IMAGE: Methane ebullition bubbles form in early winter lake ice in Interior Alaska. A yardstick is included for scale. view more
Credit: Melanie Engram photo
A University of Alaska Fairbanks-led research team has developed a way to use satellite images to determine the amount of methane being released from northern lakes, a technique that could help climate change modelers better account for this potent greenhouse gas.
By using synthetic aperture radar, or SAR, researchers were able to find a correlation between "brighter" satellite images of frozen lakes and the amount of methane they produce. Comparing those SAR images with ground-level methane measurements confirmed that the satellite readings were consistent with on-site data.
SAR data, which were provided by UAF's Alaska Satellite Facility, are well-suited to the Arctic. The technology can penetrate dry snow, and doesn't require daylight or cloud-free conditions. SAR is also good at imaging frozen lakes, particularly ones filled with bubbles that often form in ice when methane is present.
"We found that backscatter is brighter when there are more bubbles trapped in the lake ice," said Melanie Engram, the lead author of the study and a researcher at UAF's Water and Environmental Research Center. "Bubbles form an insulated blanket, so ice beneath them grows more slowly, causing a warped surface which reflects the radar signal back to the satellite."
The new technique could have significant implications for climate change predictions. Methane is about 30 times more powerful than carbon dioxide as a heat-trapping gas, so accurate estimates about its prevalence are particularly important in scientific models.
Previous research had confirmed that vast amounts of methane are being released from thermokarst lakes as the permafrost beneath them thaws. But collecting on-site data from those lakes is often expensive and logistically challenging. Because of that, information about methane production is available from only a tiny percentage of Arctic lakes.
"This new technique is a major breakthrough for understanding the Arctic methane budget," said UAF researcher Katey Walter Anthony, who also contributed to the study. "It helps to resolve a longstanding discrepancy between estimates of Arctic methane emissions from atmospheric measurements and data upscaled from a small number of individual lakes."
To confirm the SAR data, researchers compared satellite images with field measurements from 48 lakes in five geographic areas in Alaska. By extrapolating those results, researchers can now estimate the methane production of more than 5,000 Alaska lakes.
"It's important to know how much methane comes out of these lakes and whether the level is increasing," Engram said. "We can't get out to every single lake and do field work, but we can extrapolate field measurements using SAR remote sensing to get these regional estimates."
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The study was published May 11 in the journal Nature Climate Change. UAF researcher Franz Meyer also contributed to the paper, along with Torsten Sachs, Katrin Kohnert and Andrei Serafimovich from the GFZ German Research Centre for Geosciences, and Guido Grosse from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research.
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