Since the beginning of agriculture, humans have drastically altered land use on the face of the earth. Humans have created deserts, leveled forests, grown crops, eroded soil, and grazed cattle. We don’t know the precise role of livestock agriculture in all of this, but it’s doubtless a very significant part.

Does any of this land use have an effect on climate, and specifically on atmospheric carbon dioxide (CO2)?

According to William Ruddiman’s well-regarded book Plows, Plagues, and Petroleum (Princeton, revised edition 2010), land use did have a significant effect. It was enough to raise CO2 levels about 40 ppm (parts per million) in the atmosphere even before the industrial age began. In 1700, without human intervention, we would have had CO2 levels of about 240 ppm, but in fact — well before the massive burning of fossil fuels began — CO2 levels were 280 ppm.

The excess of 40 ppm isn’t nearly as much as the increase since 1700, and we are now well over 400 ppm of CO2. On the other hand, agriculture today in 2018 is much larger than it was in 1700, so we have reason to suspect that CO2 emissions due to land use changes are correspondingly larger as well.

Exactly how is this happening, and how much are the annual land use contributions to CO2?

Some people have looked at the content of radioactive carbon-14 in the atmosphere and concluded that fossil fuels are responsible for most of the “excess” CO2 in the atmosphere — that port of CO2 above the natural level. The argument here is a bit complex, but the premises are valid, and I’d suggest that interested readers read this valuable article. This article concludes: “scientists calculate that about a quarter of the CO2 present today must come from fossil fuels. That conclusion is confirmed by the fact that this fraction amounts to most of the growth in CO2 over the last 250 years, when fossil-fuel burning has really taken off.”

Since carbon-14 has a relatively short half-life and therefore is not found in fossil fuels, it appears that about a quarter of the atmospheric CO2 is due to fossil fuels, which is about the amount of the “excess” CO2. The logic would run roughly like this:

Current CO2 levels= 407 ppm (https://www.co2.earth/, accessed 10-05-2018)
“Natural” CO2 level = 280 ppm (the value of atmospheric CO2 in 1700)
“Excess” CO2 = 407 ppm – 280 ppm = 127 ppm (attributed to humans)
CO2 due to fossil fuels = 25% of current CO2 = 407 ppm * 25% = about 102 ppm

One quarter of 407 is about 102. Therefore, 102 ppm of the excess of 127 ppm is due to fossil fuels, or about 80%. Conclusion: 80% of the “excess” CO2 is due to fossil fuels. That leaves 20% from land use changes.

There are a couple of problems with this logic. The main one is that the “natural” CO2 level may not be 280 ppm. If we accept Ruddiman’s idea, the natural level is 240 ppm. This forces us to recalculate: the excess is not 127 ppm, but 167 ppm, and therefore the fossil fuel portion is not 102 / 127, but 102 / 167, or about 61%. That leaves nearly 40% due to land use.

On top of that, what is the influence of ocean absorption of CO2? Ocean absorption of CO2 is both good news and bad news; good news, it blunts the increase in atmospheric CO2, but bad news, it also creates ocean acidification. The bottom line for our calculations is that the greater the amount of ocean acidification, the greater the TOTAL amount of human-caused CO2 emissions must be. Ocean absorption is at least 25% of the CO2 emitted, and might be as high as 48%. Depending on your assumptions about CO2 absorption by the oceans, this could boost the portion of land-use CO2 still further.

We are not quite up to the Goodland and Anhang figure of 51% of all greenhouse gas emissions from livestock, but the total from land use is quite significant and makes their estimate more plausible. I am not proposing a new figure for CO2 emissions due to land use. But just a quick examination of CO2 in the atmosphere from fossil fuels shows that humans have a lot of explaining to do. The “excess” human-caused CO2 is doubtless much higher than the simple calculation of “current CO2 levels minus the CO2 level in 1700” would suggest. We were already well into the game of climate change even before the industrial age began.