Methane Concentrations and Emissions over the past three decades: Human activity in energy exploitation and livestock farming drive the growth

◆ Surface, aircraft and satellite measurements show that concentration of greenhouse gas methane (CH₄) varied widely in the past 3 decades (1988-2016), with accelerated growth rates in the most recent decade.

◆ This study shows key CH₄ emission sectors and their long-term variations using inverse analyses with atmospheric chemistry transport modeling to contribute to effective emission reduction strategies.

◆ Emissions from oil and gas exploitation and natural climate events slowed down CH₄ growth rate (1988-1998) and led to the temporary pause (1999-2006).

◆ The renewed growth since 2007 is driven entirely by human activities, coal mining in China, livestock farming in South/Southeast Asia, South America, Africa regions.

An international research group led by Naveen Chandra of National Institute for Environmental Studies (NIES), Tsukuba, and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka performed the first quantitative study of CH₄ growth rate in Earth’s atmosphere covering the past 3 decades. Methane is an important greenhouse gas and plays a significant role in tropospheric and stratospheric chemistry. Despite the relevance of CH₄ in human-induced climate change and air pollution chemistry, there is no scientific consensus on the causes of changes in its growth rates and variability over the past three decades (Figure 1).

We use a well-validated chemistry-transport model for simulating CH₄ concentration and estimate of regional CH₄ emissions by inverse modelling for the period of 1988-2016 (※1). Our findings provide a robust set of explanations about the processes and emission sectors that led to changes in the global CH₄ growth rates over the past three decades (※2), covering the three distinct periods of slowed (Period 1: 1988-1998), quasi-stationary (Period 2: 1999-2006) and renewed (Period 3: 2007-2016) phases (Figure 1). The inversion results are well validated against an independent set of extensive vertical aircraft measurements over Japan covering the three decades. We show decadal shifts in CH₄ emissions toward the lower latitudes from the northern midlatitudes, relative to earlier decades (Figure 2b) and a priori estimates (Figure 3a), which can be confirmed using the global distributions of the total column CH₄ (※3) observations by satellite remote sensing (Figure 3b-d).

We show that reductions in emissions from Europe and Russia since 1988, particularly from oil-gas exploitation and enteric fermentation, aided by lower emissions from the natural wetland due the effects of Mount Pinatubo eruption and frequent El Niño, led to the Period 1 slowed CH₄ growth rates in the 1990s (Figure 4). This period was followed by the Period 2 quasi-stationary state of CH₄ growth in the atmosphere during the early 2000s. CH₄ resumed growth in the Period 3 from 2007, which we attribute 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 (Figure 4). As per the emission inventories, CH₄ emissions from Japan decreased appreciably from 3.5 Tg/yr in the 1980s by about 35% in 2000s, but remained relatively unchanged in the post-2000 period. 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, which would require a much denser observations in space and time than the present.