Image above: Methane bubbles form throughout the year on thermokarst lakes. During winter, such bubbles become entrained in the lake ice, thus allowing researchers to readily visualize the phenomenon..
Photo Credit: Melanie Engram.
Northern high latitude wetlands and thermokarst lakes in permafrost regions have been identified as strong sources for methane (CH4), a powerful greenhouse gas. Quantifying the spatial distribution and magnitude of CH4 sources in these regions has become increasingly important in the current scenario of global warming and amidst concerns of partial release of the large permafrost soil carbon pool through thawing by thermokarst lakes. Ebullition (bubbling) is an important mode of CH4 emission from thermokarst lakes to the atmosphere. However, due to its sporadic behavior, large uncertainties remain in estimating the magnitude of ebullition emissions from lakes. Synthetic Aperture Radar (SAR) remote sensing of lake ice is a potentially valuable tool to constrain bottom-up estimates of lake ebullition in regions where lake ice forms. Here we explored various SAR imaging parameters as they correlate to field measurements of CH4 ebullition bubbles in the ice of ten thermokarst lakes on the northern Seward Peninsula, Alaska. We found that ebullition bubbles trapped in frozen lakes were strongly correlated with L-band single polarized horizontal (HH) SAR (R2 = 0.70, P = 0.002) and with the ‘roughness’ component of a classic Pauli decomposition of PALSAR L-band quad-polarized signal (R2 = 0.77, P = 0.001). We found no such correlation with ERS-2 C-band single polarized vertical (VV) SAR. We present the results of our single-pol and quad-pol SAR geospatial analysis, a discussion of probable scattering mechanisms of ebullition bubbles in frozen thermokarst lakes, including results from a March 2013 field experiment. Our results indicate that calibrated L-band SAR could be a valuable tool for estimating methane ebullition in lakes on a regional scale by evaluating the backscatter intensity from early winter lake ice.