The COVID-19 pandemic forced researchers to move academic conferences to a virtual format; but also brought attention back to the carbon footprint of their physical format. In general, while conferences can follow different formats with a different carbon footprint, the related factors of influence remain unclear, hence hindering informed decisions on how to organize and attend them.This work provides a preliminary study of the carbon footprint of academic conferences and the trade-offs between alternative conference types. First, we conducted a systematic literature review (SLR) to identify factors that contribute to the carbon footprint of on-site, virtual, and hybrid conferences. Second, we conducted an interview survey among steering committee members of a pilot of prominent international conferences to complement the SLR.There is agreement in the literature and the research community that on-site conferences suffer from travel-related emissions among many other factors. While the on-site type benefits from strong networking possibilities, the virtual and hybrid types can reduce carbon emissions significantly. Notwithstanding, we miss a generic framework that accounts for all revealed carbon footprint factors in each conference type. Also, compared to carbon offsetting, carbon handprinting as a footprint reduction option is considered in neither the literature nor the research community. Among the results, we provide a first sustainability model to compare current and future conference types according to their sustainability trade-offs. The model can be used as a decision-making tool by, e.g., conference organizers.