How much methane do small Arctic lakes produce?
High-resolution satellite images allowed a detailed inventory of Arctic lakes and wetlands. Contrary to previous assumptions, there are far fewer of them and their methane emissions are therefore significantly lower, as a research team discovered.
According to a study by a research team from Brown University in Providence, Rhode Island, USA, the number of small, unmapped lakes in the Arctic is significantly lower than previously assumed. The two researchers used newly available high-resolution satellite images to determine the area of lakes no larger than 1,000 square meters – a crucial factor in calculating methane emissions. In the current issue of the journal Geophysical Research Letters, they describe that the small lakes make up only 0.5 percent of the total pan-Arctic lake area.
As a result, the contribution of small lakes to methane emissions is only just under three percent, far below the previous assumption of around 40 percent. Previous calculations of methane production in Arctic lakes and wetlands relied on area estimates based on low-resolution satellite images and statistical extrapolation, with the area share of small lakes significantly overestimated at nearly nine percent.
“What the research has shown is that these smaller lakes are the greater emitters of methane on a per area basis, which means even that though they take up a small amount of the landscape they have a disproportionate level of emissions,” Ethan D. Kyzivat, who led the current study as part of his doctoral work at Brown University and is now a postdoctoral researcher at Harvard University, said in a university news release. “Traditionally, we haven’t had a good picture of how much area they take up, but this new high-resolution dataset helped us scale it up to finally make those estimations much more accurately.”
As a greenhouse gas, methane plays a crucial role in climate change, as it is at least 25 times more effective at trapping heat in the atmosphere than carbon dioxide. The Arctic – especially in the permafrost and in lakes and wetlands – harbours particularly high levels of the gas, which is increasingly released into the atmosphere as the climate warms.
The original aim of the research project was to find hidden lakes in the Arctic. When the researchers combined high-resolution aerial image data with a global map of the lakes in the Arctic, they realised that many of the large and small lakes were still being counted double as wetlands. The estimates of methane emissions in the past were therefore significantly too high.
The study also helps to close the gap between two different methodological approaches. In the “bottom-up” method, methane emissions are modeled on the basis of maps of the earth, as in the current study. In contrast, the “top-down” method models emissions based on atmospheric measurements. According to Laurence C. Smith, professor of environmental studies and of earth, environmental and planetary sciences at Brown University and co-author of the study, there has been a discrepancy between the results for more than ten years. Thanks to the new calculations, this gap has now been closed.
“It’s probably been 15 to 20 years of loggerheads, but the takeaway is the satellite resolution is now there for the ‘bottom-up’ community to take a much better look at how much methane is actually being emitted,” Smith said. “We can now actually see the very smallest of these water bodies and they’re not as abundant as we were extrapolating. The end result of all of this is going to reduce the bottom-up estimates to make them fall more in line with the top-down estimates. It’s going to unify these two communities.”
The two researchers see their concept confirmed and plan to transfer their modeling method for methane emissions to other parts of the world.
“The next step is to go global,” says Kyzivat.
Julia Hager, PolarJournal
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