A past Paper 3 question for Option E : Environmental chemistry asked students to identify a natural source of methane. Several students gave the journalistic answer “cows' farts” which earned them zero marks. To gain the marks the answer needed to include mention of the anaerobic decomposition of organic material.

In fact grazing animals such as cows are thought to account for around one fifth of the total global emission of the greenhouse gas methane but the precise amount is difficult to quantify.  Researchers from the University of Bristol in the UK have recently been trying to use a marker to estimate the exact amount.  In a recent article published in Animal Feed Science and Technology they have been looking for  the presence of archaeol (see below) which is produced by the symbiotic or 'friendly' microbes that live in the foregut of ruminant animals. These organisms are called archaea and they produce methane as a by-product of their metabolism which is then released by the animal as burping and flatulence.

Structure of archaeol

What the researchers have shown is that cows fed on grass silage require less feed but form more archaeol and emit almost twice as much methane compared to cows fed on a concentrate based diet. This work confirms that manipulating the diet of cows may be one important way of controlling the emission of greenhouse gases.