In my recent Farmers Weekly article on methane, criticisms that I made of the GWP*metric, pronounced “GWP-star”, stirred up responses from some of my agricultural friends and colleagues. Many farmers and also important farmer organisations would like to see GWP*used as the methane accounting metric. I received some long emails setting out where they thought I had gone wrong.
My response here is first to emphasise a point I have made many times: a prosperous agriculture is fundamental to New Zealand’s economic future. Primary industries are what pays for the majority of our imports. So we have to get things right.
My second point is to acknowledge that there are sound arguments why the concept of carbon dioxide equivalence (CO2e), at least as currently used, is seriously flawed. But it is important to focus on proposals that will stand widespread scrutiny, and not live in an echo chamber.
The key argument put forward for GWP* in place of the widely used GWP100 (without the star), is that it accounts more accurately for the warming caused by methane. But the answer is not quite as straightforward as many of my friends think.
If it is just a case of measuring the outgoing radiation that is blocked by new methane emissions, in comparison to outgoing radiation that is blocked by new carbon dioxide emissions, then the existing GWP measures that very well within the limits of known science. However, there is a caveat that the time horizon over which the comparison is undertaken is of fundamental importance.
Conventionally, this has been on a 100-year horizon (GWP100), which captures more than 99% of the methane heating units, but captures only a small proportion of long-lived carbon dioxide emissions. So, the GWP100 screws the scrum by ignoring all of the effects of long-lived carbon dioxide that will occur thereafter. That assumes, of course, that current society has a responsibility to leave behind a liveable climate that goes well beyond 100 years. My own value judgment is that we would be better to use a 500-year (GWP500) basis for comparisons, thereby recognising the long-lived effects of carbon dioxide. By doing so, we would not be reducing the assessed value of the methane’s radiation-blocking effect, but we would be acknowledging that currently accepted science says that carbon dioxide lasts much more than 100 years. The numbers for GWP500 are set out in the IPCC AR6 document as laid out in my last article.
The opposing perspective of some of my friends is “Ah, but this is just the emissions effect and not the warming.” To which my response is that it all depends on how you measure the warming. Don’t get taken in by the notion that new methane emissions are benign.
The way I like to explain it is to get people to think of a “methane cloud”. We cannot actually see the cloud, because methane is colourless and has no smell. But there it is, up in the sky, and also all around us. It is a consequence of historical emissions. It is close on three times the size of the global methane cloud 150 years ago, although most of that increase has nothing to do with agriculture.
The currently accepted science is that methane has an atmospheric lifetime of around 12 years, or more precisely 11.8 years as recorded in the IPCC’s AR6 report. But this does not mean that all the methane disappears over 11.8 years. Rather, it means that half of the radiation-blocking effects occur in the first 11.8 years. It also means that 75% of the radiation-blocking effects will occur within 23.6 years, and about 88% will have occurred in the first 35 years.
For those who are mathematically focused, the decay is considered to be a first-order exponential decay function with a consequent long tail. For those who are not mathematically focused, don’t worry. Just accept that only half of the warming effects occur within the first 12 years.
Now, where does this all fit in within the GWP* effect?
The answer is that when claims are made that further methane emissions will not cause the temperature to further increase, referred to as “no warming”, the claimants are saying that these new emissions will do no more than balance out the ongoing decay of historical emissions.
The scientists who developed the GWP* equation have not said that new methane emissions are benign. But they have said that by their calculations, which do include some further assumptions, that as long as “global” methane emissions reduce by 0.33% per annum, then by 2050 new emissions would be in balance with the decay of historical emissions. Note that this is on a global basis. Other sources have come up with somewhat higher figures and a clear consensus has yet to emerge.
What the physicists who developed GWP* have not considered is how these concepts could be applied at the country level or at the level of individual businesses. That is not where their expertise lies. Moving from a rugby analogy to a cricket analogy, it is at the country and individual business levels where the wicket gets real sticky.
There is a very nice 2022 exposition developed by authors Rugelj and Schleussner and published in the journal Environmental Research Letters that illustrates the sticky wicket. They develop the example of three young farmers, called Abraham, Bethany and Christopher. Each of them has 10 cows and each produces the same amount of milk.
Farmer Abraham inherited his farm and the 10 cows from his grandfather, who had farmed that way for more than 20 years. Farmer Bethany came from a poor family with no cows but managed to secure a loan to buy 10 cows. Farmer Christopher inherited both land and 20 cows from his grandfather who had farmed those 20 cows for many years, but Christopher quickly reduced the cow numbers to 10 to fit in with his other activities.
Over the next 20-year period they each produce the same amount of milk from their 10 cows and each farm emits one tonne of methane per year. However, using GWP*, Abraham is assessed as producing 140 tonnes of CO2e over 20 years of farming, whereas Bethany produces 2240t of CO2e and Christopher is assessed as producing a negative 1960t of CO2e.
How is it that all of these farms are assessed differently under GWP* for the next 20 years just because they had different grandfathers? Well, GWP* gives Christopher a huge credit for the decline in his methane cloud relative to his grandfather, whereas poor Bethany has no prior methane cloud on which to call for credits. Abraham has enough methane decay in his cloud to almost balance his ongoing emissions so his assessed liability is very small.
Generalising to the underlying principle of “grandfathering” as it applies to environmental issues, it refers to situations where actions of a business or country in the past gives rights to the future. It shuts the door on the “Bethanys” of this world and leaves the door wide open for the Christophers to maintain existing behaviours and do so profitably. That notion of “to those who have shall be given” goes very much against the Paris Agreement.
Well, that is enough on GWP* for now, but the issue is not going to go away. I simply say to my farming friends that it is best to fight battles that are winnable.
GWP* provides useful insights as to the global temperature effects of methane taking into account both the warming from the current emissions and the decay from the historical emissions. It can also highlight at the global level that if global warming really is the threat that many consider it to be, then by far the most effort has to be on reducing CO2 rather than methane. But it does not provide a basis for working out how each country and business therein should bear the costs.
So, what are the other arguments that methane decision-makers need to be aware of?
In my opinion, the most interesting new science is a paper published in Nature Geoscience in 2023 demonstrating how current science has largely ignored the absorption by methane of UV energy from the sun.
When the effects of this absorption in the troposphere are included, their modelling indicates, somewhat counterintuitively, that within an atmospheric-systems framework that clouds increase such that less UV rays reach earth. The effect is estimated to reduce the overall radiation-forcing caused by methane by about 30%.
I am not aware of any reporting in the NZ media of this research. However, the international science community has reacted positively to these findings, while recognising that they need to be confirmed using other models before a high confidence level is placed in the results.
I have said enough for this article, but I still have not got to the end of the story. In particular I have not laid out a path to a future that protects NZ’s primary-industry led economy and does so in a way that will be internationally acceptable. That will be the next article before moving on to issues other than methane.
In Focus Full Show | Friday 19 January. Listen Now!