Past Climate Variability and Change in the Arctic and at High Latitudes

–112.50
–90
–67.50
SURFACE BROADBAND ALBEDO, JUNE.
–135
-45
–180 180
–157.50 157.50
energy balance (Peixoto and Oort, 1992). Open
ocean, on the other hand, has a low albedo; it
absorbs almost all solar energy when the Sun
angle is high. Changes in albedo are most
important in the Arctic summer, when solar
radiation is at a maximum, whereas changes in
the winter albedo have little influence on the
energy balance because little solar radiation
reaches the surface then. In general, warming
reduces ice and snow whereas cooling allows
them to extend, so the changes in ice and snow
act as positive feedbacks to amplify climate
changes (e.g., Lemke et al., 2007).
3.2.2 Ice-Insulation Feedback
In addition to its effects on albedo, sea ice also
causes a positive insulation feedback, primarily
in the wintertime. Ice effectively blocks heat
transfer between relatively warm ocean (at or
above the freezing point of seawater) and cold
atmosphere (which, in the Arctic winter, averages
–40°C (Chapman and Walsh, 2007). If sea
ice is thinned by warming, then the ocean heats
the overlying atmosphere in winter months,
amplifying that warming.
90
85
80
75
70
–22.50 22.50
65
60
Past Climate Variability and Change in the Arctic and at High Latitudes
135
45
112.50
67.50
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
ALBEDO
I. Open ocean
α=0.06
II. Bare ice
α=0.5
III. Ice with snow
Figure 3.4. Albedo values in the Arctic. A) Advanced Very High Resolution Radiometer (AVHRR)derived
Arctic albedo values in June, 1982–2004 multi-year average, showing the strong contrast between
snow- and ice-covered areas (green through red) and open water or land (blue). [Image courtesy of
X. Wang, University of Wisconsin-Madison, CIMSS/NOAA.] B) Albedo feedbacks. Albedo is the fraction
of incident sunlight that is reflected. Snow, ice, and glaciers have high albedo. Dark objects such as the
open ocean, which absorbs some 93% of the Sun’s energy, have low albedo (about 0.06). Bare ice has an
albedo of 0.5; however, sea ice covered with snow has an albedo of nearly 90% [Source: http://nsidc.org/
seaice/processes/albedo.html.]
90
α=0.9
Feedbacks involving snow insulation of the
ground are also important, through their effects
on vegetation and on permafrost temperature
and its influence on storage or release of greenhouse
gases, as described in the next subsections
(e.g., Ling and Zhang, 2007).
3.2.3 Vegetation Feedback
0.94
A related terrestrial feedback involves changing
vegetation. A warming climate can cause tundra
to give way to shrub vegetation. However,
the shrub vegetation has a lower albedo than
tundra, and the shrubs thus cause further warming
(Figure 3.5) (Chapin et al., 2005; Goetz et
al., 2007). Interactions involving the boreal
forest and deciduous forest can also be important.
When, as a result of warming, deciduous
forest replaces evergreen boreal forest, then
winter surface albedo increases—an example
of a negative feedback to the warming climate
(Bonan et al., 1992; Rivers and Lynch, 2004).
Alternatively, if warming allows evergreen
boreal forest to advance northward replacing
tundra or shrub vegetation, then the lower
0.5
0.1
If sea ice is thinned by
warming, then the ocean
heats the overlying
atmosphere in winter
months, amplifying
that warming.
37