Environmental conditions of the Southern Ocean around Antarctica have varied little for >5 million years but are now changing. Here we investigated how warming affects competition for space. Little considered in the polar regions, this is a critical component of biodiversityresponse. Change in competition in response to environment forcing might be detectable earlier than individual species presence/absence or performance measures (e.g. growth).Examination of fauna on artificial substrata in Antarctica’s shallows at ambient or warmed temperature found that, mid-century predicted 1°C warming (throughout the year or justsummer-only), increased the probability of individuals encountering spatial competition, as well as density and complexity of such interactions. 2°C, late century predicted warming, increased variance in the probability and density of competition, but overall, competition did not significantly differ from ambient (control) levels. In summary only 1°C warming increased probability, density and complexity of spatial competition, which seems to be summer-only driven.