Farming top cause of global warming?

Patrick Metzger at Green Daily noticed a new twist about the causes of the greenhouse effect:

A new report from Greenpeace says that agriculture is one of the biggest sources of the greenhouse gases thought to cause global warming.

The report from researchers at the University of Aberdeen estimates that between 17% and 32% of all human-generated greenhouse gases come from farming.

The largest part of the gases (about 32% of the farm total) comes from nitrous oxide produced by chemical fertilizers, with cow flatulence in second place at 27%.

One part of this Greenpeace report requires fact checking.
Flatulence is the burping of ruminant livestock. It’s methane created by fermentation bacteria in one of the animal’s stomach chambers. A cow burps 280 liters (75 gallons) per day, but the carbon in this gas is not new carbon.

The critical point that is being missed is where this carbon comes from.

Cars, ships, etc.:
This carbon comes from carbon that has been buried deep underground. It is unearthed, burned, and then released into the atmosphere. Therefore, any carbon released is added to carbon already present in the atmosphere = global warming.

Cows, agriculture:
This carbon comes from the atmosphere. The plants take up the carbon dioxide. Livestock then eat the plants and release the carbon back into the atmosphere. Therefore, any carbon released by the cows was already in the atmospher to begin with (carbon neutral) = no global warming.

The problem isn’t the re-releasing of carbon that was already in the atmosphere (livestock, agriculture), but the unearthing of new carbon sources (gas and coal) and then adding these to the atmosphere.

Food for thought
Phillip Barker posted an interesting comment at a BBC story on the impact of livestock:

“Your findings and calculations cause me to wonder if human contributions to global warming gases are a wash considering the billions of wild bison, water buffalo, rhinos and elephants no longer producing these dangerous gases due to our eliminating them from the planet.”