Putting gas-masks on cows - how to measure the methane output of ruminants

One day last year I was in an archaeology lecture discussing whether the methane output of domesticated cattle was greater than that of the American Plains bison because if one species just replaced another then the methane contribution of cattle had to be offset by that of the bison. Then some-one asked the inevitable question: "How do you measure the methane?". Following a witty exchange of banter between students and tutor, I set out to research the question. Here is what I found out.

Cattle, which are ruminants, generate methane through the process of biological fermentation that takes place in the rumen and reticulum, the front two parts of the stomach which are involved in the familiar ruminant behaviour called "chewing the cud". This grinding and chemical processing is needed to extract the nutrients from the cellulose in grass. The good news is that over 95% of methane is expelled through the mouth in a process called 'eructation' - belching. I'll pass over the other 5%.

A jersey cow undergoing methane measurement (www.afbini.gov.uk)

One method researchers used for measuring methane output involved placing the animal in a respiration chamber and measuring the build up of the gas. Another involved placing a hood over the head of the test beast to capture the exhaled gases. These methods have now been replaced with a system which uses a chemical marker, sulphur hexaflouride (SF6), which is released at a constant rate from a bolus placed in the rumen and which is captured using a pipe system which connects the beast's nostrils to a collector. This system has been shown to provide an accurate measurement of the methane output of cattle and may be capable of measuring carbon dioxide emissions if correctly calibrated. The method has been successfully applied to cattle and sheep, though there are variations relating to breed, diet and lactation which may cause issues if we try and apply these measures retrospectively to Neolithic pastoralist practices.

The annual contribution of methane to the atmosphere from all this belching by the main domesticated species was estimated in 1983 as 74 Tg/ year of which 54 Tg/year came from cattle and compared with ~6 Tg/year emitted by wild ruminants. The contribution from 36 million cattle across 10 US states in 2008 was estimated at 2.5 Tg compared with a notional displaced population of 30 million bison which would have produced 2.2 Tg/year. This suggests that the cattle and buffalo methane contributions balance out. However, in 1983 there were 115 million cattle in the whole USA while the maximum population estimate for North American bison before 1492 is 60 million. The net difference means that American cattle account for around 8% of the total anthropogenic methane contribution from domestic cattle with about another 4% from Australia and 3% from Canada.

Of course, we can't just project these figures back 8,000 years; the number of cattle appears to be linked to the exponential rise in the human population and the ratio of people to cattle has undoubtedly varied over time. However, there is a promising area of research to understand to what extent early domestication of animals has contributed to the anomalous atmospheric concentrations of carbon dioxide and methane.

What do the archaeologists have to say - where did farming start?

For the Ruddiman hypothesis to work, it is necessary to show that there was sufficient anthropogenic activity starting around 8,000 BP to initiate the kind of environmental changes suggested. Evidence is needed to show that agricultural behaviour had spread over a wide enough area of the planet, early enough for it to manifest the suggested impacts on the atmospheric concentrations of CO2 and CH4. We need to look to archaeological evidence to understand where agriculture originated, what form it took and how far it had spread by when. Then we need to use that evidence to calculate how much of a contribution agriculture made to CO2 and CH4 concentrations.

To model the spread of agriculture we need to establish a time and place for the point of origin and then estimate the rate of diffusion. The problem is that there is significant debate and disagreement between researchers over when and where farming started and how far and fast it spread. What are the alternatives?

The classic "fertile crescent" origin proposed that farming emerged between 12,000 and 8,000 years BP, after the Younger Dryas cool period, in the valleys of the Taurus mountains of southern Anatolia and of the Levant. While that is the likely earliest origin, archaeological evidence now supports the theory that farming originated at possibly more than twenty locations across most continents over time and involved a number of species of animal and plant.


From: Current perspectives and the future of domestication studies (Larson et al, 2014)

All these animals and plants originated in wild species. Zooarchaeologists and palaeobotanists have developed ways of detecting whether a plant or animal has been domesticated, that is the species has been intentionally modified by human selective actions to change it genetically, morphologically or behaviourally. For example, archaeological wheat seeds, often found in dumps and pits created by Neolithic peoples, are examined microscopically to see if they are shattering or non-shattering; the non-shattering form is indicative of domestication as it is the form which is harvested most efficiently with scythes.

The rate of change during domestication is another area of research which has shown that plant domestication can be an extended process taking thousands of years. The early rice project not only seeks out the origins and nature of rice domestications but also links the effects of rice with increased atmospheric CH4. We are now starting to understand and quantify the environmental effects of irrigation, terracing, slash-and-burn forest clearance and other anthropogenic behaviours that come with farming.

There are many reasons why people may have started to farm. The warm period after the Younger Dryas favoured plants. The human population was increasing and with it came increased competition for land and resources. Humans were developing a shared knowledgebase of experience which was passed between generations and enriched through cultural transmission. Once the commitment to farming had been made by investing energy in modifying landscapes, was it too hard to revert to the hunting and gathering strategies for food procurement that humans had used for 200,000 years? A comfortable niche had been constructed, how far and fast did it spread out across the planet?