Synthetic aperture radar (SAR) backscatter response from methane ebullition bubbles trapped by thermokarst lake ice

Thermokarst lakes, formed by permafrost thaw, are an important source of atmospheric methane (CH4), a powerful greenhouse gas. Ebullition (bubbling) is often the dominant mode of lake CH4 emission. Because extrapolating spatially limited field measurements of CH4 ebullition induces large uncertainties in regional emission estimates, there is a need for remote sensing based approaches to detect and quantify CH4 ebullition at larger spatial scales in lakes. We examined the relationship between spaceborne synthetic aperture radar (SAR) pixel values of lake ice and biogeochemical field measurements of CH4 ebullition on ten lakes on the northern Seward Peninsula. Among lakes, ebullition ranged from low to high. We found that both the area of ice-bound ebullition-bubble clusters and the bubbling rates that generated the clusters were correlated with L-band single-polarized (HH) SAR (R-2 = 0.70, p = 0.002, n = 10) and with the roughness component of a Pauli decomposition of L-band quad-polarized SAR (R-2 = 0.77, p = 0.001, n = 10). No relationship was found between ERS-2 C-band single-polarized (VV) SAR and ice-trapped CH4 bubbles. Results of this study indicate that analysis of L-band SAR backscatter intensity from winter lake ice could be a valuable new tool for constraining estimates of regional ebullition in lakes.