Greenhouse gases differ greatly in their chemical composition and atmospheric warming impact over time.

For convenience most reporting uses a Carbon Dioxide equivalent (CO2-e) effect over a 100-year period to describe global warming potential (GWP100). Other time periods such as GWP20 can also be used. It is acknowledged that GWP100 is a poor indicator for methane (CH4) due to its conversion to CO2 within a 12 year period. An alternative measure, GWP*, is now considered more appropriate. 

The Paris Agreement, adopted in 2015 at the Conference of the Parties to the United Nations Framework Convention on Climate Change (UNFCCC), represents the primary global consensus for action to combat the threat of climate change. This requires a rapid transition to net zero anthropogenic carbon dioxide (CO2) emissions.

Carbon dioxide emissions are critically important to climate stabilisation because they are the dominant greenhouse gas (GHG) contributing to climate change and their impact on global
mean temperature is highly persistent, lasting for millennia The principal greenhouse gases (GHGs) are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) which are reported as CO2 equivalents (CO2-e) in most reporting systems. The CO2-e of methane is calculated as 28 times CO2, and N2O as 265 times CO2.

These multipliers indicate the relative warming impact (radiative forcing) per unit of each gas when added to the atmosphere. The degree of effective warming however differs greatly with the “life” or radiative forcing impact differing greatly. CO2 can remain in the atmosphere for up to 1000 years unless removed by various carbon dioxide “sinks” whereas methane is a short-lived climate pollutant (SLCP), a category of pollutants that have lifetimes much shorter than CO2, from days to a few decades. IPCC assessments primarily report global warming potential as GWP100, a calculation of the warming effect over a 100 year timeframe.

However, for some sectors of the economy, such as ruminant livestock production where methane is a major emission source, climate change impacts are largely related to emissions other than CO2. The use of a single scaling factor, GWP100, to equate emissions of GHG with dramatically different radiative intensity and lifespans can be misleading for short-term climate forcers, for which a steady emissions profile over time can be consistent with climate stabilization, in contrast to the need for net zero emissions in the case of CO2 and other persistent GHGs.

An alternative approach is to report emissions as ‘warming-equivalents’ that result in similar warming impacts without requiring a like-for-like weighting per emission. GWP*, an alternative application of GWPs where theCO2-equivalence of short-lived climate pollutant emissions is predominantly determined by changes in their emission rate, provides a straightforward means of generating warming-equivalent emissions.

GWP* provides a useful indication of warming, whereas conventional application of GWP100 falls short in many scenarios and particularly when methane emissions are stable or declining, with important implications for how ‘zero emission’ or ‘climate neutral’ targets for sectors emitting different compositions of gases are defined. 

GWP* allows warming-equivalent emissions to be calculated directly from CO2 equivalent emissions reported as GWP100, consistent with the Paris Rulebook on condition that short lived and cumulative climate pollutants are aggregated separately, which is essential for transparency. It provides a direct link between emissions and anticipated warming impacts, supporting stocktakes of
progress towards a long-term temperature goal and compatible with cumulative emissions budgets.

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 oxide still ultimately need to be reduced to zero to stabilize global temperatures.

The United Nations recognises that GWP100 is inaccurate when measuring the warming potential of short-lived GHGs such as methane, however, GWP100 remains the standard for international reporting for the time being.

Emissions per unit of food nutrients is also advocated as a comparison measure where climate impact is assessed in relation to meeting global dietary requirements to prevent confusion between foods with a low climate impact per kg and low nutritional value relative to high nutrient density foods that may have a greater environmental impact per kg.