Environmental Engineering Reference
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warming; and the strong signal of warming in the semi-arid to arid regions
especially in the Mediterranean basin and nearby portions of the Asian
continent in June-July-August. Areas of disagreement between models and
observations remain, particularly in the form of more homogeneous patterns
of warming at the subcontinental scales in the modeled patterns than in
observations, with one example being the area of cooling in the southeast
of the United States appearing in the HadCRU data and not represented in
the models.
4.2 PRECIPITATION RESPONSE
As described in Section 4.1, it is reasonable to assume that the local
temperature response to an increase in a well-mixed greenhouse gas is
proportional to the global mean temperature response, with a well-defined
spatial and seasonal pattern. There are also good reasons to assume that
the local precipitation response scales with the global mean surface tem-
perature response, although the uncertainties are greater, both with regard
to the spatial and seasonal structure of the pattern and with regard to the
limitations of this pattern scaling assumption.
Using the CMIP3 archive and computing the precipitation response,
measured as a percentage change, divided by the global mean warming
and then averaging over models and scenarios, just as for the temperatures
in Figure 4.1, one obtains the pattern shown in Figure 4.6, both globally for
the annual average and the summer and winter seasons, and focusing on
North America (which is not found in the IPCC AR4 report [IPCC, 2007a]).
The patterns are very similar to those shown for a particular scenario and
time frame in the Summary for Policy Makers of the AR4/WG1 report (IPCC,
2007e).
There is a general increase in precipitation in subpolar and polar lati-
tudes, and a decrease in the subtropics, and an increase once again in many
equatorial regions. The boundary between the subtropical decrease and
subpolar increase cuts through the continental United States, but with the
boundary moving north in the summer and south in the winter. As a result,
this ensemble mean projection is for an increase in precipitation in much of
the continental United States in the winter and a reduction in the summer.
In contrast, Canada is more robustly wetter and Mexico drier, being located
closer to the centers of the subpolar region of increasing precipitation and
the subtropical region of decreasing precipitation, respectively. The Mediter-
ranean/Middle East and southern Australia are other robust regions of drying
in these projections.
