What was done
Current assessments of the potential impacts of climate change on the world's permafrost are typically based on a two-layer model that incorporates a seasonally frozen active layer and an underlying perennially frozen soil.  In the present study, Shur et al. examine the virtues of adding an in-between or transition zone layer to produce a more realistic three-layer model.

What was learned
Through a review of the literature and theoretical and data analyses, the authors showed, among other things, that the transition zone alternates between seasonally frozen ground and permafrost over sub-decadal to centennial time scales, functioning as a buffer between the active layer and the underlying perennial permafrost by increasing the latent heat required for thaw.  Consequently, in the words of Shur et al., use of a two-layer conceptual model in permafrost studies "obscures effective understanding of the formation and properties of the upper permafrost and syngenetic permafrost, and makes a realistic determination of the stability of arctic geosystems under climatic fluctuations virtually impossible."  As a result, they conclude that "the impacts of possible global warming in permafrost regions cannot be understood fully without consideration of a more realistic three-layer model."

What it means
In light of the authors' findings, there would appear to be little foundation for putting any faith in current model forecasts of future permafrost trends under various global warming scenarios.  Furthermore, if the transition zone does indeed act as a buffer at sub-decadal to centennial time scales, then current permafrost trends are likely to be manifestations of past climatic trends, some of which may have taken place several decades ago, or more!