It's not zero-sum, the carbon taken out of the atmosphere in the form of CO2 to make the feed is partly put back in the form of methane, which is a stronger gre

It's not zero-sum, the carbon taken out of the atmosphere in the form of CO2 to make the feed is partly put back in the form of methane, which is a stronger greenhouse gas than CO2. This means a cow is a net greenhouse gas producer. The half-life is not that long but not very short either, 10 years, but the lifetime is completely irrelevant: as long as we keep emitting the gas, the levels will stay the same or rise.