Environmental Engineering Reference
In-Depth Information
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 pat-
terns 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
t
(
x
).
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 forc-
ing is significant, not only because aerosols and greenhouse gases can have
different spatial footprints, but also because the effects of aerosols them-
selves 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 equilib-
rium 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 warm-
ing 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
