Environmental Engineering Reference
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Maps similar to those in Figure 3 of BN09 will result from each shift
in mean temperature, except they will correspond to different atmospheric
concentrations of CO 2 (and their implied expected global average warming)
rather than to different future periods. The main difference in our approach
compared to the analysis in BN09 is the choice of shifting the distribution
uniformly to the right rather than trying to build a new distribution of aver-
age temperature anomalies on the basis of model output. This is a choice
dictated by the use of pattern scaling, which lacks the availability of an
ensemble of fully coupled model runs for each CO 2 target.
It could be argued that our analysis represents a conservative estimate of
the expected changes in extreme seasonal temperatures, because we are as-
suming no change in the climatological distributions of temperatures besides
a shift of their central locations. In particular, no change in the variability of
seasonal average temperature is taken into consideration. Some studies (see
for example Scherrer et al., 2005; Fischer and Schar, 2009 for the European
region; Giorgi and Bi, 2005; Kitoh and Mukano, 2009 for global patterns)
have shown that future variability of summer temperature is projected to
increase, in association with drying.
In Figures 4.8 and 4.9 we show the resulting likelihoods of exceeding
the 95th percentile, or the warmest anomaly of current average JJA and
DJF temperatures (1971-2000 of 20C3M simulations) for the three levels of
global warming. The patterns become redder (higher likelihood) as we look
down each column (larger global average warming implies greater chances
of exceeding the thresholds) and bluer (smaller likelihood) as we look across
rows (higher thresholds make exceedances rarer).
4.6 PRECIPITATION EXTREMES
A general increase in atmospheric water vapor is predicted by essentially
all climate models as temperatures increase, and an upward trend in column
integrated water vapor has been observed in many regions (Trenberth et al.,
2005). The consequences of this increase for the distribution of mean pre-
cipitation has been discussed in Section 4.2. As outlined below, models and
simple theories also suggest that this increase in water vapor will increase
the intensity of heavy rainfall events. Increasing trends in extreme precipi-
tation have been documented in many regions, including much of North
America (USCCSP, 2008c), but evidence for associated increases in floods
is not compelling to date (Lins and Slack, 1999, 2005; WWAP, 2008).
As articulated by Trenberth (1999) and many others, precipitation in
storms is related mostly to atmospheric moisture convergence rather than
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