A key cause of climate change

There is now more megafauna biomass (the total biomass of all large animals that are heavier than about 100 pounds) than there has been in recent earth history — indeed possibly in all of earth’s history. It seemed to be fairly constant at 200 million tons for literally hundreds of thousands of years. Then, starting with the industrial revolution and the huge surge in human population and the population of domestic livestock, megafauna biomass has exploded. It is currently about 1500 million tons, over seven times as much. And almost all of this increase has happened just since the industrial revolution.

Whoa! How did this happen? And do you think that all this extra animal biomass would affect carbon dioxide levels?

Some people think that it does. One of the more interesting ideas in the 2009 WorldWatch article “Livestock and Climate Change” (by Robert Goodland and Jeff Anhang) is that the carbon dioxide exhaled by livestock should be counted as human-caused greenhouse gas emissions.

After untold millennia, total megafauna biomass has increased just in the last 300 years, mostly due to livestock

Two facts stand out concerning the huge uptick in megafauna: (1) this increase began with the industrial revolution, involving massive use of fossil fuels to power human civilization; (2) almost all of this huge increase in biomass is one species, namely us, and our livestock. Well, all of these humans and their livestock are living, breathing creatures, and they are breathing out carbon dioxide. Don’t we have to count this new carbon dioxide from these additional animals as a human-caused greenhouse gas?

People do a double-take when they first hear about this idea, and often object to this conclusion. They think of plants and animals as being roughly in balance, in terms of carbon dioxide and oxygen. Isn’t the carbon dioxide exhaled by cattle exactly equal to the carbon dioxide originally taken up by the grass that they eat? So how could this process increase carbon dioxide in the atmosphere?

To see the fallacy here, we could imagine a thought-experiment. Suppose that an alien civilization with vast powers wanted to test the hypothesis that “more cows on the planet means that the carbon dioxide they breathe out will add to the total carbon dioxide in the air.” They could construct an earth-like planet A on which there are a constant number of cows (all eating grass), and a fixed amount of natural grassland. They would tweak the number of cows and the amount of grassland such that the carbon dioxide / oxygen ratio was constant. The cows breathing out CO₂ would be exactly balanced out by the grass uptake of CO₂. Then, they construct another earth-like planet B exactly like planet A, except with twice as many cows. Would the carbon dioxide increase on planet B?

This is an empirical question. It’s logically possible that the additional CO₂ breathed out by cows on planet B would be absorbed somehow. For that matter, we don’t know that the additional CO₂ from any other source, such as burning coal, might be absorbed somehow. We don’t understand all the dynamics which maintain the CO₂ / oxygen balance.

For example, the extra CO₂ might stimulate the growth of forests on planet B, and this additional forest growth might absorb the excess CO₂. Or, the extra CO₂ might stimulate the growth of grassland, which would spring up more aggressively. Or some other process might stimulate and increase forest or grass production to compensate for the CO₂ from the cows.

Actually, we can imagine just such a process; excess CO₂ may stimulate the growth of plant matter, although the additional plant matter is likely able to absorb only a small percentage of the excess CO₂. We sometimes hear climate change skeptics saying that “CO₂ is plant food” and implying that agriculture, and plant growth in general, will flourish as CO₂ increases. However, this is not what we have observed in previous climate regimes in the past; extra CO₂ seems rather to further the growth of the insect population.

Prima facie, we would have to conclude that doubling the number of cows on planet B would approximately double the CO₂ in the atmosphere. It is indeed true that the CO₂ exhaled by the cows exactly equals the CO₂ which was absorbed by the grass that the cows ate, but the problem is that the total amount of grasslands would decrease on planet B.

In case there is doubt, let’s construct another hypothetical planet C. On planet C, the number of cows is not merely double the number on planet A, it is seven times the number on planet A. Do you think that this would increase the amount of CO₂ in the atmosphere on planet C, compared to planet A?

Planet C is the planet which reflects pretty much what has happened on earth. And on earth there has been no compensating increase in plant matter to take this excess CO₂ out of the atmosphere. On the contrary, there seems to be a decrease in plant matter. We are chopping down the forests like crazy, and forests incorporate the most carbon of all the natural ecosystems; and we are overgrazing the grasslands into desert. The total land turned into wasteland by human actions during the last 10,000 years is approximately equal to the land now in cultivation. Human influence has not increased plant growth; rather, we are devastating natural plant growth.

It’s true, also, that in our version of Planet C on planet earth, we’ve thrown in quite a bit of additional burning of fossil fuels, which makes the problem even worse. But we can measure this additional CO₂ from fossil fuels — and even then we find that the CO₂ from cows (and various other greenhouse gas emissions, like methane) mean that livestock contribute roughly half of all human-related greenhouse gas emissions. This is precisely the point that Goodland and Anhang were making in their celebrated 2009 article.

All the cattle on the planet, not to mention a human population explosion (which Goodland and Anhang don’t even factor in), are a mostly-unnoticed influence of humans on both the environment and on carbon dioxide in the atmosphere. This increase of cattle and people has to have a huge influence on climate. How can you increase the total megafauna biomass over seven times and not affect the total CO₂ content of the atmosphere? And this increase of CO₂ is in addition to all the methane produced by cattle. This human / livestock population explosion is the “cow in the room” of all climate change discussion.