Present climate and biological change - Climate Change: Biological and Human Aspects - page 234

and reflected less, solar energy than land with snow (or sea with ice). Forests too,
in real life (as opposed to a computer model), see areas prone to snow have a lower
albedo (are darker) than snowfields without trees. Conifers have evolved to allow
snow to fall off of them comparatively easily. All well and good, and most of the more
sophisticated models at the beginning of the 21st century accounted for such factors
and included a (coarse) model map of land cover. These models naturally took into
account one of the positive-feedback cycles of warming reducing the area of snow
cover, so reducing the solar energy reflected, so in turn causing warming, which then
further reduces snow cover (see Figure 1.8b). However, only a handful of models in
the first half of the 2000s decade included the effects of a changing regional ecology
and hence a changing and dynamic landscape. For, in addition to the above feedbacks
(instead of having a static biological landscape), there is the biological dynamic. As
the Arctic region warms then so it is open to colonisation from species that could
not have survived the previously cooler climate. Even if the ground is snow-covered,
if it has shrubs and trees that have migrated into the area due to warming, then the
albedo will change. No doubt, as we come to the end of the decade in 2010, such
biological dynamic factors have (at least on a coarse level) been included in models.
But in 2005 it was becoming clear that such factors were important in the field, in real
life (as yet not so much with model makers). At that time a wide range of innovative
Alaska data sets were assembled covering the decades from the 1960s (and in some
cases from the 1930s) to 2004. They included surface-temperature records, satellite-
based estimates of cloud cover, ground-based estimates of ground cover and albedo,
field observations in changes of snow cover and, importantly, changes in vegetation
cover. Alaska was a worthy study area for it has seen a number of thresholds crossed
with regional warming that relate to abrupt physical and ecological change near
the freezing point of water. And the region has warmed. Palaeoclimatic evidence
shows that Arctic Alaska was in 2005 warmer than at any time in at least the past
400 years and so there should be a large impact on water-dependent processes. The
warming, and especially summer warming, cannot be attributed to climatic cycles
such as the North Atlantic Oscillation or the Arctic Oscillation, while cycles such
as the El Ni no mainly affect Alaskan winter temperatures and not summer ones.
Changes in sea ice around Alaska would have an effect in the spring and autumn,
when the ice melts and freezes, and so, whereas this time may have changed by days
or weeks with regional warming, it would not affect matters in the depth of winter or
in the middle of summer. The study team (Chapin et al., 2005) state that the degree
of change in Arctic Alaskan summer warming, above that of the global average,
is best explained not by landscape drivers but by the lengthening of the snow-free
season.
Indeed, snow melt has advanced by 1.3 days per decade at Barrow (a coastal site)
and an average of 2.3 days per decade over several other coastal sites. Inland, in the
northern foothills of the Brooks Range, warming had taken place at a rate of 3.6 days
per decade, and 9.1 days per decade for the entire Alaskan North Slope. Since 1950
the cover of tall shrubs within the North Slope tundra has increased by 1.2% each
decade from 14 to 20%. The study team pooled, in a meta-analysis, field-warming
experiments showing that increasing the summer temperature by 1-2 ◦ C generally
triggers increased shrub growth within a decade (which is consistent with both recent