Simulating heat source effect of a thermokarst lake in the first 540 years on the Alaskan Arctic using a simple lake expanding model

Abstract

Thermokarst lakes are an important heat source to ground surrounding the lake, a strong greenhouse gas methane source if taliks develop under the lakes, and a sensitive indicator of climatic and environmental changes. This study proposes a simple lake radius expanding model (EM) by imposing a step increase process of thermokarst lake radius to the lake radius unchanged model (UM) as an external condition, and simulates the temporal and spatial variation of ground temperature and talik thickness beneath an expanding thermokarst lake on the Alaskan Arctic during the period that the lake radius expands from the initial length of 15 m to a length of 150 m. The differences of the simulated ground thermal regime and talik development process between using the EM and the UM under the identical computation conditions are compared. The sensitivity of talik thickness to lake expansion time interval is also analyzed. The simulated results by using the EM show that the maximum permafrost temperature increase and talik thickness increase below the expanding thermokarst lake is 6.4 °C and 17.5 m respectively during the simulation period of 540 years, the corresponding ground temperature increase of >1.0 °C beneath the original lake bottom occurs at a depth of 175.3 m. The maximum talik thickness simulated by using the UM is observably greater than that calculated by using the EM. The mean talik increase rates by using the EM and the UM are 2. 4 cm yr−1 and 3.1 cm yr−1 in the first 420 years, and 1.6 cm yr−1 and 1.3 cm yr−1 in the last 120 years from year 420 to year 540. The UM may overestimate the heat source effect of the thermokarst lake when lake radius in the EM is less than the radius in the UM, and underestimate the heat source effect when the radius in the EM is greater than the radius in the UM, and the magnitudes of the overestimate and underestimate depend on the difference of the radius in the two models. The EM can be applied to simulate permafrost thermal regime and talik development beneath an expanding thermokarst lake easily as the method is simple and the needed data to drive the model are easy to obtain. The selection of a field measurements based lake expansion time interval or lakeshore collapse width is essential for numerical simulation since decreases in lake expansion time interval have limited increases in talik thickness.