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Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO₂). Its concentration in the atmosphere has increased more than twice since the preindustrial era due to enhanced emissions from human activities. While the global warming potential of CH4 is 86 times as large as that of CO₂ over 20 years, it stays in the atmosphere for about 10 years, much shorter than more than centuries of CO₂. It is therefore expected that emission controlling of CH4 can benefit for relatively short time period toward the Paris Agreement target to limit the global warming well below 2 degrees.

A study by an international team, published in Journal of Meteorological Society of Japan, provides a robust set of explanations about the processes and emission sectors that led to the hitherto unexplained behaviors of CH4 in the atmosphere. The growth rate (annual increase) of CH4 in the atmosphere varied dramatically over the past 30 years with three distinct phases, namely, the slowed (1988-1998), quasi-stationary (1999-2006) and renewed (2007-2016) growth periods (Fig. 1). No scientific consensus is however reached on the causes of such CH4 growth rate variability. The team, led by Naveen Chandra of National Institute for Environmental Studies, combined analyses of emission inventories, inverse modeling with an atmospheric chemistry-transport model, the global surface/aircraft/satellite observations to address the important problem.

They show that reductions in emissions from Europe and Russia since 1988, particularly from oil-gas exploitation and enteric fermentation, led to the slowed CH4 growth rates in the 1990s (Fig. 2), where reduced emissions from natural wetlands due the effects of Mount Pinatubo eruption and frequent El Niño also played roles. This period was followed by the quasi-stationary state of CH4 growth in the early 2000s. CH4 resumed growth from 2007, which were attributed to increases in emissions from coal mining mainly in China and intensification of livestock (ruminant) farming and waste management in Tropical South America, North-central Africa, South and Southeast Asia. While the emission increase from coal mining in China has stalled in the post-2010 period, the emissions from oil and gas sector from North America has increased (Fig. 2). There is no evidence of emission enhancement due to climate warming, including the boreal regions, during our analysis period.

These findings highlight key sectors (energy, livestock and waste) for effective emission reduction strategies toward climate change mitigation. Tracking the location and source type is critically important for developing mitigation strategies and the implementation the Paris Agreement. The study also underlines need of more atmospheric observations with space and time density higher than the present.

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