Author: Dr. Christine Jones
Wetlands, rivers, oceans, lakes, plants, decaying vegetation (especially in moist environments such as rain forests) – and a wide variety of creatures great and small – from termites to whales, have been producing methane for millions of years. The rumen, for example, evolved as an efficient way of digesting plant material around 90 million years ago.
Ruminants including buffalo, goats, wild sheep, camels, giraffes, reindeer, caribou, antelopes and bison existed in greater numbers prior to the Industrial Revolution than are present today. There would have been an overwhelming accumulation of methane in the atmosphere had not sources and sinks been able to cancel each other over past millennia.
Although most methane is inactivated by the hydroxyl (OH) free radical in the atmosphere,another source of inactivation is oxidization in biologically active soils. Aerobic soils are net sinks for methane, due to the presence of methanotrophic bacteria, which utilize methane as their sole energy source. Methanotrophs have the opposite function to methanogens, which bind free hydrogen atoms to carbon to reduce acidosis in the rumen. Recent research has found that biologically active soils can oxidize the methane emitted by cattle carried at low stocking rates. The highest methane oxidation rate recorded in soil to date has been 13.7mg/m2/day which, over one hectare, equates to the absorption of the methane produced by approximately one livestock unit (LSU).
In Australia, it has been widely promoted that livestock are a significant contributor to atmospheric methane and that global methane levels are rising. There is no evidence, however, to suggest that methane emissions from ruminant sources are increasing. Indeed, it would seem there has been no clear trend to changes in global methane levels, from any source, over recent decades.