Fossil-fuel-related emissions of methane, a potent greenhouse gas, have been miscalculated and may be twice as high as previously thought. Researchers say the emissions have been at this high level for the past three decades (Nature 2016, DOI:10.1038/nature19797).
However, the researchers find that total fossil-fuel-related methane emissions, although previously underestimated, remained relatively stable between 1985 and 2013, despite an increase in oil and natural gas drilling and production activities.
The study provides another piece in a puzzle of global methane emission sources and emission levels. Methane is the primary component of natural gas and a by-product of oil and coal production and use. It is also released through agricultural practices and decay of organic material. The hydrocarbon has a global warming potential 28 to 36 times that of carbon dioxide over 100 years, according to the Environmental Protection Agency.
Researchers used a combination of atmospheric measurements and a detailed data set of long-term global methane emissions coupled with methane carbon isotope records. They found total fossil-fuel methane emissions from industry and leakage from natural geological sources are 60 to 110% greater than past estimates and inventories.
Methane emissions from natural gas, oil, and coal production and use are 20 to 60% higher than previously thought, the study says. Seepage of the gas from geological formations was also higher than earlier calculations.
Leaked methane as a percentage of natural gas production shows an 8% decline from around 30 years ago to some 2% today, according to Stefan Schwietzke, the study’s lead author and research scientist with the Cooperative Institute for Research in Environmental Sciences.
But, Schwietzke adds, methane emissions globally have increased by about 5% per year since 2007. Contribution from fossil fuels appears to remain flat, however.
“It helps to see absolute methane emissions from all sources as a pie,” Schwietzke explains. “Our study found that the fossil fuel slice of the pie is larger than previously thought and suggests that the individual slices were wrong. But we know the total pie size pretty well.”
The study says improvements in the natural gas industry’s production methods may have already helped cut methane emissions, but more needs to be done because of the increase in natural gas production. Schwietzke points out that reduction in methane, because of its high global warming potential, can have an immediate and substantial impact on climate change.
A related study published separately (Global Biogeochem. Cycles 2016, DOI: 10.1002/2016gb005406) also identifies the increased methane emissions in the past decade. It concludes that they may be driven by biological sources—swamps, cattle, or rice paddies—rather than fossil-fuel emissions. That study notes that most of the emission increases have been from the hot, wet tropics.