Climate Gas Budgets Highly Overestimate Methane Discharge

There are effectively only two seasons in the High Arctic — a long winter and a milder summer season. Arctic Ocean bottom water temperatures vary greatly from winter to summer season. This is also evident in the study area offshore Svalbard. (Image: B. Ferré / CAGE, UiT)

The atmospheric concentration of methane, a potent greenhouse gas, has almost tripled since the beginning of industrialisation. Methane emissions from natural sources are poorly understood. This is especially the case for emissions from the Arctic Ocean. The Arctic Ocean is a harsh working environment.

That is why many of the scientific expeditions are conducted in the summer and early autumn months, when the weather and the waters are more predictable. Most extrapolations regarding the amount of methane discharge from the ocean floor are thus based on observations made in the warmer months.

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Oceanographer Benedicte Ferré, a researcher at CAGE Centre for Arctic Gas Hydrate, Environment and Climate at UiT, The Arctic University of Norway, said:

A frozen lid on top of large methane accumulations

The study was conducted west of the Norwegian Arctic Archipelago Svalbard — an area affected by a branch of North Atlantic Ocean current called the West Spitzbergen Current. The observations were made at 400 meters water depth, where the ocean floor is known for its many methane seeps. Ferré said:

The area in question is at the limit of the so-called gas hydrate stability zone. Gas hydrates are solid, icy compounds of water and, often, methane. They remain solid beneath the ocean floor as long as the temperatures are cold and the pressure is high enough. The bottom water temperatures affect the extent of the boundary of this stability zone. Ferré added:

Seasonal changes strongly affect methane consuming bacteria

Luckily, more than 90 percent of the methane released from the ocean floor never reaches the atmosphere. Partly due to the physical properties of the ocean itself such as currents and water column stratification. Methane is also consumed by specific bacteria (methanotrophs) in the water column. These are greatly affected by the seasonal variations described here. To a surprising extent. Co-author of the study published in Nature Geoscience, Helge Niemann, professor in geomicrobiology at Royal Netherlands Institute of Sea Research (NIOZ), said:

The seasonal changes have been important for understanding primary production in the ocean for a long time. But biogeochemical processes, such as methane oxidation by bacteria, have not been considered to be strongly influenced by seasonal changes.

“In this paper we prove that assumption wrong,” states Niemann. The next step is to do more winter cruises to account for seasonal changes related to West Spitzbergen current all the way from the Norwegian Arctic to East Siberian shelf.

Potential tipping point

How methane will react in future ocean temperature scenarios is still unknown. The Arctic Ocean is expected to become between 3°C and a whopping 13°C warmer in the future, due to climate change.

The study in question does not look into the future, but focuses on correcting the existing estimates in the methane emissions budget. However, Ferré said:

Provided by: Center for Arctic Gas Hydrate, Climate and Environment [Note: Materials may be edited for content and length.]

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