Professor Barry McMullin:

If I understand the question correctly, the argument that was made earlier was that the residual CO2 that methane breaks down into, in the case of methane from biogenic sources, effectively was already in the atmosphere before it was absorbed into grass, eaten by the cattle and then turned into methane. Therefore, there is no net increase in CO2. That is absolutely correct. In contrast, when it comes to fossil methane, natural gas being released into the atmosphere or emissions from coal mining or whatever, that is not the case. When fossil methane gets into the atmosphere and eventually breaks down into CO2, that CO2 represents a net increase. However, the mass of CO2 resulting from the degradation of methane emissions from fossil sources is tiny compared with the CO2 being produced from the direct combustion of fossil fuels.

In terms of the distinction between methane from biogenic sources and methane from non-biogenic or fossil sources, there is a difference in long-term impact in terms of CO2 lingering in the atmosphere, but that difference is tiny compared with the CO2 impact of burning fossil fuels. The major impact of methane is not due to the residual CO2 in the atmosphere. It is due to the warming impact while the methane is in the atmosphere. That is what we need to be concerned about. It is what will affect the peak temperature over the next three to six decades. It is that time while the carbon is trapped in methane molecules in the atmosphere that it is increasing the global temperature or contributing to maintaining increased global temperature over the next several decades. That is the critical question in the context of methane emissions, not the residual destination of those carbon atoms in CO2. I apologise because that is scientifically complex and I do not know that I can simplify it any better than that.