August 5, 2017

Just a temporary inconvenience

From Radio NZ

The book explores the widely held view that farm livestock are responsible for an enormous net production of new global warming gases.

“Once you take into account the entire cycle of the life of a cow, it’s actually impossible for the cow to omit even one extra atom of carbon to the atmosphere that wasn’t there already there, they are carbon neural in the end.” he says.

As you’d expect, there’s a sense in which this is completely true. It’s just not a sense that contradicts the standard views of methane and global warming.

What’s going on is easier to see if you consider carbon outputs from the other end of the cow.  Some of the carbon a cow takes in comes out as cowshit. This carbon doesn’t lie around for ever; it returns to the skies and the soil as part of the Great Circle of Life. Hakuna Matata. This doesn’t happen instantaneously, though. In the short term, you still need to wear sensible footwear or watch your step when you cross the field.

There’s an equilibrium between the production and decay of cowshit. When you increase the number of cows, the ambient cowshit level increases, and settles in at a new, higher equilibrium. When you decrease the number of cows, it decreases towards a new, lower equilibrium. The time this takes is governed by how long cowshit takes to decay, so it’s pretty fast.

In a similar, but more serious way, some of the carbon that goes into a cow comes out the front end as methane.  The methane doesn’t hang around in the air for ever; it turns back into carbon dioxide and water. As with cowshit, this doesn’t happen instantaneously.

There’s an equilibrium between the production and decay of methane. When you increase the number of cows, the ambient cow-derived methane level increases, and settles in at a new, higher equilibrium. When you decrease the number of cows, it decreases towards a new, lower equilibrium. The time this takes is governed by how long methane takes to decay: over each passing decade about half of it goes away.

Carbon emitted as methane, unlike carbon emitted in cowshit, is more than a local nuisance.  Per atom of carbon, methane has 24 times the greenhouse warming effect of CO2, and while it doesn’t last for ever, it lasts long enough to make an important contribution to climate change.  There’s more than twice as much methane in the atmosphere now as there was two centuries ago.

Cows are long-term carbon-neutral: that means reducing cow numbers (or finding ways to reduce their methane production) would, in mere decades, roll back the increases they’ve caused in an important greenhouse gas.

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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

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