Environmental Engineering Reference
In-Depth Information
a reduction in Northern Hemisphere snow cover of 15% by the end of the
21st century under the B2 scenario. Individual, projected reductions range
from 9-17%, and are greatest in spring and late summer/early winter, result-
ing in a shorter snow cover season. The timing of snow accumulation and
snow melt is also projected to change. Accumulation is projected to begin
later and melt to begin earlier. Fractional snow coverage is also predicted
to decrease (Hosaka et al., 2005).
Regionally, the changes are a response to both increased temperature
and increased precipitation and are complicated by the competing effects of
warming and increased snowfall in those regions that remain below freez-
ing. Taken across models, snow amount and snow coverage decrease in the
Northern Hemisphere. Although a number of assessments have been per-
formed of the implications of changes in snow cover on run-off, especially in
the western United States (e.g., Hamlet and Lettenmaier, 1999; Christensen
et al., 2004; Payne et al., 2004; Christensen and Lettenmaier, 2007), they
have mostly been based on off-line simulations wherein hydrology models
have been forced with downscaled output from GCMs, rather than as sen-
sitivity analyses, for instance, per degree of global warming. One study that
has adopted a sensitivity analysis approach is Casola et al. (2009), which
estimated that for each increase in (local, not global) temperature of 1°C the
snowpack should decrease by about 20%. Precipitation changes (precipita-
tion is expected to increase slightly as the climate warms) complicate the
snow cover response, which varies with latitude and elevation. When the
expected global rate of increase in precipitation was considered, Casola et
al. (2009) estimated that the sensitivity of Cascades snowpack would be re-
duced to 16% per degree C of local warming. Snow amount is projected to
increase in other regions, for example, in Siberia. Such increases are thought
to be due to increases in precipitation (snowfall) during autumn and winter
(Hosaka et al., 2005; Meleshko et al., 2005).
The snow cover variables that are most sensitive to a warming climate
are snow cover duration (SCD) and snow water equivalent (SWE). This sen-
sitivity varies with climate regime and elevation, where the largest changes
are projected to occur over lower elevations of regions with maritime winter
climate, that is, moist climates with snow season temperatures in the range
of -5°C to 5°C, and the lowest sensitivities are in mid-winter in cold conti-
nental climates. In general the models predict an enhanced early response
of snow cover to warming over the western cordillera of North America
and maritime regions. Over continental interiors, snow cover changes are
slower. However, during the 21st century, annual mean SCD is expected
to decrease. Although few models show any significant decrease in SCD
