Summary:. Methane (CH4) is a greenhouse gas. Some methane comes from atmospheric carbon-dioxide which is fixed in grass, eaten by livestock, belched into the atmosphere and eventually returns to atmospheric carbon-dioxide. Before it breaks down, the methane molecules form a ‘cloud’ in the atmosphere which adds to global warming at a much higher rate per carbon molecule than does a carbon-dioxide molecule.
This paper focuses on the methane ‘cloud’, the stock of methane in the atmosphere which contributes to global warming. What is unusual about this cloud compared to, say, the carbon-dioxide one is that it has a relatively short half-life. Focusing on it leads to a different measure of the impact of methane emissions from livestock. Their contribution to the size of the methane cloud was substantially in the past. Today their methane emissions are being (largely) offset by the methane already in the cloud breaking down
New Zealand’s carbon emission accounts include methane emitted from livestock. However, in them methane is currently measured as a gross emission, rather than as a net addition to the cloud, which allows for the fact that past methane emissions are breaking down. When this is included it seems that New Zealand’s net contribution to the methane cloud is much smaller than as recorded on a gross basis.
Even so, New Zealand should be aiming to reduce its methane emissions too as a contribution to reducing global warming.
This paper does not address other sources of methane (which may be subject to a similar analysis) nor that there are other greenhouse gases emitted by farming, such as nitrous-oxide.
A column commentary on this report is here.
Gross Methane Emissions
We have estimates of the gross methane emissions from livestock going back to 1800 (effectively from the beginning because there was no livestock in New Zealand much before that date). Projections of the size of the methane cloud to 2050 are based on current forecasts of the gross emission rate.
The pattern is shown as the blue line in Figure 1. There is a fairly steady growth of emissions to the late 1960s – say of 4 percent a year. After that, the rate of gross emissions is almost flat. The methane is measured in millions of tonnes (MTs). (Later, when being compared to other sources of global warming it will be measured in million tonnes equivalent of CO2 (MTEs) for which a tonne of methane is multiplied by about 25, reflecting at its greater contribution to global warming than carbon-dioxide.) The red line shows net additions to the methane cloud. It is explained below.
Sources: Gross emissions: A Note on Methane Emissions from Livestock:– Andy Reisinger(2018);
NAMC: Ressinger (2018) and figure 2 as explained in and derived in text.
The Size of the Methane Cloud
Once in the atmosphere the methane breaks down into carbon-dioxide at a rate of about 9 percent a year, thereby diminishing the size of the methane cloud. Meanwhile the cloud size is being increased by new emissions.
We use a perpetual inventory method to calculate the size of New Zealand’s methane cloud. Essentially it involves adding the annual gross methane emissions to the cloud at the beginning of the year which is depleted by 9 percent for the methane molecules which have returned to the atmospheric carbon-dioxide from whence they came.
For instance, at the end of 2016 New Zealand’s methane cloud from livestock amounted to 12.9 MTs. During 2017 1.1 MTs converted back into CO2. But gross emissions in the year were also 1.1 MTs so at the end of 2017 the methane cloud was still 12.9 MTs. (This is not entirely coincidental – something like this is bound to happen when the gross emission rate is constant over a long time.)
Figure 2 shows the size of New Zealand’s livestock methane clouds since 1800 measured in MTs. .It, too, shows a steady growth up to the early 1970s and then there is a flattening out.
Source: Derived from Figure 1 as explained in text.
Net Additions to the Methane Cloud
The difference between the sizes of the cloud in any two years measures the net additions to the methane cloud (NAMC). Thus while the gross additions to the methane cloud (GAMC) – i.e. the gross methane emission –from belching livestock in 2017 was 1.12 MTs, the net rate, after deducting for the methane in the cloud which returned to carbon-dioxide, was near zero. (There is some rounding in the last few paragraphs.)
It makes sense to measure New Zealand’s livestock methane impact on global warming in terms of NAMC rather than GAMC terms, just as we measure the carbon sink in trees by deducting the carbon released from their harvesting from the carbon absorbed in growing trees. The methane cloud is a kind of ‘anti-sink’ – a negative forest.
(While the methane breaks down to carbon-dioxide, a greenhouse gas, that carbon-dioxide was originally in the atmosphere, unlike that which comes from fossil sources. So over the total cycle the carbon-dioxide nets out.)
Figure 1 also shows – in red — NAMC together with GAMC. (The net series is a five-year moving average to soften some of the year to year volatility.
New Zealand’s Total Carbon Emissions
Currently gross methane emissions are used when calculating New Zealand’s contribution to the global warming cloud. In 2016 the estimates (in MTEs) are as follows:
New Zealand’s contributions to the Global Warming Cloud: 2016
Sector With GMAC With GMAC With NMAC With NMAC
MTE Share MTE Share
Energy 31.3 56.0% 31.3 142.9%
Processes 4.9 8.7% 4.9 22.4%
Waste 3.8 6.9% 3.8 17.4%
Methane 33.8 60.4% -0.2 -0.1%
Agriculture 4.9 8.8% 4.9 22.4%
Land use, land-use change
& forestry -22.7 -40.7% -22.7 -103.7%
TOTAL 56 100% 21.9 100%
(Agriculture 38.7 69.2% 4.6 22.3%
(Note: It assumed in the calculation that the story for methane generated from sources other than livestock is similar to the livestock-generated methane. This is unlikely to be exactly true but there is not the data to be more precise. In any case, the other methane is only about a fifth of that by livestock-generated methane, so greater precision will not make a major difference.)
Including only net additions to the methane cloud dramatically changes the picture cutting back New Zealand’s contribution to atmospheric carbon by over half and reducing the role of the agricultural sector substantially. One consequence is that New Zealand is not as relatively high in the international pecking order of per capita carbon emissions as is sometimes claimed.
Other years show a different net balance. Not every year since 1990 has had negative NAMC. But positive or negative, their effect is small compared to overall emissions.
This paper is an exercise in improving measurement. Policy responses need not immediately follow from it. For instance, there is nothing in this paper which implies that New Zealand should desist from seeking to reduce its carbon emissions.
However, a couple of comments may be relevant.
First, the revised estimate does not let agriculture off the hook. Its past activity has put almost 13 MTEs of methane currently up in the atmosphere which is contributing to global warming. If gross methane emissions could be reduced there would be less global warming. For instance, were gross methane emissions to decrease by 3 percent each year, the methane cloud in 2050 would be halved, about the same size as it was a hundred years earlier in 1950. The reduction would be to offset a year’s emissions from other New Zealand carbon sources. (Sadly, that option is not possible for other major greenhouse gases such as atmospheric carbon-dioxide sourced from fossil fuels.)
Second, as the next table shows, the use of gross methane emission estimates has obscured the real problems in New Zealand’s rec
New Zealand’s contributions to the Global Warming Cloud: 1990-2016
Sector MTE 1990 MTE 2016
Energy 23.8 31.7
Industrial Processes 3.9 4.9
Waste 3.9 3.8
Methane (NAMC) -1.0 -0.2
Other Agriculture 1.6 4.9
Land use, land-use change
& forestry -29.5 -22.7
TOTAL 2.7 21.9
(Agriculture 0.6 4.7)
In 1990 (the first year for which we have detailed records) New Zealand’s aggregate carbon-equivalent emissions (after allowing for methane breakdown) amounted to about 2.7 MTEs. In 2016 (the last year for records) they were 21.9 MTEs, about eight times as much. Most sectors contribute to the rise (waste is an exception) but there has been two major contributors. First, energy emissions are about 30 percent higher. Second, land use, land-use change and forestry are absorbing about 25 percent less carbon in 2016 than they were in 1990. Methane hardly comes into the equation.
Acknowledgments: I am grateful for assistance from and discussions with Andy Reisinger, John Carrad, Elizabeth Caffin, Alan Gray, Suzi Kerr, Jacqueline Rowarth and David Young.<>