The lakes in the northern latitudes are considered to be an important source of the greenhouse gas methane. In order to improve the existing models for assessing climate change, it is important to know the amount of methane released from the millions of northern lakes. A german-American research team led by the University of Alaska Fairbanks, which included researchers from the Alfred Wegener Institute, has now developed a method to use satellite imagery to determine the amount of methane released.
Methane acts about 34 times more as a greenhouse gas over a 100-year period and 86 times stronger than carbon dioxide over the 20-year period. Thus, accurate calculations of its sources are particularly important in scientific climate models.
Previous research has confirmed that huge amounts of methane are released from thermokarst lakes when the permafrost thaws beneath them. However, collecting on-site data from these lakes is often logistically demanding and expensive and can only be done on a few lakes. For this reason, information on methane production is only available for a very small percentage of Arctic lakes.
There has been a discrepancy between these data, extrapolated from a small number of individual lakes, and estimates of Arctic methane emissions from atmospheric measurements. This discrepancy can be resolved with the new method developed by the researchers.
The research team used radar satellite data, which, by actively scanning the Earth’s surface with electromagnetic waves, also reveals detailed physical information about the state and properties of winter ice on the northern lakes. The special advantage of radar satellites is that they can collect data independently of weather and daylight due to the active sensors and thus even penetrate clouds and dry snow. The radar signal is influenced by the properties of the ice, such as the thickness and amount of gas bubbles in the ice of the lakes.
Using radar data, the researchers were able to establish a link between satellite signal and the field measurements of methane bubbles that had caught up in the ice. The comparison of different areas of the frozen lakes in the satellite images with near-ground methane measurements confirmed that the satellite measurements were very consistent with the data on the ground.
“We found that the backscattering of the radar signal is brighter when more bubbles are trapped in the lake’s ice,” said Melanie Engram, a researcher at the UAF’s Environmental Research Center and the study’s lead author. “The bubbles form an insulated ceiling, so that the ice under them grows more slowly, resulting in a distorted surface that reflects the radar signal more strongly back to the satellite.”
To verify the radar data, the researchers compared satellite images with methane measurements on site at 48 lakes in five different regions of Alaska. By extrapolating these results, the researchers were able to estimate the methane production of more than 5,000 lakes in Alaska for the first time.
“It’s important to know how much methane is coming out of these lakes and whether the level increases over time,” Engram said. “We can’t go out to every single lake and do field work, but we can extrapolate field measurements using radar remote sensing to get these regional estimates,” he said.
“The new method, applied to the vast northern permafrost regions with their millions of lakes, can help closely monitor changes in methane emissions in the lakes, but also better take into account the effects of methane emissions in climate models,” said Guido Grosse, co-author of the study and permafrost researcher at the AWI, in an assessment of the new method.
Source: Guido Grossen, AWI