Are cattle really contributing to global warming?

Question

Let me be straight up. I'm a physicist, I've no doubt about climate change and the part we play in it. I had been reading about cattle being one of the alleged main causes of climate change.

I get that they make methane, and we have a LOT of them, So I ran some numbers:

Roughly 93 million cattle in the United states in 2017 (some sources claim lower, but I'm going with the higher one).

Prior to humans killing the Buffalo off, there were approximately 50-100 million of them across the united states.

A quick Wiki search, estimates that buffalo (bison) produce significantly more methane than domestic cattle (though I don't know for sure that this is true).

These numbers seem to match up pretty well insofar as they're greenhouse gas contributions, so now I'm wondering if the argument is viable? I know there were other herds around the world too, so I just found myself rather curious. Has anyone run the numbers thoroughly?

On the flip side, I could see the sheer number of humans being an issue, I'll try and run that later.

Answer 1

Let's just talk about cattle here and ignore other factors related to cattle production for now.

The current cattle number (93 million) you found is probably the total number of dairy cow and beef cattle. In your argument, it is possible that bison has much more emission than cattle on pasture. But the emission from cattle in feedlot and dairy farm is much larger than the cattle on pasture because of their diet chosen by us (human).

Typical beef cattle (steers and heifers) in the U.S. spend one year on pasture, and 5-6 month in feedlot before slaughter (unlike Argentina beef cattle which are fully raised on pasture). In feedlot, these young cattle (yearlings) are intensively fed with high protein feed in order to get them fatten up in a very short time. The U.S. dairy cows spent most of their time on dairy farm (unlike the Swiss cows who are raised on pasture). They live for about 5 years and are continuously fed with high protein forage to ensure their milk production. Both beef cattle and dairy cows have a high protein diet that result in high emission of methane (the impact of methane is 25 times worse than CO₂ in terms of warming).

The closest comparable cattle to bison is beef cow which spend most of its life time (about 10 years) on pasture and are only occasionally fed with low quality hay when there is a shortage on pasture. But again, the life span of a beef cow may be different from the buffalo and beef cows may eat more because they are basically pregnant very year(!).

California has this new policy that tries to cope with dairy cow methane emission from their manure. If you are interested, there is a recent study done by University of California Agricultural Issue Center here (https://aic.ucdavis.edu/wp-content/uploads/2019/01/DairyManure-1.pdf).

Answer 2

There have been a few papers published on that topic. Johnsons & Johnson, 1995 for example estimate the amount of methane produced in a ruminant livestock to be 250 - 500 L/day. They conclude that cattle will contribute to little less than 2% of the global warming that we might face in 50 - 100 years time.

In contrast in a paper by Oliver et al., 2010 the authors looked at the per person CO₂ emission of cruise ships traveling from and to New Zealand. The found that:

Emissions factors for individual journeys by cruise ships to or from New Zealand in 2007 ranged between 250 and 2200 g of CO₂ per passenger-kilometre (g CO₂ per p-km), with a weighted mean of 390 g CO₂ per p-km. The weighted mean energy use per passenger night for the “hotel” function of these cruise vessels was estimated as 1600 MJ per visitor night, 12 times larger than the value for a land-based hotel.

As you can see cruise ships are probably far more of a „global warming machine“ than cattle - but serve only very few people. I personally think (and I might be wrong with this) that methane production in cattle can and is easily used by political groups to fight against the conditions in which the cattle are held and against the massive consumption of meat.