Atmospheric methane concentration has reached a record, but the exact reason has been difficult to determine. Atmospheric methane concentration increased 8 ppb per year during the 1980s, 6 ppb per year in the 1990s, then the trend was static from 2000 to 2007, but now increasing at 9 ppm per year since 2007 (Figure 1). The reason for the increased accumulation of methane in recent years is likely due to several factors. The methane budget includes both sources of emissions and sinks that remove methane from the atmosphere. The primary sources include agriculture, natural wetlands, fossil fuels, biomass burning, and other natural sources (oceans, lakes, termites). The primary sinks are chemical reactions in the atmosphere and soils. The increase in atmospheric methane concentrations means that emissions were larger than sinks, but which source has been the cause of the recent increase.
Ruminant animals account for the largest proportion of man-made methane emissions and cattle are by far the largest contributor. Estimated enteric methane emissions have increased since 2000, but the global cattle population has remained constant questioning the reason for the increased enteric methane emissions (Figure 1). Wetlands are the largest natural source of methane emissions and methane emissions from wetlands have also been increasing since 2000. Methane leakage during oil extraction is also a source of methane emissions into the atmosphere and was thought to possibly be the cause of increased methane due to the increase in shale oil extraction.
Based on the change in radio isotope ratio of atmospheric methane, the increase in methane emissions is likely from microbial sources which rules out fossil fuel extraction leaving enteric and wetland methane emissions. The largest increases in atmospheric methane coincide with the largest increases in global temperature. The largest methane growth rates (> 10 ppb) occurred in the tropics and subtropics through 2014 to 2017, which had average temperatures > 1°C warmer than the 1880-1909 baseline. Methane emissions from wetlands increase with increasing temperature because of increased microbial activity, but microbes in the rumen of cattle are at a constant 38°C such that global temperature would not be affecting microbial activity in the rumen. Additionally, the largest increases in methane emissions have come from the tropical and subtropical latitudes, where increased precipitation, flooding and temperature coincided between 2014 – 2017. Wetlands are the largest global source of methane emissions (Figure 2) and are a major driver of atmospheric methane especially with increasing global temperature.
Removal of methane through chemical reactions in the atmosphere can have a dramatic effect on methane lifetime. Hydroxyl, which is the chemical with which methane reacts in the atmosphere, concentrations in the atmosphere increased 10% between the late 1990s and mid-2000s coinciding with the plateau in methane concentrations from 2000 to 2007. But hydroxyl concentrations have decreased approximately 10% from mid-2000s to 2014 coinciding with the renewed increase in atmospheric methane concentrations.
The methane budget is not as simple as once thought and changes in relative amounts of sources and sinks can readily change the atmospheric concentration. Enteric emissions from ruminants is not always the primary driver and is not the largest emissions source. As global temperatures increase, wetland emissions may become a larger proportion of global methane emissions.
