The red meat industry and many who support it have recently been highlighting an alternative method for measuring the global warming potential (GWP) of methane, a potent greenhouse gas that is prominent in animal agriculture. The new metric is known as GWP* (or GWP Star).
Is the attention justified? This article considers that question under the following headings:
- What is GWP?
- What is GWP*?
- A cooling impact can be reported when emission rate is falling under GWP* despite an emission pulse’s warming impact
- The shorter-lived nature of methane is already reflected in GWP
- “Flow” and “Stock” categories are subjective
- Methane concentrations have increased alarmingly
- No matter the source of methane, it is “carbon on steroids”
- Criticism from prominent climate scientists
- Other alternatives to GWP
- Other problems with methane
- Other problems with animal agriculture
What is GWP?
Under the United Nations Framework Convention on Climate Change (UNFCCC), the emissions of different greenhouse gases are aggregated for measurement purposes by converting them to carbon dioxide equivalents or CO2-e. It is analogous to converting different currencies to a common denomination.
The greenhouse gases are converted to CO2-e by multiplying the mass of emissions by the relevant GWP, representing the radiative forcing (a precursor to temperature change) of a unit mass of the gas relative to that of CO2 over a specific period. [Footnote 1]
Over a 100-year time horizon (GWP100), the GWP for methane is 28 before allowing for the effect of climate-carbon feedbacks and 34 with those feedbacks. 
The GWP over a 20-year time horizon (GWP20) is 84 before allowing for feedbacks and 86 with them.
Why are the multipliers higher over the shorter time horizon than the longer? It is because methane breaks down to a large extent within around twelve years. The longer the time horizon used for measuring the impact of a single pulse emission, the more diluted the average warming impact and the lower the GWP. [Footnote 2]
What is GWP*?
The new metric, GWP*, was proposed by Prof. Myles Allen and Dr Michelle Cain from the University of Oxford and fellow researchers in papers published in 2018 and 2019. Although their work was relevant to what they consider to be short lived greenhouse gases in general, they have focused on methane for the purpose of explaining the concept.   
The GWP* formula incorporates conventional GWP along with adjustments reflecting the changing rate of emissions over a given time period.
The converted emissions are expressed as CO2-we (CO2-warming equivalent) rather than CO2-e.
The authors focused on the supposed “flow” quality of methane. They contend that a kilogram of the gas emitted today is simply replacing a kilogram released previously, which has since largely broken down, with no change in temperature from the new emission pulse.
They contend that such is not the case with a long-lived “stock” gas such as CO2, which they note accumulates in the atmosphere over time.
In terms of atmospheric concentrations, the authors treat a permanent increase in the emission rate of a short-lived pollutant in the same way as a single pulse of a long-lived pollutant because each would result in an ongoing presence in the atmosphere of the relevant gas.
The findings from the first paper were fine-tuned in the second, allowing for the fact that some additional long-term warming will occur from methane emissions because of the climate system adjusting in response to those emissions in order to maintain equilibrium between incoming and outgoing radiation.
Another distinction between GWP* and conventional GWP is that the former indicates temperature impacts while the latter indicates radiative forcing, which (as mentioned earlier) is a precursor to temperature change. The distinction is illustrated in Figure 4 (adapted from Balcombe, et al.). 
GWP* is insensitive to the time horizon used for measuring the warming impact provided the horizon is much greater than the lifetime of the greenhouse gas under review.
A cooling impact can be reported when emission rate is falling under GWP* despite an emission pulse’s warming impact
Rather than the absolute amount emitted, GWP* focuses primarily on the changing rate of emissions over a given time period.
A reduction in the rate of emissions (beyond a threshold level of around 0.3 per cent) is considered to have a cooling effect. Under the conventional GWP approach, an emission pulse of any size is considered to have a warming effect compared to what we would have experienced without it and whether or not the rate of emissions is changing.
Figure 2 shows that the CO2-we derived from GWP* will be lower than CO2-e derived from conventional GWP when emissions are steady or reducing. 
CO2-we derived from GWP* will be higher than CO2-equivalent from conventional GWP when emissions are increasing exponentially at a rate of more than 1 per cent per time period. This point, which may not be appreciated by some who promote the use of GWP*, is demonstrated by Figure 3.
Although the GWP* approach may help to highlight the benefits available from reducing methane emissions, it can also be seen to excuse an ongoing steady rate of such emissions from a particular activity even when they contribute to what is an overall increase in emissions of methane and greenhouse gases in general.
The shorter-lived nature of methane is already reflected in GWP
A key point highlighted by many who promote the use of GWP* appears to be that methane is removed from the atmosphere much faster than carbon dioxide.
That point often seems to be portrayed as some sort of revelation. However, it is a fundamental element of methane’s conventional GWP.
As indicated earlier, it is why the GWP multiplier for a 20-year time horizon (GWP20) differs significantly from the 100-year figure (GWP100). The latter causes methane’s shorter-term impacts, which are crucial in terms of climate change feedback mechanisms and tipping points, to be understated.
“Flow” and “Stock” categories are subjective
Proponents of GWP* argue that carbon dioxide is a long-lived greenhouse gas that accumulates in the atmosphere because it does not break down (a “stock” gas) while methane is a short-lived gas that breaks down and does not accumulate (a “flow” gas).
Although the long life of carbon dioxide is a critical problem, it is removed from the atmosphere to a significant degree and (as demonstrated under the next heading) methane does accumulate.
The USA’s National Oceanic and Atmospheric Administration (NOAA) regards methane as long-lived. 
The IPCC regards it as a “well-mixed” gas in that it becomes mixed throughout the troposphere (the lowest layer of Earth’s atmosphere) to the point that concentration measurements from remote surface sites can characterise its climate-relevant atmospheric burden. It notes that methane and certain other well-mixed gases are sometimes referred to as ‘long-lived greenhouse gases’ as their atmospheric lifetimes are much greater than the time scale of a few years required for atmospheric mixing. 
The IPCC does not include methane in its other category of “near-term climate forcers” (NTCFs).
In relation to carbon dioxide, the IPCC says 33-50 per cent is removed from the atmosphere within decades. Within a thousand years, only 15-40 per cent will remain, with only 10-25 per cent remaining after ten thousand years. 
Methane concentrations have increased alarmingly
It is the concentration of greenhouse gases in the atmosphere, rather than the rate of emissions per se, that determines how much heat is trapped.
Atmospheric methane concentrations have nearly doubled in the last hundred years compared to an increase in carbon dioxide concentrations of just over one third., as demonstrated by Figure 4. 
A 2017 study published in the journal Global Change Biology estimated that methane emissions from the global farmed animal sector increased 332 per cent during the period 1890–2014. 
In July 2020, articles in Earth System Science Data and Environmental Research Letters and related media coverage highlighted the rapid increase in methane emissions from the period 2000-2006 to 2017. They reported that emissions from cattle and other farmed animals were the main cause of the increase and were almost as large as those from the fossil fuel sector.   
Although rice production is another significant contributor, its share was less than one-third that of farmed animals.
Sixty per cent of emissions were man-made and included (in order of contribution): 
- agriculture and waste, particularly emissions from ruminant farmed animals, manure, landfills, and rice farming;
- the production and use of fossil fuels, mainly from the oil and gas industry, followed by coal mining; and
- biomass burning, from wood burning for heating, bushfires and burning biofuels.
The remaining forty per cent came from natural sources and (in order of contribution) included:
- wetlands, mostly in tropical regions and cold areas such as Siberia and Canada;
- lakes and rivers;
- natural geological sources on land and oceans such as gas–oil seeps and mud volcanoes; and
- smaller sources such as termites in African and Australian savannas.
Even if farmers from any particular region or nation maintained their current level of methane emissions, they would be contributing to an overall pool of accumulated atmospheric methane that is increasing globally.
No matter the source of methane, it is “carbon on steroids”
Whether from fossil or biogenic sources, methane (comprising carbon and hydrogen) transforms carbon into a potent greenhouse gas that Professor Kirk Smith from University of California Berkeley has described as “carbon on steroids”. 
The fact that carbon may eventually be absorbed by vegetation and other sinks provides little comfort when one considers its adverse impacts while it exists in the form of methane.
Criticism from prominent climate scientists
GWP* does not appear to have been widely accepted.
An analysis utilising GWP*, prepared by an author who had not contributed to the original GWP* papers, has been said by IPCC lead author Professor Pete Smith of the University of Aberdeen and Professor Andrew Balmford of the University of Cambridge to be flawed and to represent a form of creative accounting.  The author had used GWP* to calculate a cooling effect of farmed animal production in the United Kingdom for the period from 1996.
Smith and Balmford said:
However, this creative accounting that appears to show that livestock methane emissions are not a problem is flawed. The GWP* metric, which was created to reflect methane’s relatively short atmospheric lifetime, was never intended to suggest that we should not worry about methane; it was proposed as a way of keeping the focus on the longer-lived greenhouse gas, carbon dioxide. Yes, the world needs to immediately and aggressively reduce carbon dioxide emissions, but that doesn’t mean that we don’t need to do anything about methane.Prof. Pete Smith, University of Aberdeen and
Prof. Andrew Balmford, University of Cambridge
Another IPCC lead author, Bill Hare, has argued that utilising GWP* could fundamentally undermine the integrity of the Paris Agreement’s mitigation goals by preventing it delivering an essential component of those goals. This is due to the extent it rewards a reduction in methane emissions, which could enable a nation to achieve net zero greenhouse gas emission in an accounting sense without achieving net-zero CO2 emissions. 
He has also echoed the concerns of researchers Joeri Rogelj and Carl-Friedrich Schleussner, who have raised concerns over equity and fairness in applying GWP* at a national, as opposed to global, level. Because GWP* considers future emissions in the context of past emissions, it could reward nations with a history of high methane emissions who transitioned to a lower, but still high, level. 
Rogelj and Schleussner provided examples of many countries, including New Zealand (the world’s highest methane emitter per person), Australia and USA.
Other alternatives to GWP
GWP* is not alone in being proposed as an alternative to the conventional GWP metric. No single metric is perfect, and value judgements will always play a part in determining which should be used.
Alternatives include: global temperature change potential (GTP); sustained-flux global warming potential (SGWP); instantaneous climate impact (ICI); cumulative climate impact (CCI); technology warming potential (TWP); Integrated global temperature change potential (IGTP); temperature proxy index (TEMP); and climate change impact potential (CCIP). 
Other problems with methane
Methane reacts with nitrogen oxides, carbon monoxide and other non-methane volatile organic compounds to form tropospheric (ground level) ozone, which is the third most prevalent greenhouse gas after carbon dioxide and methane (not allowing for water vapour).   [Footnote 3]
Although the indirect effects of methane on tropospheric ozone have traditionally been included in its GWP, tropospheric ozone involves another massive problem in that it adversely affects vegetation physiology, and therefore crop productivity and the ability of vegetation to absorb atmospheric carbon dioxide. It impedes plant growth and seed production, reduces functional leaf area and accelerates ageing. 
The impacts could be catastrophic given the extent of the climate crisis, species extinction and the growing global population.
Efforts to reduce methane emissions through the use of feed supplements face key logistical difficulties and may primarily be limited to dairy and feedlot animals, where their inclusion is a straightforward process. The emissions intensity of products derived from those animals is already lower than that of the grass-fed specialised beef herd, meaning the benefits of supplements may be lower than they may otherwise have been.
Other problems with animal agriculture
Producing animal-based foods affects the environment in dramatic ways, with methane being only one of many critical problems.
The impacts arise from many inter-related factors, such as the gross and inherent inefficiency of animals as a food source; the massive scale of the farmed animal sector; land clearing far beyond what would otherwise be required; greenhouse gases such as carbon dioxide and nitrous oxide (in addition to methane); and other warming agents such as black carbon.
Those factors cause animal agriculture to contribute on a massive scale to (among other critical problems) the existential threats of climate change and species extinction.
Addressing the issue of animal agriculture comes without the need to confront a great dilemma of fossil fuel emissions, involving the formation of aerosols.
Although lower pollution levels are generally welcome, they can represent a double-edged sword in terms of global warming and climate change. The reason is that the burning of fossil fuels that contributes to carbon dioxide and other emissions also releases aerosols, which are airborne particulates including (but not limited to) sulphates and nitrates.
Although some types of aerosol can have a warming impact, the overall global impact is one of cooling, sometimes referred to as “global dimming”.  This has offset some of the warming effects of greenhouse gases, keeping warming to lower levels than we would otherwise have experienced.
Aerosols only remain in the atmosphere for days or weeks, so their cooling impact will be short-lived in any transition away from fossil fuels to less carbon-intensive energy sources.
This dilemma has been referred to by leading climate scientist Dr James Hansen as a “Faustian bargain”, alluding to Doctor Faustus of folklore and legend, who sold his soul to the devil in exchange for knowledge and power. 
The dilemma is almost non-existent in efforts to address the impacts of animal agriculture.
Using the GWP* metric or other factors to justify ongoing animal-based food production would almost certainly prevent us from overcoming the climate crisis. With the future of a habitable planet at stake, we must face reality and transition away from animals as a food source.
- For the planet’s temperature to be stable over extended periods, incoming solar radiation and outgoing thermal infra-red radiation need to be equal. This state of balance is referred to as radiative equilibrium. If the balance is upset by greenhouse gases which absorb some of the outgoing heat energy, then the planet will gradually heat up in order to restore it. The initial change in balance is known as radiative forcing.
- The lifetime mentioned is the perturbation lifetime, which allow for the effects of a greenhouse gas arising from chemical feedbacks. Around two-thirds of methane is broken down roughly every twelve years. The turnover time of methane (the length of time a methane molecule remains in the atmosphere) is around nine years.  
- Major sources of carbon monoxide are agricultural waste burning, savanna burning and deforestation. Farmed animal grazing is one of the main drivers of deforestation and savanna burning.
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Featured image: Cameron Watson, Shutterstock ID 1125122366
Capsule: Lightspring, Shutterstock ID 1539353021
Methane molecule: LoopAll, Shutterstock ID 560965879