Geoscience Reference
In-Depth Information
However, there are competing interactions. Although warming leads to more
melt ponds, reducing albedo, less snow cover means fewer melt ponds, which
increases albedo. The sign of other interactions is poorly understood or changes
seasonally. Sorting out the sign and relative magnitude of the different link-
ages represents a major challenge to the modeling community. The study by J.
Curry, J. Schramm, and E Ebert (
1995
), using a one-dimensional sea ice model,
suggests that the magnitude of the positive ice-albedo feedback mechanism is
increased by the inclusion of melt ponds and diminished by the inclusion of
ice thickness distribution and ridging (see
Chapter 7
for a discussion of sea ice
characteristics).
5.11.3
Cloud-Radiation Feedback
Numerous modeling studies suggest that changes in cloud fraction that would
accompany global warming will modify the surface temperature. However, the sign
of the feedback between different models ranges from negative (clouds cause cool-
ing) to strongly positive (clouds cause warming). The issue is complicated in that
there are feedbacks not only related to cloud fraction, but to cloud height, optical
depth, and cloud microphysical properties. In turn, because of its unique thermo-
dynamic and radiative environment, conclusions drawn from global studies may be
inappropriate in the Arctic.
The cloud-radiation feedback mechanism in the Arctic is illustrated conceptu-
ally in
Figure 5.16
. A perturbation in the surface radiation balance may arise from
increased greenhouse gas concentrations and/or increasing aerosol amounts. A
perturbation in the surface radiation balance of the snow/ice changes the snow/
ice characteristics (e.g., ice thickness and areal distribution, surface temperature,
and surface albedo). These changes, especially in the surface temperature and
fraction of open water, modify fluxes of radiation and surface sensible and latent
heat, modifying the atmospheric temperature, humidity and dynamics. This will
in turn modify cloud properties (e.g., cloud fraction and optical depth), which
will in turn modify the radiative fluxes. Resolving the interactions requires an
understanding of changes in cloud fraction coverage and vertical distribution as
the vertical temperature and humidity profile changes and adjustments in cloud
water content, phase, and particle size as atmospheric temperature and composi-
tion change.
As outlined earlier, for most of the year, the observed net cloud radiative forcing
at the surface is positive in the Arctic (clouds cause warming). However, with respect
to feedback processes, a major uncertainty is how cloud characteristics (e.g., optical
depth, microphysical properties) will be altered in a changing climate. Because of
the impact of clouds on the surface radiation flux, cloud-radiative feedback pro-
cesses in the Arctic are very much intertwined with the ice-albedo feedback. At
present, the sign of the cloud radiation feedback is still uncertain, but appears to be
positive.
