Friday, August 12, 2011

Owen McShane: What is it with Bovine Eructation?

Imagine an atom of carbon floating around in the atmosphere just above a farm. That is one atom of C in the air – but packaged in the gas called carbon dioxide or CO2. (If it was an atom of carbon it would be either diamond or graphite powder and would be lying on the ground.)

The molecule of carbon dioxide gets captured by a nearby blade of grass and gets turned into a useful protein or carbohydrate by photosynthesis, and helps build the blade of grass. (There is a chance it will be exhaled as CO2 by the process of respiration but let's ignore that.) Then along comes Daisy the Cow who eats the blade of grass and starts digesting it in her rumen, and in other parts of her intestinal tract. Some of these atoms of carbon will be stripped off the oxygen and ingested into the cow, turned into protein, and finally turn up on your dinner plate as steak or whatever.

However, we are told that the rumen fermentations of the cellulose means that, when chewing her cud, Daisy emits little clouds of methane into the atmosphere when she quietly belches. So presumably there is a chance that the carbon atom we started with  in the molecule of CO2 can be emitted repackaged as CH4, commonly known as methane. We are told this molecule of methane is a much more potent greenhouse gas than CO2.

And it may well be. But it has a limited half-life and is soon oxidised into carbon dioxide.

So at the end of this carbon atom’s progress from the atmosphere to the grass, from the grass to the cow, and then from the cow back into the air, we have exactly the same carbon atom in the atmosphere that was there in the first case, and still bundled up as CO2.

So, how come the ruminants of the world can be changing the climate? The cows have not snatched this carbon atom from a deep hole in the ground and added it to the atmospheric “stock”. It hasn’t even been “fracked”!

Why are the dairy farmers of NZ being told they have to pay their “fair share” when they are adding no carbon dioxide to the atmosphere but leaving the number of carbon atoms in the atmosphere exactly the same as it was before Daisy the Cow came along for her morning snack.

Am I missing something?

It seems I am. The counter argument runs that while the methane molecule is in the atmosphere for a comparatively short time before it is oxidised or washed out of the atmosphere, it is a more potent greenhouse gas than CO2 and hence caused “unfair” warming in spite of its small volume and short life.

However, a paper by Dr Wilson Flood, “The Methane Misconceptions, published in Energy and Environment suggests my instincts are correct, and are backed up by some powerful numbers.

His summary reads:

·      A doubling of the amount of methane in the atmosphere with its present composition would produce a warming equal to only about one thirtieth of the warming produced by a doubling of carbon dioxide.
·      At present rates of increase it would take about 360 years for atmospheric methane levels to double.
·      Molecule for molecule, methane is 7 times more effective at being a greenhouse gas than carbon dioxide.
·      Present changes in atmospheric methane levels pose no environmental risk whatsoever.

·      Research into altering the diet of farm animals to reduce dietary methane is hugely wasteful of resources.

This last finding is surely most important for the New Zealand Government, given our dependence on Dairy exports and our delicate financial position. Why waste resources that could be better spent on making our farms more productive while dealing with real environmental impacts? The following is an example of the actual number-crunching which surely reveals the trivial consequence of our cows contentedly chewing on their cud.

At present rates the atmospheric methane concentration would need 360 years to double. Methane levels are not rising rapidly and present no conceivable threat of any kind.

The amount of methane in the atmosphere is increasing by a mere 14 million tonnes (0.014 gigatonnes) per annum . Only a small fraction of this will be due to farm animals and one is led to the conclusion that research into methane emissions from this latter source is wasteful of resources since the amount is insignificant when compared with the total mass of the atmosphere which is approximately 4.5 million gigatonnes. 

And if you are one of the farmers being accused of not paying your “fair share” you might like to read these papers that present the latest research in this contentious area and which nail home the fact that our farmers are simply not guilty of their assumed sins.

1 comment:

Anonymous said...

I understand that the total biomass on the planet remains the same. The species may change from time to time however. It may not matter whether the land is covered with forest or has grass and cows. The process of rotting vegetation on the forest floor approximates the emissions from grass digesting stock. What am I missing? have not had an answer to this question?