RESEARCH HIGHLIGHT

Arctic Ocean Methane Contributes to Global Warming

July 10, 2006

Global warming and its effects on the environment and human life are being studied by scientists at the International Arctic Research Center. One primary effect of global warming is an increase in the average global temperature due to the greenhouse effect. Special attention is being given to methane, a greenhouse gas that warms the earth 23 times as much as the same amount of carbon dioxide (for a given weight averaged over a 100 year time period). In 1998, the average concentration of methane at the Earth's atmosphere was measured at 1.745 ppb with higher concentrations in the northern hemisphere where most sources (both natural and anthropogenic) are larger. Even higher concentrations were registered over the Arctic/Sub-Arctic regions where human activities are considered to be negligible (Figure.1). It is the widespread opinion that water-saturated terrestrial arctic ecosystems (wetlands) are primarily responsible for the higher concentrations of methane over the Arctic region. During the winter, however, the wetland ecosystems are dormant whereas the concentrations of methane remain high. Scientists from Dr. I. Semiletov's group surmised that there must be another major natural source of methane in the northern hemisphere and considered it might be the Arctic Ocean.

Until recently it was thought that bacteria, which inhabit marine sediments and produce methane, could not survive under the severe conditions of the Arctic Ocean and that methane could not reach the atmosphere where the ocean is covered with sea ice. Recent data, however, shows that the arctic microorganisms are well adapted to the extreme temperatures and can produce methane in marine sediments all year round. During the cooling periods of the climatic cycle this methane becomes preserved in form of gas hydrates (methane gas associated with ice) beneath the sea bottom. Gas hydrates deposits are the largest natural source of methane and during the warm epochs their stability becomes vulnerable due to the temperature increase. They remain stable under the bottom of the Arctic Ocean as long as they are sealed by sub-bottom permafrost (annually frozen sediments).

During fieldwork conducted in the East-Siberian Sea in the Arctic Ocean, IARC scientists Dr. I Semiletov and Dr. N. Shakhova learned that sub-bottom permafrost in this region is not as stable as it was once considered. The summertime (Figure.2) and annually averaged temperature of bottom sediments are slightly above 0°C. Methane producing bacteria are able to keep themselves viable even while preserved in permafrost. Upon thawing they restore activity and begin to produce a lot of modern methane from ancient organic matter preserved within the permafrost. Together with ancient methane coming from gas hydrates deposits this methane is responsible for anomalously high concentrations of methane measured within the water column. The geological structure of the ocean bottom creates various pathways that allow methane to pass through permafrost and get into seawater. Such pathways come from geological faults, boundaries of tectonic plates, taliks of submerged thermokarst lake, paleo-river valleys, sea lagoons and ancient volcanoes. Methane is released into the water in the form of large buoyant bubble groups called flares that float to the surface of the water and accumulate beneath the sea ice. Through ice brines and polynyas (areas of open water surrounded by sea ice, see Figure.3) the methane is released into the atmosphere. During the period of ice break-up, methane release into the atmosphere abruptly.

Figure 1 (click on image for Wikipedia reference)

Figure 2

Figure 3