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”).  

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”.