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transport into the Arctic from Atlantic-layer waters which has been linked to the
warm phase on the Atlantic Multidecadal Oscillation. It has been argued (e.g.,
Polyakov et al.,
2010
) that this has fueled a substantial additional heat flux to the
bottom of the sea ice cover, contributing to thinning, in turn making the ice more
vulnerable to disruption by extreme atmospheric events. Although the role of this
extra Atlantic-layer heat on the observed evolution of the sea ice cover remains
unresolved, the question also remains as to whether this change in ocean circula-
tion represents natural low-frequency ocean variability or contains an anthropo-
genic influence. Analysis of paleoclimate evidence suggest that the early twenty
first century temperatures of Atlantic water entering the Arctic are unprecedented
over the past 2,000 years (Spielhagen et al.,
2011
). The issue of natural variability
versus forced change also holds with respect to evidence for a link between declin-
ing sea ice extent and thickness and warmer Pacific Surface Water entering the
Arctic Ocean via the Bering Strait (Shimada et al.,
2006
).
The sun, of course, is the ultimate source of energy to the climate system. Space-
borne measurements indicate that the solar constant (~1361.5 Wm
−2
) is not entirely
constant. These data show a variation in the annual mean of about 1 W m
−2
in
total irradiance between the minimum and maximum of the eleven-year sunspot
cycle, which has a discernible effect on global average temperatures. However, as
discussed, there is evidence of periods with considerably larger changes in solar
output, such as during the Maunder Minimum (sunspot minimum) from 1680 to
1730, and it is increasing recognized that solar influences can be amplified within
the climate system through changes in stratospheric and tropospheric atmospheric
circulation and feedbacks in the ocean-atmosphere system (Lean,
2009
). Although
explosive volcanic activity is known to have impacts on climate, a period of volca-
nic activity working in conjunction with altered solar output and attendant climate
feedbacks, could have strong climate influences.
11.2
Greenhouse Gas Growth and Aerosols
Another element of uncertainty surrounding the Arctic's future lies in the growth
of greenhouse gas concentrations through the twenty-first century. Looking back to
Figure 9.12
, in terms of projected changes in surface air temperature, the growth
rate scenario over the next twenty years or so is not especially important, the reason
being that the spread in projected temperature change attributed to natural climate
variability and model-to-model differences in sensitivity are as large as the spread
attributed to the differences in climate forcing. The separation in projected tem-
peratures between the scenarios becomes apparent later in the century. The same
general statement can be made with respect to sea ice extent, which, as we have
seen, responds sensitively to changes in winds linked to shifts on the phase of the
NAO and other teleconnection patterns as well as changes in ocean circulation. The
future rate of greenhouse gas emissions itself depends on many factors, including
population growth, economic growth (particularly in developing countries such as
