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The North Sea: high levels of methane emissions

6/14/16

A team of researchers from Belgium has identified important concentrations of methane in the surface waters of the North Sea, mainly near the Belgian and English coasts. In order to understand the origins of this methane concentration, it is necessary to go back 16,000 years in time when forests and peatlands connected England and Ireland to continental Europe. Trapped in marine sediment today, this organic matter produces methane which is easily released into the atmosphere from the shallower zones of the basin. This ground-breaking study includes the coastal regions in the quantification of the methane cycle. This quest was made even more difficult by the many sinks and sources of this hydrocarbon of both anthropogenic and natural origin. A better understanding of methane, the second most efficient greenhouse gas after carbon dioxide could be key to slowing down climate change.

A team of researchers has measured a significant quantity of methane dissolved in the surface waters of the North Sea. This strong concentration was observed mainly along the Belgian and English coasts. “We recorded up to 1000 nanomoles of methane (CH4) per litre, which represents 16 milligrams per cubic metre”, explains Alberto Borges, of the Chemical Oceanography Unit of the University of Liege and the leading author of the recently published work in the Scientific Reports journal(1). “This figure may seem insignificant, but  the ocean surface waters contain an average of 3 nanomoles of methane per litre. This represents a concentration that is 300 times weaker but the fact that methane has such potential to cause climate change means that it contributes significantly to the phenomenon”. These measurements were taken during the course of six research campaigns that were conducted between 2010 and 1013. These campaigns enabled scientists to highlight a seasonal variation in the concentration of methane and therefore a direct influence on warmer seasons corresponding to an increase in emissions of this gas. 

Mer du nord1

Peat trapped in sediments 

 In order to understand the singular nature of this source of methane it would be necessary to travel back in time: 16,000 years into the past, when the North Sea and the entire Celtic Sea were covered in peatlands and forests and then to travel to a point in the Atlantic Ocean beyond the westernmost part of the current island of Ireland. “This was a time of significant glaciation. Sea levels were much lower than today. Sea levels then became progressively higher but 7,000 years ago the Southern Bight of the North Sea was still dry”.

While the sea began to cover these lands again over the millennia, layers of sediment accumulated, covering and trapping these large peatlands. “There are therefore large quantities of organic matter trapped in the sediments”, continues the oceanographer. “This organic matter was degraded by bacteria. In oxygenated environments, the bacteria that destroy organic matter consume the oxygen. From a thermodynamic point of view, this is the most efficient way. Therefore the more oxygen there is, the more competitive the aerobic bacteria are”.  But in an environment where oxygen is not renewed in the deepest layers of sediment, they consume all the oxygen and give way to other bacteria which develop in anaerobic conditions (absence of oxygen). New reactions are then created and these degrade the organic matter. At the end of this chain, fermentation occurs and this is the least effective way of breaking down organic matter. This fermentation produces methane. This is what occurs in the digestive systems of cattle and termites and is also what happens in humid areas saturated with organic matter such as marshes, flood plains, rivers or in these peatlands trapped under the sea floor.  (Read First report on GHG emissions from African rivers)

The water column trap

This production of methane generates pockets of gas which are progressively released into the water column. Only waters close to the coasts show abnormally high levels of this gas. However, this biomass is not confined to the Belgian coast but to all lands that were above water in the past. “Other gas pockets associated with peat were identified in the northern section of the North Sea as is the case at Dogger Bank, but in deeper waters. In summer, the surface waters heat up while the lower layers remain cold. This difference in density linked to temperature creates a stratification, a cover-lid through which the waters do not mix with each other. This is known as the thermocline. The methane remains trapped in the deeper layers, is transported laterally and is not released into the atmosphere”.

When it approaches the Belgian and English coasts, this peat accumulates at depths of between 10 and 30 metres which is quite close to the surface.  The tidal currents are strong there and mix the different water column. In other words, the surface waters are heated but the water column is not stratified. This heat is redistributed from the surface down to the sea bed. Instead of leading to the creation of a cold layer and a warm layer, lukewarm water therefore extends from the surface to the seabed. The methane released from the sediment easily reaches the surface and can be released into the atmosphere. This phenomenon is likely to accelerate with climate change. Not only do the warmer waters activate the bacterial production of methane, but they also enable it to be released more easily. 

This is one of the first times that a study has shown the behaviour of methane in permanently-mixed bodies of water that are rich in organic matter such as the waters of the Belgian coast. This study therefore marks the beginning of a new avenue of research into the methane cycle and includes the coastal environments and the shallower areas in general. “We knew that the coasts played an important role in the marine methane cycle, but this remains difficult to quantify. One coast is not the same as another, these are particularly heterogeneous environments. However, we estimate that today the coastal areas alone constitute a source of methane that is 5 to 10 times higher than all the deep oceans”. This is an advance that will make it possible to fine-tune knowledge about the contributions of the different sources of methane which remain imprecise today due to the fact that the methane cycle is so complicated to quantify.

CH4  concentration North Sea 

A global ecological problem…

The emission of methane, in terms of mass, is much less significant than that of carbon dioxide (CO2). But in terms of global warming, the molecules of CH4 are twenty-five to thirty times more efficient than CO2. This rather competitive factor means that methane as a greenhouse gas, is second only to CO2. It is responsible for one third of global warming linked to human activity. In addition, methane is a gas which has many sources and sinks to the extent that its cycle easily defies attempts to quantify it. It is therefore difficult to establish a clear estimate of the sources of emission of this gas and to act accordingly. One piece of data is very solid however: the total accumulation of methane in the atmosphere. “This is quite easy to calculate, says Alberto Borges. “We know the volume of the atmosphere. By measuring the concentration of methane in the atmosphere over time, we can determine its accumulation”. 

Studies on methane aim to include the sources and sinks identified above with this overall accumulation of methane in order to better identify the causes of this known consequence. The overall picture is like a balance sheet with deposits, the natural and man-made sources, and withdrawals, the sinks which consume the gas. Each new piece of data identified must be included in the balance sheet. “These are therefore just rough estimates”, comments the researcher, “But they are becoming more and more precise and enable us to understand the main aspects of the methane cycle and to identify the most polluting activities. For example, the fourth report by the IPCC, which dates from 2007, estimates the level of emission of methane to be 143 million tonnes per year.  Two thirds come from humid zones such as marshes or flood plains. But these natural sources have been working in the same way for millennia and therefore do not contribute to an increase in CH4 emissions into the atmosphere. What interests us even more, are the anthropogenic sources that contribute to an increase in greenhouse gases. These are estimated to be around 358 million tonnes per year. 106 million tonnes are due to the extraction of fossil fuels, 81 million tonnes are due to cattle, 61 million tonnes due to dumping of waste, 60 million tonnes from rice-growing and finally, 50 million tonnes are due to the combustion of the biomass such as forest fires, for example”. 

The anthropogenic part played in methane sources therefore accounts for 70% of its total emission. According to the same report, the methane sinks absorb 515 million tonnes each year. These sinks are of different sizes. The troposphere is the main sink, with 445 million tonnes each year. The methane released into the atmosphere is quickly broken down by oxidation. There is a stratospheric loss of 40 million tonnes and oxidation by soils of 30 million tonnes. “By comparing these figures, we can conclude that in 2007, the atmosphere accumulated 14 million tonnes of methane. This figure must today be revised upwards to be closer to 30 million tonnes”. Today we are back to a level of methane emission equivalent to that of the nineteen-eighties for several reasons, particularly the extraction of shale gas (See article: “A high price to be paid for shale gas”).  

Tourbe Mer du nord

An estimate tinged with hope

This figure, which indicates a growth in the quantity of an outrageously efficient greenhouse gas, is observed by Alberto Borges with a note of optimism. “The largest sources of methane are in the order of around one hundred million tonnes while the balance sheet is of the order of around ten million tonnes. The scales are not the same. This difference in magnitude indicates that a small variation in the sources or sinks of the gas can cause a relatively significant variation in the overall quantity. And as these sources are very numerous, it is possible to take action on several levels. Conversely, if we take carbon dioxide as an example, its principal source is the industrial combustion of fossil fuels. As citizens, we can have an important influence on some of its sources”.  The emissions due to cattle, for example, are three times higher than the current accumulation of methane in the atmosphere. By reducing our consumption of meat by one third, we would succeed in reversing the trend and would create a negative balance sheet. That is to say that the system would consume more methane than it would emit. It is also possible to imagine better recycling management or to find ways of producing rice that cause less methane emissions or better still, to imagine a combination of these different measures. 

The impact would be all the more rapid and significant given that the methane cycle is particularly dynamic. “This is due to its molecular composition. Methane is made up of an atom of carbon and four atoms of hydrogen. The atomic bonds between hydrogen and any other atoms are very high in energy. Methane is therefore very reactive and rapidly oxidises in the atmosphere”. This property makes a big difference. The atmosphere needs one hundred years to eliminate a molecule of CO2, it only requires ten to eliminate a molecule of methane. Within a ten-year period, we could already benefit from measures taken today. “As individuals, we often have the impression that these phenomena are so vast that we cannot play a significant role. But by putting all these figures into perspective we realise that more demanding scenarios could reduce or stop the accumulation of methane in the atmosphere. To my eyes, methane is one of the most promising keys to efficiently slow down global warming in the short-term”. 

(1) Alberto V. Borges, Willy Champenois, Nathalie Gypens, Bruno Delille, Jérôme Harlay, Massive marine methane emissions from near-shore shallow coastal areas, Scientific Reports, 6:27908, doi:10.1038/srep27908. 


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