The calculation of a reasonable global average temperature structure is the first step in the simulation of the dynamics and observed characteristics of Venus's upper mesosphere and thermosphere. Various physical processes that influence Venus temperatures are examined using updated inputs and new parameterizations. We demonstrate, using a one-dimensional NLTE radiative transfer code, that temperatures observed during the Pioneer Venus mission can be reproduced using an O-CO2 collisional excitation rate coefficient of 4¿10-13 cm3 s-1 for strong 15-&mgr;m cooling balanced by 9.5%-efficient EUV heating. Cooling by eddy mixing is at best a minor contribution to the total cooling required. Exospheric temperatures are calculated to vary by 60 K or less over a solar cycle, in rough agreement with observations. We conclude that CO2 cooling effectively buffers against such solar perturbations, due to its nonlinear temperature dependence.