Atmospheric methane (CH4) concentration plateaued leading up to 2006, but began to rise again thereafter by about 8 parts per billion per year or 25 million tonnes of CH4 per year. There have been several explanations of the increase. Some studies have found that rising methane emissions from oil and gas operations are a key source. Others have found evidence that it is primarily due to increased methane emissions from biogenic sources such as agriculture, especially livestock, or tropical wetlands. A recent study revisits the contributions to the methane increase from various sources including biomass burning, fossil fuels and biogenic sources such as agriculture and tropical wetlands and suggests that the increase can be ascribed to increased fossil fuels emissions and biogenic sources in roughly equal measure. However, more recent research suggests that this is not conclusive and the source of the increasing methane remains an open question.
The evidence used to identify the source of the increase in atmospheric methane of 25 million tonnes of CH4 per year is based on topdown or bottomup observations. Two types of topdown observation have been used to identify the source of the methane increase. When methane is released from natural gas and oil operations, it is accompanied by the releases of other hydrocarbons including ethane. Tracking the rise in ethane in the atmosphere provides a way of identifying how much of the increase in atmosphere methane can be assigned to oil and gas operations. Another set of topdown observations that can help identify the source of the increase is the change with time in the ratio of methane with the carbon-13 isotope (13C) to methane with carbon-12 (12C). Biogenically produced methane has a smaller ratio of 13C/12C than does methane produced from oil and gas operations. The observed change in the ratio since 2006 reveals a decrease in the heavy isotope content of methane which points toward a biogenic source.
A recent study has estimated that contribution of oil and gas operations to the increase in atmospheric methane by measuring atmospheric ethane as a tracer for this source of methane emissions. After years of weak decline the ethane time series showed in 2007 a sudden onset of a significant positive trend. Based on this result the oil and natural gas emission contribution to the methane increase was estimated to be at least 39 %.
Another recent article studied the 13C/12C ratio over the last few decades and found that the increased atmospheric methane since 2006 is predominantly biogenic in origin. The study concluded that fossil-fuel emissions stagnated or diminished in the 1990s and were a minor contributor to the renewed CH4 rise. It suggests that either food production or climate-sensitive (biomass burning) natural emissions were the most probable causes of the current CH4 increase.
These apparently incompatible results have been resolved in a new recent study that revisited the estimated contribution from biomass burning. Using satellite measurements it was found that methane emissions from biomass burning had dropped significantly post-2006. After updating both the total and isotopic budgets for atmospheric methane with these revised biomass burning emissions it was found that fossil fuels have contributed an increase of 12–19 million tonnes of CH4 per year and biogenic sources an increase of 12–16 million tonnes of CH4 per year. The new estimate of the biomass contribution brings the estimates of the fossil fuel and biogenic contributions based on atmospheric ethane and on the 13C/12C ratio into agreement.
A bottomup study has recalculated livestock methane emissions factors for enteric fermentation in cattle, and for manure management in cattle and swine. Using the new emissions factors, researchers have found that in 2011 the global estimates for annual livestock CH4 emissions comprise one-fifth of total annual global methane emissions. They calculate that the annual increase in livestock emissions accounts for one half to three-fourths of the global increases from all sources since 2007.
In a related study which analyzed GOSAT satellite observations of atmospheric methane, it found that U.S. methane emissions have increased by more than 30% over the 2002–2014 period. It was estimated that growing U.S. anthropogenic methane emissions could account for 30–60% of the renewed growth in global atmospheric methane. The satellite results revealed that the spatial distribution of CH4 emissions over the U.S. was largest in the central part of the U.S. However, the limited resolution of the spatial pattern of the methane increase seen by satellite was not able to distinguish the source of the methane because of the geographic overlap of livestock and oil and gas production in the U.S.
Research has shown that the proportion of carbon-13 in methane has been decreasing since 2007. Since carbon-12 is taken up preferentially by photosynthetic processes, the decreasing carbon-13 fraction suggests biogenic sources such as wetlands or agriculture including livestock as opposed to fossil fuel burning or biomass burning. Alternatively, a shift from biomass burning to fossil fuels may contribute, for example, there is a major program in India to replace solid fuels with gas for indoor use. There is evidence that the source of the increased methane could be tropical wetlands where record high temperatures increased methane production. Another possibility is reduced chemical reactions in the atmosphere, primarily with hydroxyl ions, that destroys methane.
Hausmann, P., Sussmann, R., and Smale, D.: Contribution of oil and natural gas production to renewed increase in atmospheric methane (2007–2014): top–down estimate from ethane and methane column observations, Atmos. Chem. Phys., 16, 3227-3244, https://doi.org/10.5194/acp-16-3227-2016, 2016.