Two greenhouse gases, carbon dioxide and methane, contribute the most to global warming. If we're going to control them, we need to understand where they come from and how they end up in our atmosphere. That has been relatively easy for carbon dioxide, but we still lack a solid understanding of the global atmospheric methane budget.

Recently, a team of researchers has undertaken a new investigation into the global atmospheric methane budget. The researchers found that, contrary to expectations, emissions of methane from fossil fuel production appear to be dropping. But the overall flux of methane is much larger than we'd thought.

Main methane contributors

Until now, the global fossil fuel industry has been thought to contribute 15 to 22 percent of the total methane emissions. This industry includes both the production and use of energy sources including natural gas, oil, and coal. Atmospheric methane also comes from natural sources, both biological and geological. Unfortunately, natural and human-driven sources are often located in the same geographic areas. This presents a problem for anyone trying to piece together the global atmospheric methane budget.

In order to reevaluate the methane budget, the team compiled a database of the isotopic signatures of different methane sources. Methane released from coal will have a different ratio of carbon isotopes than the methane that comes from biological sources, and so on; these signatures can be used to help identify where methane has come from.

Their expanded database contained information on the country-specific carbon isotope distribution based on oil, natural gas, and coal, providing a solid data set to identify contributions from the fossil-fuel industry. In addition, the database included multiple natural sources: more than 1,000 isotopic carbon samples from wetlands, termites, ruminants, rice agriculture, and waste/landfill. It also contained over 1,000 measurements on C3- and C4-plants (which use carbon differently) based on global vegetation and biomass-burning fluxes.

The scientists applied specific atmospheric models that leveraged this expanded database in order to re-evaluate where the methane was coming from, starting their analysis in the 1980s.

New insights

The researchers concluded that the total atmospheric methane flux is about 60 to 110 percent greater than current estimates. The increase comes from both natural and human activities.

The team’s analysis revealed that the total fossil fuel methane emissions have not been increasing over time, as has been previously thought, but are actually decreasing. The team attributed this large difference to large revisions in carbon isotope source signatures. Still, methane emissions from the fossil fuel industry, including both production and use of natural gas, oil, and coal, are 20 to 60 percent greater than was previously thought.

The team took its analysis one step further, breaking down the budget by energy source. From this evaluation, they found that methane emissions from natural gas as a fraction of production have declined by two to eight percent.

They also broke down the atmospheric methane budget by source, finding that methane emissions from microbial sources has steadily increased over time. This figure includes both microbial emissions due to wildlife and those produced by agriculture and waste.

These findings suggest that the ongoing fight to reduce greenhouse gas emissions, which has resulted in regulatory changes across multiple industries, may actually be working. However, the analysis as a whole suggests that there is still a lot of room for better efficiency in the fossil fuel industry, as the methane emissions from natural gas have only declined marginally since the 1980s.

Of course, there's room for efficiency further down the line. If end users were more efficient about how they used natural gas and gas-generated electricity, there'd be less in the production and distribution networks to begin with.

Nature, 2016. DOI: 10.1038/nature19797 (About DOIs).