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Does Grass-Fed Beef Help Sequester Carbon?

February 27, 2010

Some advocates of grass-fed beef argue that well-managed pasture land builds soil carbon.  But when you look at the sources behind  these claims, the evidence just isn't there.  

A recent Time magazine article states, "Indeed, although grass-fed cattle may produce more methane . . . their net emissions are lower because they help the soil sequester carbon." They cite no sources, but they are evidently quoting groups such as the American Grassfed Association, "Animal Welfare Approved" (AWA), and the Soil Carbon Association

If true, this means that grass-fed beef might actually help fight climate change, and this would be quite surprising, given the fact that some scientists and even one study funded by Meat and Livestock Australia have reached the opposite conclusion. So what’s going on?

The source for the AWA and the Soil Carbon Association are some reports from the U. K. Soil Association on organic agriculture.  The summary report states:

"According to a recent European Commission report, grasslands have the potential to be sequestering large amounts of carbon on an ongoing basis. In the UK, the potential sequestration is said to be 670kgC/ha/year, (10) which, if true, would offset all the methane emissions of beef cattle and about half those of dairy cattle. (11)"

But both footnotes give a somewhat different story.

(1) The reports of carbon sequestration by grasslands (footnote 10) are extremely tentative, and the net CO2 production difference between grasslands and forest lands (which is what most grasslands in the world would revert to if not grazed by cattle) is ignored.

(2) The estimates of greenhouse gas emissions by livestock (footnote 11) only considers methane emissions, not total carbon dioxide. It also seems to include factory-farmed beef, which we know emits less methane than purely grass-fed beef.

Footnote 10 is enigmatically described as "Janssens et al, 2003," and there are no further details about this report in the summary. I think they are referring to the article "Europe's Terrestrial Biosphere Absorbs 7 to 12% of European Anthropogenic CO2 Emissions" which appeared in Science magazine.  If you read this article by Janssens et. al., it turns out to be a report on the ability of the land in Europe to absorb carbon dioxide. The report states: "grassland ecosystems may constitute a net C sink . . . although the uncertainty surrounding this estimate is larger than the sink itself" (emphasis added).

Janssens, et. al. further states: "Within this land-based estimate, forests represent the largest C sink, whereas agricultural soils dominate the large uncertainty. This large uncertainty is attributable mainly to the lack of data and the simplifications assumed in aggregating fluxes from the mosaic of vegetation types, land-use histories, soil types, climates, and management regimes. If we assume a normal probability distribution, the probability that the land-based sink estimate is positive (i.e., a net C sink) only amounts to 0.66."

Given the fact that forests represent a large carbon sink and grasslands much smaller, and given the fact that most pasture land would revert to forests if left to its own devices, shouldn’t we consider the net CO2 effect of maintaining pasture instead of forests? The carbon difference is probably huge, something which Janssens et al. themselves point out. In fact, calculating this difference is a key point in the recent WorldWatch article "Livestock and Climate Change."

Footnote 11 comes from the annexes of the U. K. Greenhouse Gas Inventory for 2007.  If you go to  page 374 to find out about the methane emissions of beef and dairy cattle, it appears that they consider the animals to be at least partially factory farmed (page 376 indicates that the animals are only grazed half the time).

This of course invalidates the use of this data for information on grass-fed beef, for which the methane emissions are presumably higher. Furthermore, this only gives the methane emissions, not the total greenhouse gas emissions, so that the claim to "sequester" carbon is not supported at all.

It is possible is that some grasslands are a net carbon sink. But even if this is the case, whether grazing cattle on such grasslands would result in GHG emissions less than the sink value is highly debatable.

This claim depends on:

considering grass-fed and grain-fed beef to be equivalent in terms of methane emissions,
ignoring carbon dioxide emissions due to livestock,
tolerating an uncertainty greater than the claimed "sink" value, and
ignoring the net CO2 difference between pasture land and forests (which we know are the biggest CO2 sink).

If you consider the consequences of converting forest land to grassland (the "foregone" carbon absorption that would occur if pasture was allowed to revert to forest), it is highly unlikely that most grasslands are a net carbon sink at all.  The claim that grass-fed beef helps sequester carbon is as yet unproved.