We nominated Ken Buesseler for the AGU Ambassador Award for his innovative contributions to the measurement and impacts of natural and artificial radionuclides in marine environments. Over the past 30 years, he has pioneered measurement and analysis of radionuclides in marine environments, leading more than 25 international research cruises and authoring over 200 peer-reviewed papers, with scientific and societal impacts spanning seas, oceans, and cultures. Ken’s leadership was instrumental after the 1986 Chernobyl and 2011 Fukushima nuclear accidents. He organized rapid response sampling campaigns coupled with innovative tracer techniques to track radioactive fallout across seas and published foundational work with international teams, even under politically tense conditions. Following Fukushima, he led efforts to monitor the dispersal of marine radioactivity across the Pacific, guided public communication through media and educational resources in English and Japanese, and persistently advocated for government transparency and international access to contaminated areas, despite personal risk. To build expertise worldwide, Ken founded the Woods Hole Oceanographic Institution’s (WHOI) Center for Marine Radioactivity and launched training for more than 160 scientists worldwide, making workshops and resources freely accessible, especially for developing nations. His mentorship extends both formally and informally, empowering careers across the field and fostering collaboration among diverse groups. Ken’s expertise in radionuclide techniques has been critical in the scientific evaluation of ocean iron fertilization and other marine carbon dioxide removal methods. He organized key interdisciplinary cruises and forums, contributed to national strategy documents, and led the Exploring Ocean Iron Solutions group to emphasize responsible research on climate interventions. Beyond research, Ken shaped policy and scientific culture as a National Science Foundation program manager, journal editor, and leader in international research programs. His tenure as department chair at WHOI was defined by growth, harmony, and advocacy for equity. As a science diplomat, Ken brings together scientists of different backgrounds to solve critical, globally relevant problems. His contributions exemplify the spirit of the AGU Ambassador Award in scientific leadership, societal impact, mentoring, and the promotion of talent worldwide. —Willard S. Moore, University of South Carolina, Columbia, South Carolina