Lakes in the northernmost latitudes are widely considered to be a significant source of the greenhouse gas methane. In order to improve currently available climate-change projection models, it’s essential to have some idea of how much methane will be released by the millions of northern lakes. A German-American research team led by the University of Alaska Fairbanks (UAF), which also included members from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), has now developed a method for determining the amount of methane released, drawing on satellite images to do so.
Over a 100-year time period, the greenhouse gas methane is roughly 34 times as potent as carbon dioxide, and over a 20-year period, it is even 86 times as potent, making it especially important to accurately represent its sources in climate models. Previous research has confirmed that thermokarst lakes release massive amounts of methane when the permafrost below them thaws. However, gathering data on these lakes on site is often logistically challenging and expensive, and is only feasible on comparatively few of them. Accordingly, information on methane production is only available for a small percentage of all Arctic lakes.
To date, there has been a discrepancy between this projected data, extrapolated from a small number of individual lakes, on the one hand; and estimates of Arctic methane emissions based on atmospheric measurements on the other. Yet the new method developed by the team makes it possible to mitigate that discrepancy.
The team used data from radar satellites, which actively scan the surface of the Earth with electromagnetic waves, yielding detailed physical information on the current condition and properties of winter ice on northern lakes. The greatest advantage of the radar satellites is that, due to their active sensors, they can continue gathering data regardless of the weather conditions, day or night, and can even penetrate cloud cover and powder snow. The radar signal reflected back to the satellites is influenced by the ice’s characteristics, e.g. the thickness of the ice on the lakes and the amount of gas bubbles inside it.
Thanks to the radar data, the experts were able to establish a relation between the satellite signal and the field data on methane bubbles trapped in the ice. Comparing various areas of the frozen lakes as captured in the satellite images with near-surface atmospheric methane measurements confirmed a high level of agreement between the satellite readings and data gathered on site.
“We found that backscatter is brighter when there are more bubbles trapped in the lake ice,” says Melanie Engram, an expert at the UAF’s Environmental Research Center and first author of the study. Bubbles form an insulating blanket, so ice beneath them grows more slowly, causing a warped surface which reflects the radar signal back to the satellite.”
In order to check the radar data, the researchers compared satellite images with methane readings taken on site at 48 lakes scattered across five regions of Alaska. By making projections on the basis of the results, they were able, for the first time, to estimate the methane production for more than 5,000 Alaskan lake: “It’s important to know how much methane comes out of these lakes and whether the level is increasing,” says Engram. “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.”
“The new method, when applied to the vast northern permafrost regions and their millions of lakes, can help to more closely monitor changes in the lakes’ methane emissions, but also to better integrate the effects of methane emissions in climate models,” says Guido Grosse, a co-author of the study and permafrost expert at the AWI, with regard to the new method’s relevance.
Original Publication
Engram, M., K. M. Walter Anthony, T. Sachs, K. Kohnert, A. Serafimovich, G. Grosse, F. J. Meyer, Remote sensing northern lake methane ebullition; Nature Climate Change, 11 May 2020, DOI: 10.1038/s41558-020-0762-8
Original publication
Engram, M., K. M. Walter Anthony, T. Sachs, K. Kohnert, A. Serafimovich, G. Grosse, F. J. Meyer, Remote sensing northern lake methane ebullition, Nature Climate Change