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I recently came over some numbers of GHGs released per energy in food produced, and the result is pretty flooring. I can see how animal-based food production is quite inefficient so let's ignore that for now and just look at plants.

For example rice releases 2 kg carbon = 7.6 kg CO2-equivalents per 1000 kcal. This would actually mean that it is more environmentally friendly to drive a car 10 km than to bike 10 km on a rice-based diet, because biking burns around 300 kcal = ~2.28 kg CO2e, while a normal compact car burns 0.5 liters of gasoline = ~1.9 kg CO2e. Are my numbers wrong? If not, how can this be?

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There are several contributing factors,

  • livestock, Livestock is easy animals produce co2 but their methane is a bigger problem, methane is a better greenhouse gas than the co2 that the feed plants took in. Livestock waste is also a problem , people do not realize how much waste modern factory farms produce, specially when stored as a liquid is it produces all the problems of rice production with addition release of methane from the waste itself.

  • transportation, transportation of ag products to market are almost all petroleum based. Depending on the source this may or may not be included in agricultural emissions.

  • Landuse, changing land, especially things like draining wetlands and burning forest for farmland are big contributors, Wetlands in particular produce huge amounts of co2 and keep doing so for decades if not centuries, all the organics built up in wetland start oxidizing when drained, this is basically the same as burning petroleum without actually getting any usable energy out of it.

  • Fertilizer produces a plethora of problems, and could justify an question all its own.

  • Rice, rice production has been called out as particularly problematic. You would think this would produce the opposite effect to draining wetland but it is actually worse. Bacteria in flooded soil produces methane instead of co2 and as I said methane is better greenhouse gas, too top it off rice fields are supplied with enough nutrients to keep the bacteria producing way above normal wetland levels and keep it from being trapped the way carbon in mostly anaerobic wetlands will be.

Source

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If you want to exclude animals, better to say why does arable farming release so much greenhouse gas. I have heard the argument about rice fields before, and it may well be true, but you have to remember that arable farming is almost carbon neutral. It removes CO2 from the air and gives it back in the ways you mention. You could even have tractors powered by biofuel, and some tractors already are. There is a very small downside, which is that plants, rice fields in particular, return a small amount of the carbon in the form of methane, which is a more potent greenhouse gas. Whatever the arguments against arable, it is more environment friendly than livestock. One thing you are never going to do is persuade people to give up eating. You aren't even going to persuade many to give up eating rice. With regard to livestock farming, its effect is not as damaging as some people allege, because you need to remember that the Earth has always had vast herds of herbivores releasing methane, it's just that the previous herds of bison etc have now largely been replaced by cattle.

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  • $\begingroup$ humans raise livestock in a very different manner than wild herds. Also a source that arable farmland is neutral is needed, esspecally given abundant sources that say the opposite. $\endgroup$ – John Jul 7 at 16:40
  • $\begingroup$ tractors powered by biofuel cause negative productivity as the amount of crop needed to create that fuel is higher than the amount of crop you can hope to produce using that tractor. That's why tractors on farms growing the crops needed to create biofuels run on diesel (plus diesel is way cheaper). $\endgroup$ – jwenting Oct 15 at 3:38

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