This is the second of a series of articles discussing some of the difficult issues that have to be understood and resolved in relation to New Zealand’s proposed Zero Carbon Bill. The first article is here
For many years, we have been told that agriculture contributes approximately half of New Zealand’s greenhouse gases. This supposed fact has been seared into the minds of every New Zealander who reads newspapers, listens to radio or watches television.
What very few people understand is that this supposedly simple fact is based on a dodgy assumption that the effects of methane can somehow be turned into equivalent units of carbon dioxide.
I liken it to the silly statement that if you have one carrot and one banana then you have three banana equivalents. The problem with such a nonsensical statement is that carrots have a totally different nutritional profile to bananas in terms of fat, starch, sugars, fibre and various micro nutrients.
Everyone can therefore quickly see that making a global assessment about how many carrots equals one banana is nonsense. It all depends on what nutritional component you are talking about.
Similarly, the number of units of carbon dioxide that is equivalent to one unit of methane depends totally on the time horizon on which the comparison is made. One is a short-lived gas and the other is a long-lived gas. This means that a short time horizon captures all of the effects of the short-lived methane but only a small proportion of the long-lived carbon dioxide.
When methane and carbon dioxide are bundled up into units of carbon oxide equivalents, it is conventional to use a 100-year time horizon. The relative number for each gas is given as a GWP100 index figure, with GWP being shorthand for ‘global warming potential’.
By definition, the GWP100 for carbon dioxide is 1. The relative value for methane, according to the latest estimate of the IPCC is 28, although all accept that there is uncertainty around the precise number. Back in 1995 it was supposedly 21, by 2007 it was believed to be 25, and there are some who believe a more appropriate figure might be 34. It seems that the New Zealand official national inventory is currently calculated using a figure of 25.
This illustrates that although there is general agreement among scientists that methane has strong global warming potential, the specific value is far from settled science.
The key point for the current discussion, however, relates to the choice of 100 years as the relevant time horizon. This is not a matter of science but a value judgement.
For those who do not care about what happens to the world beyond 100 years, then a GWP100 would be reasonable. For those who believe it is important that the world survives beyond 100 years then a longer time horizon is appropriate.
In its early reports, the IPCC used to also provide GWP500 estimates for the three key gases of carbon dioxide, nitrous oxide and methane. By definition, the values for carbon dioxide must still be ‘1’ given that carbon dioxide is the reference gas. However, the number of heating units captured within this reference value now increases by a factor of more than three compared to the GWP100 value.
Accordingly, the GWP500 for short-lived methane will drop by a factor of more than three compared to the GWP100, because all of methane’s warming is already captured in that GWP100.
In 1995, the GWP500 estimate for methane was 6.5 units of CO2 equivalent, in 2001 the estimate rose to 7.0, and in 2007 to 7.6. Thereafter no new values have been provided, a point I will return to later.
The key take-away message from what may seem a complex and thereby confusing story, is that with a GWP of 500 years, the relative importance of methane as a global warming gas compared to carbon dioxide declines by a factor of more than three. And so agricultural greenhouse gases, and methane in particular, would be considered of much less critical importance if we were using GWP500.
So, next time someone says that agriculture contributes close to half New Zealand’s greenhouse gases, ask them what time horizon they are using. If the response is 100 years, then ask the reason why they have chosen 100 years. The correct answer is actually a non-answer that ‘it is the convention’.
However, the more likely response to the first question will be a blank stare. This is because very few people appreciate the hidden assumption about time horizons in the current mantra.
I first wrote about this issue way back in 2006 in the journal Primary Industry Management. However, that article did not reach the mainstream media. Also, I was more than a little pre-occupied by many other things at that time. And so, I let the matter rest.
Coming back to the issue of why there have been no updated figures for the methane GWP 500, the answer is that it is because of increasing uncertainties around the atmospheric life of carbon dioxide. Quite simply, we know that both carbon dioxide and methane are greenhouse gases, but we don’t have the science yet to make good comparisons over long time horizons.
Accordingly, for those of us who do believe that saving the planet is indeed important beyond time horizons of 100 years, the whole concept of carbon dioxide equivalents should be considered as simplistic nonsense.
Putting the issue of time horizons aside, there is another key reason why methane needs to be considered separately. It relates to the difference between gross and net emissions.
In the case of New Zealand’s ruminant-sourced methane, the gross emissions have been close to static for the last 30 years. This is documented in the national inventory of greenhouse gas emissions and is not controversial.
Given that the average resident time of methane is believed to be around 12.4 years, we have now reached a point where the atmospheric cloud of ruminant-sourced methane is no longer growing.
What this means is that net emissions of ruminant-sourced methane into the atmosphere are effectively zero. The carbon cycle is working nicely with a new balance having been reached. To the extent that global levels of methane are still increasing, they are from sources other than New Zealand’s ruminants.
This stability of the ruminant-sourced atmospheric methane cloud contrasts greatly to the carbon-dioxide cloud. Both in New Zealand and globally, the levels of carbon dioxide continue to increase. Even if those emissions were to decline, then the atmospheric cloud would continue to increase because carbon dioxide lives there for hundreds of years.
New Zealand’s aim is to get to zero net emissions of carbon dioxide by 2050. Methane from ruminants is already at that point!
As a concluding comment, I emphasise that none of the arguments in this article say that methane is unimportant as a greenhouse gas. Nor is any claim made as to the importance or otherwise of global warming. What is said, however, is that working out how to treat methane emissions fairly in any regulatory framework requires an understanding of its unique characteristics. There is lots of work to be done.