The Scandinavian fjords are often seen as a byword for clean air and natural beauty, but it turns out they are large-scale emitters of methane.
It’s all down to storms hitting the fjords, which mixes the different layers of water - which stirs up methane buried in the sediment at the bottom.
Farming is making the problem worse and methane is among the worst greenhouse gases.
Researchers from the University of Gothenburg have estimated that the total emissions of this climate-warming gas are as great from fjords as from all the deep ocean areas in the world put together.
The world's fjords were created when the inland ice receded, and are a relatively rare natural feature, constituting only 0.13 per cent of all the oceans on Earth.
According to researchers from the University of Gothenburg, emissions of methane from the surface of fjords are comparable to the emissions of this gas from global deep oceans which account for 84% of the global sea surface area.
Stefano Bonaglia, researcher in marine geochemistry at the Department of Marine Sciences at the University of Gothenburg, says: "It's been known for some time that many fjords have anoxic environments closest to the bottom and that methane forms in the bottom sediment.
“Usually, only a small portion of this gas ever reaches the atmosphere because it gets broken down as it ascends through the more oxygen-rich waters closer to the surface. But in our research, we recorded large emissions of methane when the water in the fjord was mixed during storm events, for example.”
Researchers estimate that methane emissions cause about 30% of the greenhouse effect.
Bonaglia says, "This is because in fjords, carbon-rich sediment is deposited from marine plants and animals as well as from materials entering the fjords from the surrounding land via streams that flow into them.
“As fjords are relatively protected from ocean currents, the water tends to remain stratified in layers at different temperatures and with different concentrations of salt and oxygen. The layers closest to the fjord floor are anoxic regions where methane gas forms as the material in the sediment decomposes.”
The researchers say that farming exacerbates the problem due to nutrients flowing into the fjords.
Bonaglia says, "American researchers have seen the same types of events in fjords in Canada. We estimate that emissions from all the world's fjords are of the same magnitude — around 1 Teragram (Tg) or 1 million tonnes per year — as the budgeted emissions from global deep oceans.
“This is because the distance from the bottom to the surface of a fjord is much shorter than in deep oceans. This results in more organic matter being deposited in the sediment, and not enough time for the methane to be broken down on its way up to the surface."
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