Geoscience Reference
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China and India), and the development and adoption of new technologies. Wild
cards include feedbacks involving the release of carbon from terrestrial and subsea
reservoirs in the Arctic, discussed in the next section.
Although aerosol forcing is a key uncertainty in climate change science, aero-
sol loading owing to incomplete combustion of fossil fuels is known, in a globally
averaged sense, to be a cooling agent, offsetting some of the warming attributed
to increasing greenhouse gas concentrations. As discussed in
Chapter 2
, aerosols
have direct climate effects through scattering and absorbtion of solar radiation as
well as a number of indirect effects linked to cloud albedo, amount, and lifetime
that revolve around the role of aerosols as cloud condensation nuclei. Changes in
combustion technologies and fuel sources (e.g., a shift from coal to natural gas or
nuclear) will alter aerosol concentrations and types, and thereby affect the rate of
climate warming. However, Arctic responses may be different than for the globe as
a whole. Regarding aerosol direct effects, one must consider potential changes in
the concentration of black carbon aerosols, which strongly absorb solar radiation,
and hence unlike sulfate aerosols have a warming effect (Shindell and Faluvegi,
2009
). A related issue is soot on snow, which reduces the surface albedo. Regarding
aerosol indirect effects, it is important to remember that, in contrast to lower lati-
tudes, and except for summer, the net cloud radiative forcing in the Arctic is posi-
tive. In other words, in the Arctic, clouds are not cooling agents but rather warming
agents. This is because the impact of clouds on reducing the solar flux at the surface
through their high albedo is outweighed by the increased longwave flux to the sur-
face. Arctic cloud amount appears to be quite sensitive to a small increase in aerosol
loading when aerosol concentrations are low (Mauritzen et al.,
2011
).
11.3
Feedbacks and Responses
There are questions regarding the nature and strength of climate feedbacks and how
different feedbacks will interact with each other. As just discussed, Arctic cloud
cover may be influenced by changes in aerosol concentration and type, but warm-
ing will also influence cloud amount and radiative properties, as well as the sur-
face albedo because of changes in snow cover and sea ice extent. Whereas general
warming will alter the frequency of mixed phase versus water phase clouds, cloud
amount and radiative properties will likely themselves also be altered by an increase
in open water. Based on analysis of numerous data sets over the 2006-2008 period,
J. Kay and A. Gettleman (
2009
) find that whereas there has been no summer cloud
response to declines in Arctic sea ice extent, low clouds do form over newly open
water during autumn. They explain this in terms of the low static stability and strong
air-sea temperature gradients during early autumn that promote large upward turbu-
lent latent and sensible heat fluxes. This contrasts with summer, when near-surface
stability is strong, and air-sea temperature gradients are small.
Cloud cover will also be influenced by changes in atmospheric circulation that
represent responses to a warming climate. Although these may be attributed to
