A Mechanism for the High Rate of Sea Ice Thinning in the Arctic Ocean

Submarine measurements of sea ice draft show that the ice has thinned in some parts of the Arctic Ocean at a remarkably high rate over the past few decades. The spatial pattern indicates that the thinning was a strong function of ice thickness, with the greatest thinning occurring where the ice was thickest initially. A similar relationship between sea ice thinning and the initial thickness is reproduced individually by three global climate models in response to increased levels of carbon dioxide in the models’ atmosphere. All three models have weak trends in their surface winds and one model lacks ice dynamics altogether, implying that trends in the atmosphere or ice circulation are not necessary to produce a relatively high rate of thinning over the central Arctic or a thickness change that increases with the initial thickness. A general theory is developed to describe the thinning of sea ice subject to climate perturbations, and it is found that the leading component of the thickness dependence of the thinning is due to the basic thermodynamics of sea ice. When perturbed, sea ice returns to its equilibrium thickness by adjusting its growth rate. The growth–thickness relationship is stabilizing and hence can be reckoned a negative feedback. The feedback is stronger for thinner ice, which is known to adjust more quickly to perturbations than thicker ice. In addition, thinner ice need not thin much to increase its growth rate a great deal, thereby establishing a new equilibrium with relatively little change in thickness. In contrast, thicker ice must thin much more. An analysis of a series of models, with physics ranging from very simple to highly complex, indicates that this growth–thickness feedback is the key to explaining the models’ relatively high rate of thinning in the central Arctic compared to thinner ice in the subpolar seas.