Quantifying methane ebullition from northern lakes with space-borne synthetic aperture radar (SAR)

• Main Authors: Engram, Melanie, Walter Anthony, Katey M., Sachs, Torsten, Kohnert, Katrin, Serafimovich, Andrei, Grosse, Guido, Meyer, Franz J.
• Format: Conference Object
• Language: unknown
• Published: AGU 2018
• Subjects: Barrow; Ice; permafrost; Thermokarst; Alaska
• Online Access: https://epic.awi.de/id/eprint/50811/; https://hdl.handle.net/10013/epic.2eb51ded-f723-48fa-8833-cf55cd6bc4f1

Lakes in the northern permafrost region are a significant source of atmospheric methane (CH4), a potent greenhouse gas, yet large uncertainties exist in quantifying lake-source CH4. In thermokarst (thaw) lakes, the dominant pathway of CH4, ebullition (bubbling), is sporadic and spatially irregular. These lakes are also generally remote and difficult to access, resulting in challenging and costly field measurements. Scaling up field measurements from a few study lakes to regional and pan-Arctic scales relies on the assumption that the sampled lakes are a fair representation of all lakes across a landscape, which is not always the case. We present an innovative new method of quantifying lake-source CH4 using space-borne synthetic aperture radar (SAR), an instrument which can image at night, through clouds and dry snow, valuable attributes for Arctic remote sensing. Our recent work using satellite-based SAR data showed a significant correlation between polarimetric L-band SAR backscatter from lake ice and field-measured ebullition bubbles: L-band SAR backscatter intensity increases with the amount of ebullition bubbles trapped by early winter lake ice. We developed a regionally robust empirical model based on this correlation to quantify ebullition across surfaces of over 5,000 individual Alaskan lakes in satellite SAR scenes. We produced SAR-based ebullition fluxes from each lake across the landscape and created CH4 maps for five sub-regions in Alaska. Our SAR-based lake-source CH4 fluxes compare favorably with airborne CH4 measurements on the Barrow Peninsula and Atqasuk regions, and with scaled-up field measurements. We examine how our SAR remote sensing application can 1) improve selection of study lakes for field work, 2) provide regional estimates of CH4 ebullition from lakes in remote areas where field work is limited, 3) improve lake-size vs. flux relationships for upscaling field measurements and 4) shed light on the discrepancy of top-down vs. bottom-up CH4 flux estimates in the Arctic. This new approach ...