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Keywords:

  • Siberian shelf;
  • air temperature;
  • bottom water layer;
  • subsea permafrost;
  • water temperature

Eight thousand years ago, a rising sea inundated the vast permafrost regions off the northern coast of Siberia. Comprising the modern east Siberian shelf, the region holds enormous quantities of methane hydrates bottled up in remnant subterranean permafrost zones that are, in turn, trapped beneath the ocean waters. Records of seafloor water temperature showing a 2.1°C rise since 1985, coupled with recent observations of methane emissions from the seabed, have led some scientists to speculate that the rising temperatures have thawed some of the subsurface permafrost, liberating the trapped methane. The connection is compelling, but an investigation by Dmitrenko et al. into the sensitivity of permafrost to rising temperatures suggests the two observations are not connected. Using a permafrost model forced with paleoclimate data to analyze changes in the depth of frozen bottom sediments, the authors found that roughly 1 meter of the subsurface permafrost thawed in the past 25 years, adding to the 25 meters of already thawed soil. Forecasting the expected future permafrost thaw, the authors found that even under the most extreme climatic scenario tested this thawed soil growth will not exceed 10 meters by 2100 or 50 meters by the turn of the next millennium. The authors note that the bulk of the methane stores in the east Siberian shelf are trapped roughly 200 meters below the seafloor, indicating that the recent methane emissions observations were likely not connected to the modest modern permafrost thaw. Instead, they suggest that the current methane emissions are the result of the permafrost's still adjusting to its new aquatic conditions, even after 8000 years. (Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007218, 2011)