Shrinking Arctic Lakes are Forming New Local Permafrost, but for How Long?

Using multiple physical, thermal, and geophysical methods over two summer seasons we have observed patchy permafrost aggradation within the recently receded margin of Twelvemile Lake, in the Yukon Flats of interior Alaska. However, recent rapid recession of Twelvemile Lake over tens of years is presumed to be linked to climate warming. The counterintuitive process of newly forming permafrost in a warming climate may be a result of ecological succession: discrete bands of brushy vegetation develop on the dried lakebed, reducing local summer soil temperatures and infiltration, and favoring the persistence of ground ice through multiple annual cycles. Using a modified version of the USGS code SUTRA to account for variably saturated flow and freeze/thaw dynamics, a suite of 1-D simulations were constructed to assess the relation of permafrost aggradation at Twelvemile Lake to ecosystem-driven effects. The changes simulated included reductions in recharge during the summer (plant transpiration/interception) and peak surface soil temperatures (albedo/shading). Simulations indicate that the system is strongly responsive to reductions in peak surface soil temperature. Permafrost aggradation began after 2 years with only a 1° C reduction; after 75 years saturated and unsaturated frozen materials on the order of 7 m thick were simulated when soil peak temperatures were reduced by 2° C. The absence of summer recharge alone did not support permafrost aggradation in the simulations, but did reduce time to permafrost equilibrium for the moderate 1° C reduction in peak temperatures while having less of an effect for the 2° C reduction model. Finally, when a predicted climate warming trend of 3° C/100 yr is imposed on the simulation of strongest cooling and infiltration reduction, the aggraded permafrost thaws completely after approximately 70 yr, and the seasonal freeze/thaw layer shallows thereafter. Therefore, local permafrost aggradation in response to lake recession within the discontinuous permafrost zone may affect local processes in the short term, but is likely to be overtaken by further warming within the next century.