From there Andy moved to the United States and the University of Michigan, researching the evolution of the atmospheres of Earth and Venus as part of NASA’s Pioneer Venus mission. Returning to the UK and turning to marine science, he developed tracers that could be detected in the ocean at minute concentrations. He used these to solve diverse problems in oceanography, for example making the first direct measurements of vertical mixing rates over ocean-basin scales, directly measuring air-sea gas exchange, and enabling the iron release experiments that showed unequivocally that iron is an important limiting nutrient for marine life. He also developed methods automated instruments for measuring carbon dioxide in the surface ocean, which are now used worldwide on commercial ships. He retains a strong interest in Earth and other planetary histories, and the processes setting the concentrations of carbon dioxide and oxygen in the atmosphere, co-authoring the book “Revolutions that made the Earth” (OUP, 2011) with colleague (and former student) Tim Lenton.