• Methane is a greenhouse gas 20 times more potent than CO₂ on a century scale, according to our partner The Conversation.

  • 40% of methane emissions on Earth are of natural origin and occur mainly in wetlands (peat bogs, mangroves, swamps, etc.), melting permafrost areas, but also at seabed level.

  • This analysis was conducted by Livio Ruffine, researcher in physico-chemistry at the French Research Institute for the Exploitation of the Sea (Ifremer).

We are a long way from the spectacle of the boiling Baltic Sea following the methane leaks on the Nord Stream gas pipelines.

But this image shows the meticulous work of an Ifremer diver who, equipped with a sampler, measures the flow rate of a natural gas outlet consisting of 98% methane.

It was taken during a scientific campaign in September 2022 in Varna, on the Bulgarian coast in the Black Sea, as part of a study on the impacts of methane on the climate and marine ecosystems.

Less known than carbon dioxide (CO₂), methane is a greenhouse gas 20 times more potent on a century scale, 80 times more potent on a 20-year scale.

It is commonly accepted that 60% of methane emissions on Earth are linked to human activities (agriculture, energy, waste, etc.), and that 40% are of natural origin.

These natural emissions occur mainly in wetlands (peat bogs, mangroves, swamps, etc.), melting permafrost areas, but also at seabed level.

This is what interests the team of the METZE campaign which recently took place in the Black Sea.

​Dissolved methane in the sea

Because the Black Sea is the most important basin of dissolved methane on the planet: it is the receptacle of several large rivers of Europe and Asia (Danube, Dnieper, Dniester, Don, Kizilirmak, etc.) and of all matter natural organic matter, derived from plants and animals, which they carry without forgetting that produced by the surrounding anthropogenic activities, agriculture, agri-food industry, chemical industry... All this organic matter is deposited at the bottom, sediments and decomposes, which, over time time, generates methane.

Several studies testify to a high density of natural gas emissions in the Black Sea, from the coast to the deepest areas (more than 2000 meters).

Today, our team is trying to map these areas, count the gas outlets, measure their flow, and find out what happens to this gas in the water and in the air.

Indeed, once emitted into seawater, methane reacts with dissolved oxygen and consumes it to produce bicarbonate (a polyatomic ion, HCO3-).

This can cause local acidification as well as deoxygenation of seawater. These impact ecosystems and threaten marine biodiversity if the natural carbon cycle is disturbed by human activities (additional supply of organic matter for example) or climate change.

You should know that in the Black Sea, fish are condemned to live in the first 150 to 200 meters deep, below, the water is devoid of oxygen.

The Black Sea is the most oxygen-poor sea on the planet and methane emissions could accentuate this trend.

Studying methane emissions is therefore also useful to better understand the physico-chemical processes that cause deoxygenation in the Black Sea and to assess its consequences on fish populations.

​Who can join the atmosphere

Depending on the intensity of the emissions, a greater or lesser part of the methane is transferred into the atmosphere and contributes to its warming.

To understand all of these cascading phenomena and their impacts, we need to take stock of methane emissions in these areas and understand how and to what extent certain environmental factors (seasons, extreme events such as storms, etc.) affect flow rate variability and transfer to the atmosphere.

This information will make it possible to improve climate models, but also to better assess the resilience of marine ecosystems and thus put in place appropriate procedures for their preservation, in particular by limiting the supply of anthropogenic organic matter.

OUR “GREENHOUSE GASES” FILE

Beyond natural methane emissions from the Bulgarian coastline, most Black Sea methane is trapped in deep sediments in the form of “ice cubes” called gas hydrates.

Their proven melting worries scientists, because it accelerates under the effect of salt water infiltration into the sediment (salt water destabilizes hydrates and inhibits their formation), accentuating the release of methane into the water. .

Many other underwater sites of natural gas emissions are known in the world and in particular closer to us in the Bay of Biscay.

This analysis was written by Livio Ruffine, researcher in physico-chemistry at the French Research Institute for the Exploitation of the Sea (Ifremer).



The original article was published on The Conversation website.


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