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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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