Home Sciences Fjords already emit as much methane as the oceans

Fjords already emit as much methane as the oceans

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The world’s fjords, which represent 0.13 percent of all the Earth’s oceans, emit as much methane into the atmosphere as all the world’s seas, due to high concentrations of inorganic nutrients from agriculture, which leave no oxygen to coastal waters.

The fjords are narrow and deep gulfs, arising between mountains with steep slopes, which were formed by glaciers during the Quaternary period, about 2.59 million years ago. Norway is the country in the world where they are most abundant, with more than a thousand fjords. Sweden, Scotland, Alaska, Iceland, Canada, Greenland, Patagonia and New Zealand also have fjords.

When strong storms occur, the normally stratified layers of water in ocean fjords mix, leading to oxygenation of the fjord bottom. These storm events also cause an increase in methane emissions from the fjords to the atmosphere, which currently contribute about half of all methane emissions to the atmosphere.

This gas is produced naturally by the decomposition of organic matter and reached 1,900 parts per billion (ppb) in the Earth’s atmosphere last year, according to measurements made by the United States National Oceanic and Atmospheric Administration (NOAA). cited by Professor Euan Nisbet.

Methane Impact

Before the industrial revolution, methane was at 700 ppb. Its total impact on global warming, calculated since 1750, is about half that of CO₂. Every year about 600 million tons of methane are released into the atmosphere.

Two-fifths of these emissions are estimated to come from natural sources, primarily decaying vegetation in wetlands. The remaining three-fifths come from sources linked to human activity, according to the same source.

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Researchers at the University of Gothenburg, Sweden, have calculated that fjords, which account for 0.13 percent of all the Earth’s oceansgenerate as much methane as the emissions that come from the planet’s deep oceanswhich represent 84% of the world’s marine surface.

The researchers estimate that methane emissions cause about 30 percent of the global greenhouse effect and that the contribution of the oceans to methane emissions is significantly less than that of land areas.

However, human activity has increased the eutrophication in coastal areas, which means that the excess of inorganic nutrients (from human activities), mainly Nitrogen (N) and Phosphorus (P), have skyrocketed in coastal aquatic ecosystems.

fjord chemistry

This eutrophication has created larger areas of oxygen-depleted (anoxic) waters not only on the seabed, but also in fjords: carbon-rich sediments from marine plants and animals, as well as materials that enter the oceans, are deposited in them. fjords from the surrounding land through streams that flow into them, with all kinds of substances.

As fjords are relatively protected from ocean currents, their waters tend to remain stratified in layers at different temperatures and with different concentrations of salt and oxygen.

Consequently, the layers closest to the bottom of the fjord are anoxic regions where methane gas forms as sediment material breaks down, the researchers explain.

runaway eutrophication

To reach their conclusions, researchers at the University of Gothenburg studied Byfjorda Swedish fjord in the west of the country, near Uddevalla, during the period 2009-2021, and conducted field studies to measure methane production in the fjord.

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Byfjorden has very little oxygen concentration (it is hypoxic) due to eutrophication. The Bäve River flows into the fjord, bringing with it high concentrations of nutrients from the region’s agriculture.

The researchers found that during mixing events in the fjord, methane emissions into the atmosphere increase.

During these events, anoxic bottom water rapidly rises to the surface, taking methane with it, which can then be emitted into the atmosphere.

1 million tons per year

The researchers found that methane emissions were high in the Swedish fjord, something US scientists have also recorded in Canadian fjords.

They estimate that emissions from all the world’s fjords are of the same magnitude (about 1 million tons per year) than the emissions contributed by all the 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 break down on its way to the surface, the researchers explain.

However, they add that if global warming leads to more extreme weather events, methane emissions may increase, but only up to a certain point, because if there is a significant increase in severe storms, methane emissions would decrease, because Anoxic environments at the bottom of fjords would disappear if the water mixes frequently.

Reference

High methane emissions from an anoxic fjord driven by mixing and oxygenation. Stefano Bonaglia et al. Limnology and Oceanography Letters, 20 May 2022. DOI:https://doi.org/10.1002/lol2.10259.

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