Brian Fallow: Time to turn down the methane tap

Methane from sheep and cattle accounts for about a third of NZ's gross emissions, according to the latest national inventory. Photo / NZME

OPINION:

With an endless stream of news about the likes of flooding on the East Coast and a drought in the far south so severe that the Awarua wetlands catch fire, you would have to have your eyes firmly shut and fingers in both ears not to get the message Mother Nature is sending us about climate change.

This week's report from the United Nations' Intergovernmental Panel on Climate Change is very clear that the days are over when governments, and the rest of us, could get away with saying "We all know it's a serious problem, but now is not the time to do anything serious about it".

For the main greenhouse gas, carbon dioxide, the primary remedies are well known: wean ourselves off fossil fuels and plant a lot more trees.

But in New Zealand's case, a key source of our conspicuously high per capita emissions is the methane belched by cattle and sheep, which represents 35 per cent of gross emissions, according to the most recent national inventory.

And that presents a tricky policy problem because methane is a potent but short-lived greenhouse gas, and how hard you want to be on it depends on whether you focus on the potent part or the short-lived part.

It is extremely good at trapping heat but, fortunately, it gets oxidised into the much less potent CO2 within a few years by reacting with hydroxyl radicals in the troposphere.

The 35 per cent share of national emissions is calculated on the basis used for international accountability under, for example, the Paris Agreement.

The GWP100 metric the world has fastened on to for carbon accounting purposes estimates the heat-trapping effect a tonne of a greenhouse gas will have over the arbitrary period of 100 years after it is emitted, with CO2 as the benchmark. In methane's case, the current estimate is 27 times as much as CO2. Using the GWP20 measure instead, based on the 20 years after emission, the ratio would be over 80 times.

The latest IPCC report acknowledges the limitations of the standard GWP100 metric, especially when applied to a "short-lived climate forcer" (SLCF) like methane.

"This is because warming caused by an individual SLCF emission pulse [e.g. a bovine belch] is not permanent and hence, unlike CO2, the warming from successive SLCF emission pulses over multiple decades or centuries depends mostly on their ongoing rate of emissions, rather than cumulative emissions." This is sometimes expressed as methane being a flow gas rather than a stock gas.

On the rough analogy of running a bath, you normally start by inserting a plug in the plughole before turning on the tap.

Before long you have to turn the tap off or there will be no room for you in the bath and it will overflow. That is the situation with the long-lived gases: emissions need to go to zero.

With methane it is as if there is no plug.

What matters is how much water is coming out of the tap (the rate at which methane is being emitted) compared with how much is going down the drain (the rate at which it is then oxidised).

So long as the former does not exceed the latter, the level of the water — the concentration of methane in the atmosphere — will not rise, and no additional warming, or very little, from that source will occur.

New Zealand faces a tricky problem in cutting back emissions from livestock. Photo / NZME

Essentially this is the basis for the split gas approach which the Zero Carbon Act adopted and which sets a more lenient target for reducing methane emissions than for long-lived gases like CO2 and nitrous oxide.

MPs on the environment and primary production select committees received a briefing on this yesterday.

Professor Dave Frame, director of the NZ Climate Change Research Institute at Victoria University of Wellington, said the average lifetime of a methane molecule in the atmosphere was about a decade or so.

In the case of a short-lived greenhouse gas like biogenic methane, it is the recent and current emissions that determine its contribution to global warming, in contrast to the long-lived gases where it is the cumulative emissions since the start of the Industrial Revolution, he said.

"The issue is, when we combine these gases in a single policy instrument like the emissions trading scheme using something like GWP100, that masks the fact that some of these gases hang around forever and some disappear after 20 years," Frame said.

"If you really want to know what the temperature implications of your emissions are, then you unbundle them and report the short-lived gases separately from the long-lived gases because they contribute in different ways."

Mushing them together and pretending they are fungible made no sense.

"What you do with short-lived gases is a second order effect, behind what you do with CO2. Short-lived gases can buy you 0.3 or 0.5 degrees of warming [avoided] — which is not an insignificant chunk of warming that we should do something about — but whether you get 1.5 degrees through to 5 degrees of warming is determined overwhelmingly by choices we make about carbon dioxide," Frame said.

The IPCC report says modelled pathways that would give the world a better than 50:50 chance of limiting global warming to 1.5C above pre-industrial levels — a threshold we should dread to cross — include global methane emissions reducing by a third by the end of this decade and 44 per cent by 2040.

Unfortunately, the global picture is one of rising methane emissions. Even though only a minority — around 39 per cent — of methane emissions from man's activities come from agriculture, the IPCC report found agricultural methane emissions have risen 10 per cent since 1990 and are dominated by enteric fermentation in ruminant livestock.

If nothing is done, the combination of a rising population and increasing demand for the foods of affluence will see those emissions continue to climb.