Study uncovers link between seasonal melting of sea ice and release of methane gas
The ice-covered Arctic Ocean plays a more important role in determining the concentration of methane in the atmosphere than previously thought. Researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) recently published their discoveries regarding about the interactions between the atmosphere, sea ice and ocean in the Scientific Reports section of Nature journal.
Sea ice forms a natural cover upon the central Arctic Ocean, thereby limiting the gas exchange between the water and the atmosphere. In recent years, the sea ice cover during the summer has shrunk rapidly in the Arctic. “We're investigating how the changing conditions affect the geochemical interaction between the ocean, ice and atmosphere,” said Dr Ellen Damm, an AWI biogeochemist and first author of the study.
“We were able to confirm that the surface water in the central Arctic contains higher methane concentrations than the atmosphere, which means that the Arctic Ocean is a potential source of atmospheric methane. That makes it fundamentally different from oceans at lower latitudes, which – except for a few sporadic sources – are considered methane sinks.”
For the study, Damm and her colleagues from AWI, the Finnish Meteorological Institute and the University of Bremen evaluated the geochemical and oceanographic data which had been obtained during a 2011 expedition involving the Polarstern research vessel. They measured the methane levels in both the sea ice and the water direct below it, as well as in deeper seawater entirely unaffected by the ice.
“Our study shows that there are previously overlooked feedback between ice melting and formation, the atmosphere and the seawater influenced by ice,” said Damm. Among other things, the researchers analysed the brine, which is highly concentrated seawater formed during the formation of sea ice. Compared to the atmosphere, the methane concentration in the brine is a thousand times higher. Hence, it is possible that the sea ice is a source of methane, a greenhouse gas.
Methane in the brine channels is released during the melting and freezing processes. Due to the different densities of freshwater and saltwater, water is trapped in the stable layers. This results in the methane from the brine channels being restricted to just the uppermost layer during the summer. The stable layering prevents the methane from being introduced into the lower depths of the Arctic Ocean, as evidenced by the significantly lower methane concentrations in the deeper ocean layers.
When autumn comes and the temperatures drop, convection causes the different water layers to be mixed. Portions of the sea ice would have melted during this time of the year, allowing the methane to be released into the atmosphere through the fractional ice cover. This process continues into the winter months.
The implications of this discovery means the newly discovered and not previously accounted-for near-surface feedback mechanism can cause the direct release of methane from the sea ice and ocean into the atmosphere. In addition, the exchange between the atmosphere and the deeper layers is reduced, limiting the Arctic Ocean's capacity to act as a methane sink.
“The role of sea ice in gas exchange and gas flux is much more complex than previously assumed, and the processes at work in the Northern Ocean differ greatly from those in lower latitudes. These aspects have to be kept in mind in future climate models,” said Prof Ursula Schauer, AWI oceanographer and co-author of the study.
She added that a further question about the origins of the methane has arisen. It is conceivable that the gas could have been produced as the sea ice drifts through the Arctic, or become trapped in the sea ice as it moves in from other regions.
Link to the study: www.nature.com/articles/srep16179