Climate Feedbacks

Current GCMs' unrealistic negative feedback in the Arctic

Climate change projections show that surface warming in response to anthropogenic forcing will be greatly amplified in the Arctic (Figure 1). A major reduction of sea ice extent, especially during summer, is also expected. But if some features of Arctic climate change are now well established, major uncertainties remain concerning the exact magnitude of the changes and a better understanding of the mechanisms governing Arctic climate change is needed to address this issue.

It is well known that Arctic climate change is characterized by a large seasonal redistribution of energy as shown in Figure 2. In spring and summer, the decrease of ice cover leads to an increase of absorbed shortwave radiation, and consequently to a progressive increase of oceanic temperature. In winter and fall, thinning sea ice and decreasing sea ice cover and its associated insulation effect lead to larger heat loss from the ocean to the atmosphere: the solar energy stored in the ocean during spring and summer is finally radiated to space as longwave radiation. We show in this paper that the way models simulate the wintertime damping of oceanic temperature anomalies (i.e. the negative longwave feedback) play a very important role in the large spread of the model results in the Arctic.

The vertical thermal structure of the atmosphere is found to play a key role in this context. Arctic climate is characterized by an inversion of the vertical profile of temperature near the surface during winter (temperature at surface is colder than above) but the strength of the inversion varies greatly from model-to-model, which appears to affect the longwave feedback: models with a large inversion in the present climate tend to have a very strong negative longwave feedback and conversely. As most of climate models may overestimate the strength of the inversion, we conclude that they probably overestimate the negative longwave feedback and therefore that models may as a group underestimate future Arctic climate change.

Download the publication (Boé et. al. 2009) describing these results in more detail.

Julien Boé, Alex Hall, and Xin Qu make up the team that performed this research.