O'Neill obtained an M.S. in Applied Science and a Ph.D. in Earth Systems Science from New York University. After working on the science staff of the Environmental Defense Fund in New York, he was an Assistant and Associate Professor at Brown University's Watson Institute for International Studies. From 2005 to 2009 O'Neill was at the International Institute for Applied Systems Analysis in Austria, where he founded and led the Population and Climate Change Program. Afterwards, he returned to the United States to work at the National Center for Atmospheric Research. At NCAR, he was a Senior Scientist and led the Integrated Assessment Modeling group. In September 2018, he started working as a professor for the Josef Korbel School of International Studies at the University of Denver. O'Neill has researched the interplay of future societal development, emissions, and climate change impacts. He has written several publications on the relationship between demography and climate change, including how demographic factors like aging or urbanization affect emission trajectories and how climate change impacts are dependent on demographic composition, which affects the ability to adapt. O'Neill uses long-term scenarios in his analysis and contributes to their development. Most notable, he was involved in the development of the Shared Socioeconomic Pathways to be used in scenario analysis. These SSPs replace the Special Report on Emissions Scenarios projections and will be used to systematically assess climate change under different societal futures.O'Neill was also the driving force behind the first Scenarios Forum in 2019, which he organized at the University of Denver. O'Neill has performed research on impacts from climate change by using integrated assessment modelling impacts, as well as advancing concepts on how to best systematically assess impacts. He co-led the "Benefits of Reduced Anthropogenic Climate changE" project to study climate change impacts and co-edited a special issue in Climatic Change. O'Neill has also served as a lead author for the Intergovernmental Panel on Climate Change's Assessment Reports. For the volumes on impacts, adaptation and vulnerability, he was lead author for the chapter on "New Assessment Methods and the Characterisation of Future Conditions" and "Emergent Risks and Key Vulnerabilities", respectively. O'Neill is currently coordinating lead author of the Sixth Assessment Report for the chapter on "Key risks across sectors and regions".
