This study contributes to the debate on the environmental impacts of academic conferences by comparing the life cycle impacts of a sample of real-world in-person, virtual, and hybrid conferences with different features and organizers. Results show that virtual formats reduce impacts by two to three orders of magnitude across all impact categories (for global warming, averagely from 941.9 to 1.0 kg CO2eq per person) The hybrid case study, with a share of 69% virtual attendees, displays an average 60% reduction in indicator results, less than ideal cases where the farthest attendees join online. The cross-conference comparison allowed identifying several drivers of impact variation. For in-person conferences, some never addressed drivers were uncovered, including the energy sources and systems used to supply the venue or the number of non-local staff members and exhibitors. For virtual conferences, the main impact driver is the average time spent online by delegates, surprisingly more related to virtual experience design (e.g., synchronous vs. asynchronous presentations) than conference duration. The study further summarizes mitigation options from the literature and proposes new ones, such as selecting a venue supplied by a biomass-fueled district heating system or with a green electricity contract (around −41 and −1.9 kg CO2eq per person, respectively). Lastly, the authors' work highlights some inconsistencies that affect current conference assessments and proposes new research avenues, advocating the need to shift the focus from optimizing single conferences to considering the optimal portfolio of conferences and other activities for academic societies to meet their members’ needs while minimizing environmental impacts.