Weather and Climate Extremes in a Changing Climate. Regions of ...

The U.S. Climate Change Science Program Chapter 3
The retreat of sea
ice together with
changing sea levels
will likely increase
the exposure of
arctic coastlines to
damaging waves and
extreme erosion
produced by strong
storms, continuing
an observed trend
of increasing coastal
erosion in arctic
Alaska.
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extratropical storms by increasing the stability
of the atmosphere (Lambert and Fyfe 2006).
Analyses of strong winds (Fischer-Bruns et
al., 2005, Pinto et al., 2007), lower atmosphere
vorticity (Bengtsson et al., 2006), and significant
wave heights (Wang et al., 2004; Caires et
al., 2006) from single models suggest increased
storm strength in the northeast Atlantic, but this
increase is not apparent in an analysis using
output from multiple GCMs (Lambert and Fyfe,
2006). Differences may be due to the focus on
cold season behavior in the wind and wave
analyses, whereas Lambert and Fyfe’s (2006)
analysis includes the entire year.
The warming projected for the 21st century is
largest in the high latitudes due to a poleward
retreat of snow and ice resulting in enhanced
warming (Manabe and Stouffer 1980; Meehl
et al., 2007a). Projected seasonal changes
in sea ice extent show summertime ice area
declining much more rapidly than wintertime
ice area, and that sea ice thins largest where
it is initially the thickest, which is consistent
with observed sea ice thinning in the late 20th
century (Meehl et al., 2007a). Increased storm
strength in the northeast Atlantic found by
some may be linked to the poleward retreat
of arctic ice (Fischer-Bruns et al., 2005) and
a tendency toward less frequent blocking and
more frequent positive phase of the NAM
(Pinto et al., 2007), though further analysis is
needed to diagnose physical associations with
ice line, atmospheric temperature, and pressure
structures and storm behavior. Whether or not
storm strength increases, the retreat of sea ice
together with changing sea levels will likely
increase the exposure of arctic coastlines to
damaging waves and extreme erosion produced
by strong storms (Lynch et al., 2004; Brunner et
al., 2004; Cassano et al., 2006), continuing an
observed trend of increasing coastal erosion in
arctic Alaska (Mars and Houseknecht, 2007).
Rising sea levels, of course, may expose all
coastlines to more extreme wave heights (e.g.,
Cayan et al., 2008).
3.3.11 Convective Storms
Conclusions about possible changes in convective
precipitating storms and associated severeweather
hazards under elevated greenhouse
gas concentrations have remained elusive.
Perhaps the most important reason for this
is the mesoscale (10s of km) and smaller dynamics
that control behavior of these storms,
particularly the initiation of storms. Del Genio
et al. (2007), Marsh et al. (2007) and Trapp
et al. (2007) have evaluated changes in the
frequency of environments that are favorable
for severe thunderstorms in GCM simulations
of greenhouse-enhanced climates. In all three
cases, increases in the frequency of environments
favorable to severe thunderstorms are
seen, but the absence of the mesoscale details
in the models means that the results are preliminary.
Nevertheless, the approach and the use of
nested models within the GCMs show promise
for yielding estimates of changes in extreme
convective storms.