IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis Subsea Permafrost

Subsea (or offshore) permafrost refers to permafrost occurring beneath the seabed. It exists in continental shelves of the polar regions. Subsea permafrost formed either in response to the negative mean annual sea-bottom temperature or as the result of sea level rise so that terrestrial permafrost was covered by seawater. Although the potential release of methane trapped within subsea permafrost may provide a positive feedback to climate warming, available observations do not permit an assessment of changes that might have occurred.

4.7.3 Changes in Seasonally Frozen Ground

Seasonally frozen ground refers to a soil layer that freezes and thaws annually regardless of whether there is underlying permafrost. It includes both seasonal soil freeze-thaw in non-permafrost regions and the active layer over permafrost. Significant changes in seasonally frozen ground have been observed worldwide.