if some measure of uncertainty is sought) on the basis of available coupled model simulations, then adjusting the profile of transient warming by using a fast and simple energy balance model to predict the global mean surface temperature evolution. There are significant differences between the patterns generated by different models—for example, in the amount of polar amplification in the Arctic per degree of global warming. These differences are averaged over in the ensemble-based estimates of mean patterns, but uncertainty can be characterized by the inter-model spread in the pattern τ(ξ).

The choice of the pattern in the studies available in the literature are often as simple as the ensemble average (across models and/or across scenarios, for the coupled experiments available) of the spatial change in temperature, normalized by the corresponding change in global average temperature, choosing the end of the simulations (usually last two decades of the 21st century) and a baseline of reference (pre-industrial or current climate). Similar properties and results have been obtained using more sophisticated multivariate procedures that optimize the variance explained by the pattern.

There are limitations to this approach. It can break down if aerosol forcing is significant, not only because aerosols and greenhouse gases can have different spatial footprints, but also because the effects of aerosols themselves are more difficult to characterize in this simple way. For example, Asian and North American aerosol production are likely to have different time histories in the future. Our focus in this report is on the greenhouse gas component of climate change, making the pattern scaling assumption more justifiable.

Simple pattern scaling is regarded as especially useful for summarizing model projections of transient climate change due to well-mixed greenhouse gas increases on a time scale of a few centuries. But it is less accurate for stabilization scenarios, as the temperature changes approach an equilibrium response. From the early work of Manabe and Wetherald (1980) and Mitchell et al., (1999) it has been clear that the pattern of temperature response evolves as the slow component of the warming, associated with equilibration of the deep oceans on multi-century time scales, equilibrates. In particular, on these long time scales the warming of high latitudes in the Southern Hemisphere is much larger relative to the global mean warming than in the earlier periods. Held et al. (2010) emphasize that this slow warming pattern is present, but of small amplitude, during the initial transient adjustment phases of the response as well.

There are some regions of sharp temperature gradients, near the ice edge



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