Written for a farm journal but not published

Still winter nights without rain clouds are usually followed by a frost. The clouds reflect back the heat coming off the earth; the higher ambient temperatures and reduce 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 from waste, swamps and other ground sources, but 40 percent worldwide comes from livestock belching. Because New Zealand does not use much coal, our methane emissions, as conventionally measured, are the single largest contributor to our GHG clouds.

Atmospheric methane molecule breaks down, on average after 12 years, which means that molecules in the existing methane cloud are diminishing. (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 as methane which after a while returns to atmospheric carbon-dioxide, completing the cycle.

A paper I published quantified these effects. Instead of looking at the emissions from belching, I focused 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 to 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 reason for the stability is that the annual gross methane emissions contributing to it have been fairly constant over the last fifty years. Mathematically, 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. But they change 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 (in some years even negative) because it is being offset by the breakdown of the methane from past belching. Suppose you personally emitted a vicious gas which broke down to something benign seconds later. Would we want to include that in the calculations?

The salient change from focussing 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 rose 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 22 MTEs, 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. First, energy emissions are about 30 percent higher. Second, land-use change and forestry were absorbing about 25 percent less carbon in 2016 than they were in 1990. The methane story – gross emissions have hardly changed in the period – has hidden the overall disastrous record of these two sectors.

Farmers feel they have been unfairly blamed for contributing to global warming. They will take some comfort from this revised perspective, although the farm sector remains a net contributor to the GHG clouds from the fuel that farmers use and nitrous-oxide emissions. However, farming is no longer the headline sector. All of us are.