The conventional Global Warming Potential (GWP) can be misleading when applied to methane emissions, particularly when these are being reduced. A revised usage of GWP, denoted GWP*, which uses the same metric values interpreted in a new way, provides a more accurate indication of the impact of short-lived pollutants on global temperature.
Methane and livestock
Ruminant livestock are associated with high levels of methane emissions which is generated from two main sources - enteric fermentation and the loss of orgnaic material in their manures. Per molecule in the atmosphere, methane has a stronger global warming impact than carbon dioxide (CO2). However because of their very different lifetimes, the relationship between methane emissions and the concentration of methane in the atmosphere is very different to that of carbon dioxide.
When emissions are compared they are often compared using the global warming potential - which is where the number 28 for methane comes from (methane is 28 times more potent than carbon dioxode). However this report is saying that the way that methane behaves in the atmosphere is unlike that of carbon dioxide - as such using the 28 times more potent metric may underestimate the impact of the short lived risk of methane.
Of particular importance for ruminant livestock farming are the following observations:
- Past increases in methane emissions caused warming when they occurred, but constant methane emissions cause little additional warming. In contrast, every tonne of CO2 emitted causes approximately the same amount of warming whenever it occurs.
- Gradually declining methane emissions of 10% over 30 years, equivalent to halving over about 200 years (e.g. through efficiency savings), cause no additional warming.
- Faster reductions in methane emissions lead to cooling, presenting an opportunity for agriculture to compensate for delays in reducing CO2 emissions, although net emissions of CO2 and nitrous oxide still ultimately need to be reduced to zero to stabilize global temperatures.
- Increasing methane emissions cause very substantial warming, equivalent to very large emissions of CO2, but only while those increases are occurring.