Environmental Engineering Reference
In-Depth Information
during the 20th century, by 2020 the majority of models show significant
decrease in SCD over eastern and western North America, Scandinavia,
and Kazakhstan. By 2080 the majority of models show significant decreases
everywhere. The regions that exhibit the least consensus tend to be conti-
nental and that might be due to a slower snow cover change signal. Like
SCD, SWE decreases into the 21st century; however, model consensus on
the statistical significance of this decrease does not become clear until 2050
when significant decreases occurred over the mid-latitude coastal regions of
western Europe and North America (Brown and Mote, 2009).
Permafrost
Permafrost is defined as soil that remains at or below freezing tem-
peratures for two or more years successively. Classified as continuous,
discontinuous, and sporadic, permafrost zones occupy about 24% of the
Northern Hemisphere exposed land or about 26 million km
2
but permafrost
underlies only 13-18% of the exposed land (Nelson et al., 1997; Zhang
et al., 1999). The active layer of permafrost is the upper layer of soil that
thaws in summer and refreezes in winter (Sazonava et al., 2004). In this
layer almost all of the below-ground biological processes take place. When
it is deep, thawing increases soil moisture storage. Permafrost degradation
occurs when temperatures increase in the active layer, and as a result the
depth of thaw increases in successive summers and becomes greater than
the depth of refreezing.
Although there have been exceptions to the trend (e.g., Brown et al.,
2000), in general, observations indicate that the temperature of the per-
mafrost has increased over the 20th century. For example in northern
Alaska permafrost temperatures increased by 2 to 7°C over the 20th century
(Lachenbruch and Marshall, 1986; Nelson, 2003). Although the size var-
ies, this warming trend is repeated in northwestern Canada (e.g., Smith et
al., 2003), northwestern Siberia (e.g., Pavlov and Moskalenko, 2002), and
Scandinavia (Isaksen et al., 2000) among other regions. Along with the
higher temperatures, observations show that the active layer is deepening
and the permafrost extent is decreasing (Jorgenson et al., 2001; Serreze et
al., 2002; Zhang et al., 2005). Since 1900, the maximum area of seasonally
frozen ground has decreased by about 7%, led by decreases in spring of up
to 15% (Lemke et al., 2007).
