degree) in 1973, after completing a thesis on aircraft pollution in the stratosphere and troposphere. With his background in meteorology, Crutzen adroitly spanned atmospheric chemistry and dynamics. His Ph.D. work at Stockholm University laid the basis for his blockbuster 1970 paper showing that catalytic chemistry involving nitric oxide (NO) is the dominant control on the distribution of stratospheric ozone. He and atmospheric chemist Harold Johnston independently pointed out that NO in the exhaust of supersonic aircraft flying at stratospheric altitudes might therefore threaten the ozone layer. Crutzen also drew attention to the potential role of NO from nitrous oxide generated by fertilizers as a mechanism for controlling stratospheric ozone. In the 1970s, he led the field in numerical modeling of processes influencing stratospheric ozone. His pioneering contributions to stratospheric chemistry ultimately resulted in his sharing the 1995 Nobel Prize in Chemistry with F. Sherwood Rowland and Mario Molina. In the mid-1970s, Crutzen and his family moved to Boulder, Colorado, where he worked at the National Oceanic and Atmospheric Administration and, before long, became the director of the Atmospheric Chemistry Division at the National Center for Atmospheric Research. Crutzen's research extended from the stratosphere down into tropospheric ozone and pollution. He first identified the impact of biomass burning on pollutants, including carbon monoxide. His interests also extended up into the mesosphere and lower thermosphere and into the world of climate. He is credited with suggesting that carbonyl sulfide makes major contributions to the amount of sulfuric acid aerosols in the stratosphere, which is important not only for stratospheric chemistry but also for climate. He showed that most tropospheric ozone is made in the lower atmosphere, not transported from the stratosphere. He also conducted leading work on the sources of methane, the second most important anthropogenic greenhouse gas. I met Crutzen in 1977, when he took me under his wing as a graduate student to study the impact of solar proton events on atmospheric chemistry. His knowledge of every aspect of the chemistry that controls atmospheric ozone was breathtaking. He spent innumerable hours patiently teaching me how to analyze models and identify which chemical reaction produced which observable change in atmospheric gases. He was a skillful, supportive, and kind lifelong mentor to me and dozens of others. When one colleague was having trouble getting his (later very influential) paper on the lifetime of atmospheric methane through review, Crutzen swooped in to convince the editor to publish it by agreeing to write a commentary himself. Another colleague was named to lead a chapter in an international scientific assessment for the Montreal Protocol and needlessly worried that Nobel laureate Crutzen—one of the chapter's authors—might have a hard time working as a team member rather than the leader. He quickly learned that Crutzen was always a constructive and thoughtful team player in any group effort. A remarkable linguist, Crutzen could converse easily in multiple languages, including German, Swedish, and English. In 1980, Crutzen took a position in Mainz, Germany, as a director at the Max Planck Society. There, he and his colleague John Birks identified the problem of nuclear winter, a topic pioneered at about the same time by others, including Carl Sagan. Having lived through the devastation of World War II, Crutzen felt a deep connection to that work. He believed in the importance of scientific research to identify the risks of human conflict in the nuclear age. A few years later, when the ozone hole was discovered, Crutzen showed that nitric acid clouds were vital to its chemistry (a discovery simultaneously and independently made by atmospheric scientist Brian Toon). Crutzen's incessant curiosity led him to contribute, together with climate physicist Veerabhadran Ramanathan and others, to our understanding of the role of black carbon as an important driver of climate change. That work also helped illuminate the health risks of black carbon, especially in the developing world. Crutzen continuously churned out new and influential ideas. At a conference in 2000, he became annoyed by talk of the Holocene era as the current geological epoch, and he rose on the spot to declare that humanity had already had such a vast impact on the planet that we ought to be talking about the “Anthropocene” instead. He spent the last 20 years of his life expanding on and deepening the Anthropocene concept, which he entered into the common lexicon. Crutzen was scientifically prolific, creative, and profoundly influential in the fields of atmospheric chemistry and climate science. He was also a person of great warmth, caring, deep humility, and genuine concern for his fellow humans as well as the future of the planet we all share. As we address the urgent challenges of pollution, climate change, and sustainability, the foundation he helped to build will support the many scientists and students who follow in his footsteps.