Comment: David Clark, President Federated Farmers Mid-Canterbury, attempts to debunk what he calls the methane myths in the Zero Carbon Bill.

As a farmer I am fed up with being vilified and our industry accused of being the primary contributor to climate change in New Zealand.

The myth is that agricultural gases, primarily methane, make up 48.1 per cent of this country's emissions profile. That is nothing more than a politically and socially convenient half-truth/untruth.

So here are established, scientific facts pertinent to the Zero Carbon Bill and the Emissions Trading Scheme:

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• A pre-existing and stable level of greenhouse gases in our atmosphere is required to maintain our temperature levels and is essential to life on earth.

• The Paris Accord calls for countries to take steps to limit global warming to 2 degC and preferably 1.5 degC above pre-Industrial temperatures.

• NZ has set a target of Zero Carbon by 2050 - no mandated reduction in gross carbon emission, just an aspiration to offset by forestry plantings so that carbon emitted will be in balance with carbon sequestered.

• Total methane emissions in NZ have increased 6.2 per cent from 1990 to 2017 (they have been declining since 2006). Agricultural production has doubled in this time.

• The ZCB has an aspiration of reducing methane by up to 47 per cent by 2050.

• Methane is a short-lived gas that originated from CO2 absorbed by the growing grass and when belched by livestock rapidly breaks down into CO2 again to complete the cycle. No additional carbon enters the atmosphere.

• Fossil fuel carbon has not been circulating in the atmosphere for thousands or millions of years, but once burnt will circulate in the atmosphere for centuries to come, with constant additional warming effect.

• The key objective is to limit any further warming.

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To represent methane on the same graph as fossil carbon and state that agricultural gases are 48.1 per cent of our emissions is simply wrong and does not recognise the cyclical nature of methane. I will demonstrate why.

Methane is a short lived gas, known as a "flow gas" which rapidly breaks down compared to carbon, which is known as a "stocks gas".

So let's represent methane as water and carbon as small stones. I have a bucket that is the atmosphere and the level inside the bucket is the global warming effect.

Read more from Federated Farmers here.

I have a centimetre of water in the bucket, which has a small hole in the bottom of it.

I start tipping cupfuls of water into the bucket. So long as I do not tip in water faster than it drains from the hole, the level does not increase.

It would be wrong to count the number of cupfuls put into the bucket, only correct to pay attention to the change in level. The number of cupfuls has absolutely no relationship to the level in the bucket.

If I take the same bucket and start dropping small stones into it, none of which fit out the hole, every stone is additional and the bucket gradually fills up. The total number of stones added has a direct correlation to the number of stones in the bucket.

Methane and carbon are water and stones. So long as stock numbers remain static, or more correctly the feed fed to livestock remains static, the emission of methane today replaces the methane that degraded today. The cycle stays in balance.

Every gram of carbon emitted from a power station, factory, car, aeroplane or any other part of our life adds to all of the carbon previously emitted from all sources.

The only way of reducing that carbon is to effectively bury it by absorption into soil by plants.

Our Government need to stop telling the methane myth and stop counting the water and the stones as if they were equal. They are not.

Net methane makes up only a very small portion of NZ's total emissions. Our farmers are being asked to reduce methane emissions way beyond the equivalent of "Zero Carbon" and are being vilified in the process.

The current fixation on methane is a dangerous, politically convenient distraction taking the focus of the enormous task of eliminating our reliance on carbon for our modern existence.