A new study, led by scientists from NASA, has shown that rising emissions from the oil and gas industry are mainly responsible for the increase in methane in the atmosphere.
After remaining fairly stable around the year 2000, atmospheric methane levels began rising in 2006.
This rise could be significant for climate change because methane is a powerful greenhouse gas. It breaks down in the atmosphere more quickly than carbon dioxide but it has a greater warming effect. Scientists estimate that over 20 years it has a global warming potential that is 86 times greater than that of carbon dioxide.
Researchers have long disagreed on the cause of rising methane levels.
After studying atmospheric methane levels around and over drilling sites, some scientists have argued that the fracking industry is principally to blame, with the rise in methane levels corresponding with the rise in fracking activity in the United States. Taking methane emissions into account, Robert Howarth has suggested that shale gas may have a larger greenhouse gas footprint than burning coal.
Other studies of methane molecules in the atmosphere have suggested that the rise could be due to increased microbial activity in rice paddies due to a warming climate.
If both explanations were right, the observed methane level in the atmosphere should be substantially higher than it actually is. This led some researchers to argue that only one of the suggested causes could be correct.
The new NASA research shows that both explanations are correct.
Global fires – another source of methane
Methane levels in the atmosphere are also affected by the area of land burned in fires. Satellite surveys suggest that the land area burned dropped by 12% between the early 2000’s and the period 2007 – 2014. This would suggest that methane emissions from burning would also have dropped by 12%, and scientists took this into account when calculating the expected level of methane in the atmosphere.
However, the NASA team found that the actual drop in methane from burning was almost twice what they had expected. Taking this bigger drop into account, and adding predicted increases from both fossil fuels and wetlands, they found that the calculated methane levels now matched the observed levels in the atmosphere.
Identifying methane sources
Key to the research was piecing together clues to determine the source of methane molecules. Carbon isotopes, variations in the carbon atoms in the methane, are one indicator. Methane molecules resulting from land burning contain more heavy carbon isotopes, microbial methane emissions have the fewest. Emissions from shale gas and other gas activities usually contain ethane as well as methane, while land burning causes increased carbon monoxide concentrations.
Using these clues, researchers were able to conclude that:
- Fossil fuel methane emissions are rising at 17 teragrams (Tg) per year
- Wetland and rice farming emissions are rising at 12 Tg per year
- Emissions from land burning are falling at 4 Tg per year
Adding these figures together gave an annual overall increase of 25 Tg per year, matching the observed behaviour.
(A teragram is a thousand million kilograms which is (according to NASA) the weight of 200,000 elephants. Total methane emissions are currently around 550 Tg a year)
The report notes that
“The required FF [Fossil Fuel] emission enhancement found here is substantially larger than in previous literature”.
Previous studies had concluded that fossil fuel methane emissions were rising at 5.5 Tg per year, rather than 17 Tg found in this report.