New Zealand has got itself into a right proper muddle over methane emissions and their impact on climate change. A simple change to the proposed legislation would sort it out.

The proposed Climate Change Response (Zero Carbon) Amendment Bill treats biogenic methane emissions differently from all other carbon emissions. The latter are to be measured net so that emissions from fossil fuels can be offset by carbon stored in trees. However, methane from livestock is measured gross.

Why make the distinction? Measuring methane net both clarifies what is going on and simplifies how to deal with it. Our problem arises because we are focusing on emissions and not paying sufficient attention to the fact that global warming is the consequence of clouds of gases in the stratosphere which cause the earth’s heat to be retained rather than escaping. Emissions contribute to these clouds. But at the same time molecules in the methane cloud are breaking down.

A simple illustration of how the clouds affect the temperature is that still winter nights without rain clouds are usually followed by a frost. The clouds reflect back the heat coming off the earth, generating higher ambient temperatures and reducing the risk of frost. The clouds of greenhouse gases (GHGs) in the stratosphere similarly reflect the heat coming off the earth. However, while the visible rain clouds come and go, the GHG clouds have been increasing.

The main reason is that human activity on the ground has emitted more GHGs into the air. Fossil fuels burnt since the industrial revolutions have increased atmospheric carbon-dioxide by around 40 percent. The evidence of associated global warming is palpable. If we continue to emit GHGs, adding to the clouds, temperatures will rise further.

The largest GHG cloud (aside from water) is composed of carbon-dioxide. Next is the methane one. It is much smaller than the carbon-dioxide cloud but because methane is more reflective of the heat, its contribution to total global warming is about a third of the carbon-dioxide one.

The methane cloud comes partly from waste, swamps and other ground sources, but 40 percent worldwide comes from livestock belching. Because New Zealand does not use much coal, methane emissions are the single largest contributor to our GHG clouds.

The half-life of atmospheric methane molecules is about 10 to 12 years. (Carbon-dioxide has a much longer half-life.) The breakdown from livestock methane is, in effect, to nothing. Its carbon came from atmospheric carbon-dioxide converted into grass, eaten, ruminated, belched and as methane breaks down, eventually returns to atmospheric carbon-dioxide, completing the cycle.

I have quantified these effects. Instead of looking at the emissions from belching, it focuses on the methane cloud generated from New Zealand livestock. That cloud is about 13MTs (million tonnes) of methane. Last year around 1.1MTs of it broke down into atmospheric carbon-dioxide. Meanwhile our livestock belched another 1.1MTs – almost exactly the same as the breakdown tonnage.

The livestock-methane cloud has been at the 13MTs level for about forty years. The stability is because the annual gross methane emissions contributing to the cloud have been fairly constant over the last fifty years. That leads to an equilibrium because the additional methane is offset by the methane breaking down.

(To be clear, these calculations do not affect by one iota the actual degree of global warming, nor the urgency of restraining it; they simply change our understanding of what is going on.)

Fundamentally, it is not the GHG emissions which are the problem but the GHG clouds which reflect the earth's heat; the significance of the emissions is that they add to the clouds. We should measure our contribution to global warming in terms of the GHG clouds. That means the livestock contribution is small, zero or even negative because it is being offset by the breakdown of the methane from past belching.

The salient change from focusing on net methane emissions to the clouds is that the picture of our total GHG emissions changes markedly. Include gross methane emissions and the total GHG emissions rise from about 36MTEs (million tonnes carbon equivalent) in 1990 to 56MTEs in 2016. That is a disappointing 54 percent increase.

Measured with net additions to the methane cloud, the increase of our total GHG emissions is from 2.7MTEs to 22MTEs, about eight times as much. (Internationally, we rank high on a per capita basis measure using gross emissions but this net adjustment puts us near the middle.)

Most sectors contributed to the rise but there have been two big ones: energy emissions from fossil fuels, especially from the transport sector, while land-use change and forestry are absorbing less carbon today than thirty years ago. The methane story – gross emissions having hardly changed in the period – has hidden the overall disastrous record of these two sectors.

The consequence of this muddled thinking is that too much of the blame for the New Zealand contribution to global warming is attributed to farmers and insufficient attention has been given to the transport sector which is the number one problem. Forty years ago, livestock was contributing to global warming but it is no longer doing so today.

The remedy to the muddle in the Climate Change Response (Zero Carbon) Amendment Bill is simple. The proposed legislation currently reads:

• The target for emissions reduction (the 2050 target) requires that—

• (a) net emissions of greenhouse gases in a calendar year, other than biogenic methane, are zero by the calendar year beginning on 1 January 2050 and for each subsequent calendar year; and

• (b) gross emissions of biogenic methane in a calendar year—

• (i) are 10% less than 2017 emissions by the calendar year beginning on 1 January 2030; and

• (ii) are at least 24% to 47% less than 2017 emissions by the calendar year beginning on 1 January 2050 and for each subsequent calendar year.

The references to gross biogenic methane in subclause (a) and subclause (b) need to be deleted. The provision becomes simpler, more understandable and less arbitrary as follows:

• The target for emissions reduction (the 2050 target) requires that—

• (a) net emissions of greenhouse gases in a calendar year are zero by the calendar year beginning on 1 January 2050 and for each subsequent calendar year.

The effect of the change is to relax the target a little, by crediting New Zealand with the breakdown of methane from its methane cloud. That surely is right.. Suppose you personally emitted a vicious gas which broke down into something benign seconds later. Would we want to include that in the calculations?

In any case why allow the netting out of thermogenic carbon emissions but not those from biogenic carbon. Motorists are allowed to net their carbon emissions against carbon sinks but the farm sector is not allowed to net its methane emissions against the past methane emissions which are breaking down. Why discriminate against farmers?

As far as the farm sector is concerned, the proposed change replaces an arbitrary target with the feasible one of doing the best the sector can. While it is already in (about) balance as far livestock methane emissions are concerned, any further reduction in them eventually reduces the methane cloud, which reduces global warming. That is obviously beneficial. The proposed change to the legislation in no way inhibits that goal, but treats it in a sensible way.

Notes

The paper which explains the story of net and gross methane emissions is New Zealand’s Methane Cloud. A simplified version of the paper is Up in the Clouds. (Which repeats some of the above).

My overall view of the challenge is Towards a Low Emission Economy