The increasing frequency of very high temperatures driven by global
warming has motivated growing interest in how the probability
distribution of summertime temperatures will evolve in the future.
Climate models predict increasing temperature variance in global warming
simulations, but given their biased representations of historical
temperature variability, it is important to use simple models to
evaluate and understand these predictions. In this study we show that
the projections of increasing temperature variance are indeed credible
and are driven primarily by the magnitude of local warming. A simple
analytic theory based on the surface energy and water budgets reproduces
the increased midlatitude summertime temperature variance shown by state
of the art climate models using only the local change in summertime mean
temperature and relative humidity. The relative contributions of local
warming and relative humidity changes to the increases in summertime
temperature variance are roughly equal.