New calculations suggest that the farm sector is not adding as much to the greenhouse gas clouds as previously thought. But there remains the challenge of global warming which farmers must still take up.

Still winter nights without rain clouds are usually followed by a frost. The clouds reflect back the heat coming off the earth maintaining higher ambient temperatures, thereby reducing the risk of frost.

The clouds of greenhouse gases (GHGs) in the stratosphere similarly reflect the heat coming off the earth. Since one cannot see them it is easy to pretend they do not exist. 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 (or triggered natural processes which have that effect). Fossil fuels (mainly coal, oil and other hydrocarbons) burnt since the industrial revolutions have increased the atmospheric concentration of 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. Climate scientists have various predictions about how much. This professional economic forecaster is sceptical about the more precise predictions but unquestionably the temperature increase will continue if current emission trends continue. The consequences of the rises (which include rising sea levels) will be more than we – and future generations – can cope with.

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. (Around four-fifths of our gross methane emissions are from livestock.)

A feature of the atmospheric methane molecule is that it breaks down, on average after 12 years, which means that molecules in the existing methane cloud are diminishing, while new emissions are adding to them. The breakdown from livestock methane is, in effect, to nothing as far as global warming is concerned. 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. (In contrast carbon-dioxide has a much longer half life.)

I have just published a paper quantifying these effects. Instead of looking at the emissions from belching, I focused on the methane cloud generated from New Zealand livestock. I found that the cloud is about 13MTs (million tonnes) of methane. Last year around 1.1MTs of it broke down, back 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, reflecting the same quantity of heat as it has done in the past but not blocking further escaping heat. 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 methane adding to the is offset by the methane breakdowning down from it.

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.

It seems to me that 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 a little negative) because it is being offset by the breakdown of the methane from past belching. We can argue over this but suppose you personally emitted a vicious gas which broke down to something benign seconds later. Would we want to include that in the calculations? 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.

The salient change from focussing on net methane emissions to the clouds is that the picture of our total GHG emissions are  markedly changed. Include gross methane emissions and the total GHG emissions rose from about 36.3MTEs (million tonnes carbon equivalent) in 1990 (the first year for which we have data) to 56.0MTEs in 2016 (the last year for data). 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 21.9MTEs. Not unexpectedly, the level is lower. In fact 30 years ago  were close to zero net carbon emissions, we are now emitting about eight times as much.

Most sectors contribute to the rise but there have been two major contributors. First, energy emissions (mainly hydrocarbons) 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 (that is, the forestry sector’s absorption of carbon is making a smaller contribution). 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. But the farm sector remains a net contributor to the GHG clouds. In addition to diesel and petrol that farmers use, nitrous-oxide emissions are smaller but they are more vicious in terms of their reflecting heat and they last ten times longer than methane. However, farming is no longer the headline sector. We are, when we use carbon-emitting transport.

It would be easy to shift the blame for the methane cloud to farmers 12 years ago, but you could go on and blame those who removed the bush cover in the last 750 years of human habitation and anyway other countries are making larger contributions to global warming. (We used to rank high internationally on a per capita basis measure but this adjustment puts us near the middle of the pack.)

The blame game does not help. Have you noticed the tendency to blame others rather than do anything? What is needed is action, including from the farm sector. Hopefully farmers will not use this approach to withdraw from the challenge of reducing and restraining the GHG clouds. For instance, if gross livestock-methane emissions could be reduced by, say 3 percent a year, the cloud would diminish in size – by 2050 it would be about the same as it was a century earlier, so there would be less global warming from it. And, of course like the rest of us, farmers need to reduce those other GHG emissions which are clouding the future of the world.