Wetlands, rivers, oceans, lakes, plants, decaying vegetation (especially in moist environments such as rainforests) – 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.
The increase in global methane levels from 1930 to 1970 was due to emissions from the production, transmission and distribution of natural gas. There was a tenfold increase in the use of natural gas through the 1960s and 1970s. The source of many of the natural gas emissions, such as leakages from the Trans-Siberian pipeline, have since been rectified. Measurements over the last 25 years show concentrations of atmospheric methane are merely exhibiting natural variation, with no significant trends in any direction
There is therefore no scientific basis for selectively targeting ruminants for a ‘methane tax’, or worse, interfering with this natural process. Farming in ways that enhance, rather than inhibit, soil biological activity, would improve the capacity of agricultural soil to act as a methane sink, helping balance the greenhouse equation. The issue with today’s industrialized approach to agriculture is that methanotrophic bacteria are chemically sensitive. Their activities are reduced by nitrogenous fertilizers, herbicides, pesticides, acidification and excessive soil disturbance.