Multiphase fluid mixtures present not only fascinating examples of nonequilibrium pattern formation, but also illustrate rather general questions concerning new emergent levels of organization. To illustrate these issues, we use lattice-gas cellular automata to study a buoyant mixture of hot and cold bubbles. We find that, depending on the volume fraction of bubbles, the model can exhibit either a coarsening instability familiar from studies of sedimentation, or a convective instability similar to the large-scale flows of Rayleigh–Bénard convection.