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.
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.