Jellyfish blooms are conspicuous demographic events with significant ecological and socio-economic impact, as they alter aquatic food webs. Despite worldwide concern about an increased frequency and intensity of jellyfish outbreaks, we are challenged to predict their booms and busts. To overcome this issue, we need to identify the ecological drivers of jellyfish blooms by taking into account the complex life cycle of scyphozoans (Cnidaria). Here we present demographic rates of all life stages of the cosmopolitan jellyfish Aurelia aurita s. l. within a stage-structured matrix model to investigate the life stage-dynamics of such complex populations under different environments. We illustrate how booms and busts of the medusa stage are highly influenced by non-medusa stage dynamics. We further point out increased food availability as an important ecological driver of jellyfish blooms, as it can shift the population structure of A. aurita away from the benthic polyp stage towards more medusae. Comparatively, our projected climate change scenario caused low fluctuations in population density. Overall, our study reveals ecological and demographic key variables that regulate the intensity and frequency of jellyfish blooms, and thereby contributes to a better understanding of anthropogenic drivers of jellyfish mass occurrence, including habitat eutrophication and climate change.