Humans produce more methane emissions than previously thought

Methane is a potent greenhouse gas that may be contributing far more to climate change than previously thought. In a new paper published on 19 February in Nature, the authors suggest humans may be responsible for up to 40 per cent more methane emissions than previously estimated (1).

This presents an important opportunity: reducing methane emissions — for example, the methane that leaks out while producing and transporting oil and gas — might have a much greater impact on curbing global warming.

Methane is produced in two ways: fossil methane released from ancient hydrocarbon deposits through the extraction and burning fossil fuels; and biological methane released from natural sources like wetlands or anthropogenic sources such as landfills, rice paddies, and livestock. Together, these emissions are responsible for about 25 per cent of the global warming that’s happening. But anthropogenic methane emissions may play a much bigger role than expected.

Over the past 300 hundred years, methane emissions to the atmosphere have increased by around 150 per cent, according to the authors, but until now, pinpointing the exact source of methane emissions — humans or natural — has been difficult since the gas is emitted naturally and from human activities.

“As a scientific community we’ve been struggling to understand exactly how much methane we as humans are emitting into the atmosphere,” said Prof Vasilii Petrenko of the University of Rochester, who led the study. “We know that the fossil fuel component is one of our biggest component emissions, but it has been challenging to pin that down because, in today’s atmosphere, the natural and anthropogenic components of the fossil emissions look the same, isotopically.”

However, unlike natural methane sources, fossil methane does not contain the carbon-14 isotype. And this allowed the scientists to determine which proportion of methane originated from fossil sources and which were biological?

Armed with this information, the scientists looked to the past. They collected ice cores from Greenland, which act as time capsules: ancient air is trapped inside the ice. So, the researchers were able to study air samples from the eighteenth century (before the start of the Industrial Revolution) to the present day.

By measuring the chemical compositions of the ancient air, they found that all of the methane emitted to the atmosphere up until about 1870 was biological in nature. Then, they observed a sharp increase in the fossil fuel component that coincides with the rapid increase in the extraction and use of fossil fuels — and the new results are 25 to 40 per cent higher than previous estimates.

In other words, scientists have been vastly underestimating how much methane humans are emitting into the atmosphere via fossil fuel use. Therefore, lead author Dr Benjamin Hmiel said in a statement: “Placing stricter methane emission regulations on the fossil fuel industry will have the potential to reduce future global warming to a larger extent than previously thought”.

The new findings suggest methane is the second-largest contributor to climate change after carbon dioxide. However, this might actually be good news Hmiel explained: “Most of the methane emissions are anthropogenic, so we have more control. If we can reduce our emissions, it’s going to have more of an impact”.

(1) Hmiel, B. et al. Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions. Nature (2020). DOI: 10.1038/s41586-020-1991-8.