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Climate Feedbacks publications The
goal of our work in climate feedbacks is two-fold: First, we
aim to determine what processes cause the large variations in
responses of climate models to external forcing, such as an
increase in greenhouse gases. Second, we seek ways to
constrain these processes with currently available
observations. So far, we have focused on processes shaping the
climate system's response in two parts of the globe: (1) the
heavily-populated northern hemisphere land masses, where snow
albedo feedback is important; and (2) the high latitudes,
which may exhibit enhanced sensitivity to changes in
greenhouse gas concentrations, and, in the case of the Arctic,
have already exhibited dramatic signs of change. In addition,
we have investigated the processes controlling simulated
global warming over the course of the 21st century. Below we
describe our work in these areas. Snow
Albedo Feedback Snow
is
projected to decrease in a warmer climate, and since snow is
generally more reflective of sunshine than bare
land, this causes an increase in net incoming solar
radiation. This in turn leads to additional warming,
particularly in the mid to high latitudes of the northern
hemisphere, where snow is most common. (Insight into how
this feedback affects simulated climate change and
internal climate variability can be found in an idealized modeling
study, where we "turned off" surface albedo
feedback in a global climate model.) Our work on
snow albedo feedback involved a determination of what
causes
the nearly three-fold spread in the strength of this
feedback in current climate models. We
also examined the large consequences of
this spread for simulations of climate
change
in Eurasia and North America,
and northern
hemisphere
atmospheric circulation.
We went on to develop a
method to constrain this feedback observationally.
Finally, we undertook a
study to determine which aspects of the
models' treatment of snow lead to spread in
the feedback, an important practical aspect of
bringing the feedback in line with observational
constraints. This is the first time such a
concrete strategy to constrain a critical
climate feedback observationally has been devised,
and the work was featured in the 2007
IPCC report. High-Latitude
Climate Change The
ubiquity
of sea ice in the Arctic might lead one to think that the
large spread in simulated Arctic climate change is dominated
by the marine analog to snow albedo feedback: sea ice albedo
feedback. Surprisingly, in an
analysis
of
the processes controlling simulated Arctic climate
change in current models, we found that large variations in
wintertime longwave feedbacks are mainly responsible for the
spread and that these feedbacks may be quite unrealistic
in most models. Because these feedbacks are controlled by the
atmosphere's vertical temperature structure, we undertook a
study of the
controls on the real atmosphere's vertical temperature
structure. This will lay the groundwork for an eventual
comparison of simulated and observed behavior. Because
of the importance of sea ice to Arctic climate, we undertook
a
related study of the reasons for the
intermodel spread in simulations of Arctic sea ice loss and
the systematic bias in this metric of change compared to
observations. This allowed us to make a bias-corrected
prediction of total loss of September Arctic sea ice by
2100. Global
Climate Change during the 21st century Climate
change
anywhere on the globe scales roughly with the global mean
temperature increase. To determine what controls this
critical parameter in current climate simulations, we
studied the relative importance of conventional
climate feedbacks and the rate of ocean heat uptake in the
magnitude of simulated global warming. Surprisingly, ocean
heat uptake plays the dominant role, suggesting that an
emphasis on greater realism of oceanic processes in climate
model development may be appropriate.
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