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
change anywhere on the globe scales roughly with the global mean
increase. To determine what controls this critical
current climate simulations, we studied the
relative importance of
climate feedbacks and the rate of ocean heat uptake in the magnitude of
simulated global warming. Surprisingly, ocean heat uptake
dominant role, suggesting that an emphasis on greater realism of
in climate model development may be appropriate.