Methane emissions debate

A controversial study of Greenland ice cores reports fossil methane emissions from human activity as far higher than previously esti­mated. It suggests scientists have underestimated the quantities of methane being released from oil and gas operations (Nature, doi:10.1038/s41586-020-1991-8).

Methane is blamed for one-quarter of the warming of the atmosphere since pre-industrial times. It can be released from burning and transporting coal or gas, but also from two types of natural reservoirs: geological fossil sources like volcanoes and rumi­nant animals and decaying plants.

When animals or plants take up carbon from the atmosphere, it contains some carbon-14. However, this radioactive form of carbon decays, so after 50,000 years there is none left in fossil methane sources. This allows re­searchers to distinguish between ancient methane from fossil fuels and geological features, and newer methane from plants and animals.

But this doesn’t allow researchers to distinguish the fossil methane released naturally from geological sources, and the methane released by today’s fossil fuels. To discover how much fossil methane is not due to human activities, researchers checked ice cores from pre-industrial times. ‘We took ice core samples at 100 to 130m depth,’ says Benjamin Hmiel, an atmospheric chemist at Rochester University, New York. ‘To get enough methane from it for C-14 analysis, requires about 1000kg of ice.’

Their measurements found that natural methane releases to the atmosphere were only about 1.6 teragrams/year, with a maxi­mum of 5.4 teragrams/year, 200 to 300 years ago. (A teragram is 1m t.) This is a surprisingly small fraction of the fossil methane emission estimates today of 172 to 195 teragrams/year of methane.

This flags an underestimation of anthropogenic methane emis­sions at 38 to 58 teragrams/year of methane, or 25 to 40% more than recent estimates. ‘The total methane is still conserved, so the anthropogenic slice of the pie has to be higher to account for the methane budget,’ says Hmiel.

‘This is a salutary reminder of how uncertain our account­ing of global methane emissions actually is,’ comments Dave Reay, University of Edinburgh, UK. ‘We knew fossil fuel extraction – including fracking – was a major part of global methane emissions, but this impressive study suggests it is a far bigger culprit in human-induced climate change than we had ever thought.’

However, others argue that Hmiel and coworkers have greatly underestimated natural geologi­cal emissions. ‘[These] results are hardly reconcilable with the emission factors experimen­tally assessed and applicable to more than 3000 gas-oil seeps, more than 740 mud volcanoes, more than 30 active submarine degassing regions, diffuse degas­sing over petroleum fields and exhalations from more than 2300 geothermal-volcanic systems,’ comments Giuseppe Etiope at the National Institute of Geophysics and Volcanology in Italy.