Since 2009, the quantity of ethane in the atmosphere has been increasing by 5% per year. Prior to this, concentrations of the gas had actually been decreasing over a twenty-year period thanks to successful political initiatives. This gas tends to lead to peak ozone levels in the air we breathe and increase the atmospheric lifetime of greenhouse gases. Ethane essentially comes from the exploitation of oil and natural gas. It is also a good indicator of anthropogenic emissions of methane. A multiple study initiated by the University of Liege using measurements and modelling has clearly identified the main cause of the increase of ethane in the atmosphere: the sudden expansion of shale gas exploitation in the US.
The observation occurred almost anecdotally. Whitney Bader and Bruno Franco had just improved the technique for measuring atmospheric ethane from the Jungfraujoch station. Together with Emmanuel Mahieu, a researcher at the FNRS and head of GIRPAS (InfraRed Group of Atmospheric and Solar Physics of the University of Liege), they re-analysed and updated their ethane time series. It was at that moment, as they were observing the data gathered that they noticed an intriguing trend upturn. “At the Jungfraujoch observatory”, explains Emmanuel Mahieu, “We have been collecting atmospheric data since the mid-1980s. This data has enabled us to study the stratosphere and the troposphere, and therefore the quality of surface air. And since then, we have observed a progressive decrease in the burden of ethane (C2H6) of 1% to 2% per year. This decrease was due, in particular, to efficient political measures”. Certainly, this environmental awareness placed restrictions on the behaviour of industrial organisations, such as, for example, prohibition of the use of CFCs (see article: New threat to the ozone layer?).
“Because this decrease remained constant”, continues the researcher, ”We were quite casual in our approach to checking ethane levels until our improved time series alerted us to a startling trend reversal. Since 2009, the concentration of ethane in the troposphere has been increasing by 5% per year”. Having made this observation (see article, the butterfly effect of shale gas), the team could not remain in possession of this data without investigating further. The researchers had lots of theories, but they had no proof. In order to verify their theories the researchers needed to match their observations with models. The result of this work has just been published (1). The result was unequivocal and the cause was clearly identified. The increase in the concentration of ethane is directly linked to exploitation of shale gas by means of hydraulic fracturing.
A harmful gas for the environment
“One of the beneficial components of the atmosphere is the hydroxyl radical (OH)”, explains Emmanuel Mahieu. “We could call it the detergent of the troposphere because it is a highly reactive molecule which consumes other elements by oxidising them. Among these elements is methane (CH4), for example, which is an even more efficient greenhouse gas than CO2”. Ethane consumes this hydroxyl radical. It becomes oxidised to form carbon monoxide with the carbon it contains. Therefore, the higher the quantity of ethane in the atmosphere, the less hydroxyl radical there is to consume the methane and the longer the lifetime of the latter will be. A second harmful effect concerns the quality of the air, because ethane, during its degradation, promotes the formation of tropospheric ozone (O3). “And we have all heard warnings about ozone peaks in summer. While it is true that the ozone present in the stratosphere protects us from the UV rays of the sun, in the troposphere and the surface air, it is, in excess of a certain threshold, harmful not only for health but also for the development of plants etc.”
Ethane is a gas which is essentially emitted due to human activity. Two-thirds of its emission is caused by the exploitation and transportation of natural gas. This natural gas, trapped in rock, results from a mixture of hydrocarbons which notably include methane, ethane and propane. Every gas leak simultaneously releases all of its compounds. The last third is divided between the consumption of bio-fuels and combustion of the biomass (forest fires, for example). “As its emission is essentially anthropogenic, government initiatives taken to reduce emissions were immediately effective. Following this, each year the levels were decreasing. The atmosphere consumed more ethane than was being emitted up to 2009. But since then, we have wiped out 30 years of effort in just a few years”. This sobering fact was the object of the first publication. It still remained to understand what was happening, to identify the sources and quantify them. “We did not know what this increase represented in tonnes of gas emitted, for example”.