This article argues that Nobel Prize-winning chemist Paul Crutzen (1933 – 2021) spoke in the name of science over several decades as a public intellectual who shaped research fields, environmental policy, and public understanding of the environment. It analyses the atmospheric chemist as a case study to explain the formation and influence of the scientist as a public intellectual, tracing the trajectory of his public career, focusing on his critical contributions to four significant episodes in modern environmental politics: His warnings in the 1970s of damage to the ozone layer, his catalysing impact on the nuclear winter debates of the 1980s, his turn-of-the-century conceptualisation of the Anthropocene, and his late-career advocacy of solar geoengineering. It undertakes a textual analysis of four agenda-setting articles to demonstrate how Crutzen performed the public intellectual functions of testing the assumptions of scientific and policy elites, and framing new ways of understanding environmental problems. It argues that he was a technocratic public intellectual who viewed scientists as guides for society to understand and respond to human-caused environmental threats. As climate change becomes a defining issue of the twenty-second century, Crutzen’s career illuminates the potential and limitations of the technocratic public intellectual to shape global environmental politics.