We examine simulations of today's climate performed with a global atmospheric general circulation model run at three different horizontal grid resolutions (T42, T170, and T239 truncations, corresponding to grid cell sizes of roughly 300 km, 75 km, and 50 km, respectively). The simulations were forced with observed sea-surface temperatures and sea-ice concentrations. On the scales of a T42 grid cell and larger, most quantities we examined in all the T170 and T239 simulations agree better with observations, at least in terms of spatial patterns, than in the T42 simulation. We discuss aspects of simulated regional climates, and their dependence on model resolution. We also report results from by far the highest-resolution simulations of global warming yet performed. We find that the global climate sensitivity and large-scale patterns of climate change are similar at T42 and T170. However there are important regional scale differences that arise due to better representation of topography and other factors at high resolutions.