| Summary: | Research treating permafrost‐climate interactions is traditionally based on a two‐layer conceptual model involving a seasonally frozen active layer and underlying perennially frozen materials. This conceptualization is inadequate to explain the behaviour of the active‐layer/permafrost system over long periods, particularly in ice‐rich terrain. Recent research in North America supports earlier Russian conclusions about the existence of a transition zone that alternates in status between seasonally frozen ground and permafrost over sub‐decadal to centennial time scales. The transition zone is ice‐enriched, and functions as a buffer between the active layer and long‐term permafrost by increasing the latent heat required for thaw. The existence of the transition zone has an impact on the formation of a cryogenic soil structure, and imparts stability to permafrost under low‐amplitude or random climatic fluctuations. Despite its importance, the transition zone has been the focus of relatively little research. The impacts of possible global warming in permafrost regions cannot be understood fully without consideration of a more realistic three‐layer model. The extensive data set under development within the Circumpolar Active Layer Monitoring (CALM) program will provide a significant source of information about the development, characteristics, behaviour, and extent of the transition zone. This paper is focused on the uppermost part of the transition zone, which joins the active layer at sub‐decadal to multi‐centennial time scales. This upper part of the transition zone is known as the transient layer. Copyright © 2005 John Wiley & Sons, Ltd.
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