We calculate by Monte Carlo simulation on the lattice the energy density [epsilon] of an SU(2) Yang-Mills system at finite physical temperature. First, we study the high temperature form of [epsilon], showing that the conventional euclidean lattice formulation converges to the parameter-free Stefan-Boltzmann limit of a free gluon gas in the continuum. Secondly, we show that the specific heat of gluon matter exhibits a sharp peak at the transition point from the confined phase to the color-screened gluon gas. The resulting transition temperature is found to be 210 ± 10 MeV.