Small local variations in temperatures eyed as factor
FRISCO — Scientists with the U.S. Geological Survey say they’ve found new patches of permafrost forming in the margins a retreating lake in the interior of Alaska. The findings run counter the conventional wisdom that permafrost will shrink and disappear as the Earth’s climate warms — but don’t jump on the happy train just yet.
The new permafrost patches are small and suggest that the areas of frozen soil are sensitive to small temperature variations and other local factors, the USGS-led study suggests. Especially important is emerging vegetation around the edge of the lake. Thick willows shade the ground to the point that the soil can freeze, the scientists said.
“Central Alaskan lake shrinkage may be caused by shallow permafrost changes and not by increasing deep aquifer connections,” said Martin A. Briggs, USGS, lead author of the study. “Newly formed permafrost along the shores of shrinking lakes may reduce groundwater outflow and allow them to refill.”
Widespread lake shrinkage in discontinuous regions of permafrost has been linked to climate warming and shallow permafrost thaw.
Permafrost, or frozen ground lasting at least two consecutive years, typically forms in colder climates when average annual temperatures remain close to or below freezing. Permafrost soils accumulate ice and plant material and can impede groundwater flow. While the upper 1-2 meters may thaw seasonally, frozen soil and dead plant material continues to accumulate at depth over thousands of years, depending on the strength and duration of the colder climate.
This study considered ecological succession, the pattern of vegetation regrowth, within the receded lake margin as the driver of new permafrost through alterations in ground shading and water infiltration. This hypothesis was tested by modeling variably saturated groundwater flow and heat transport under freeze-thaw conditions.
The simulations supported new permafrost development under current climatic conditions, when the net changes effects of woody vegetation are considered, thus pointing to the role of ecological succession.
“Large lake level swings due to shallow permafrost thaw and subsequent refreezing due to ecological succession may be an important natural cycle,” said Briggs. “However, in the long term, model simulations projected into the future to reflect even moderate climate warming indicate new permafrost around similar lake sites will stop forming and recede within seven decades, possibly ending the current natural cycle of lake level waning and waxing.”
This study was conducted by team of scientists from the U.S. Geological Survey and McGill University in Montreal, Canada, and was published in the journal of Geophysical Research Letter