Scientists have shown that the global warming potential of methane is 72 times that of CO2 over 20 years.

Steven Cranston's opinion piece criticising the Carbon Zero Bill (HBToday 5/7/18) showed he is no scientist, however popular he may be as a paid consultant to Waikato dairy farmers. He clearly does not understand the meaning of half-lives or what they mean for steady state levels.

Cranston takes the low end for the estimated half-life of atmospheric methane, 7 years, and says that means the methane will be "completely decayed at 12 years".

Absolute nonsense. It means the current methane level, if nothing further were added to the atmosphere, would still be 50 per cent after 7 years, 25 per cent after 14 years, and 12.5 per cent after 21 years. It won't reach zero in our lifetimes. And if methane emissions continue at current levels, the atmospheric levels of methane will at best stay the same, or more likely continue to increase.

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Allowing for the differences in decay rate, scientists have shown that the global warming potential (GWP) of methane is 72 times that of CO2 over 20 years, and 28 times that of CO2 over 100 years.

A little methane goes a long way towards accelerating global warming, with all the subsequent costs.

The measured decay rate of atmospheric methane exaggerates the rate of reduction in GWP of methane because of several known chemical feedback loops, through which methane decay slows the rate of removal of other greenhouse gases.

But even ignoring that, it is undeniable that the massive increase in New Zealand dairy cow numbers over the last 30 years has resulted in a huge increase in steady state levels of methane in our part of the global atmosphere.

This will have to be addressed if we are to set a better example to the rest of the world, as urged so cogently by our new Prime Minister.

No doubt some dairy farmers, and the consultants who depend on them, would prefer the impact of 30 years of herd increases to be ignored, in favour of blaming car drivers, trucking companies, city dwellers, forestry owners and dairy farmers in other countries.

But from a scientific perspective, the easiest gains in reducing New Zealand's global warming impact can be made by targeting the most short-lived of our major greenhouse gases: methane.

If we halve the current level of methane emissions, within 50 years we can reduce the GWP of our accumulated methane emissions to less than 60 per cent of what they currently are. That would be a huge gain and a powerful signal to others.

Trying to bring about similar impacts by reducing net CO2 emissions will be a far lengthier and more difficult challenge.

We rely mostly on photosynthesis to reduce atmospheric CO2 levels, but given recent net deforestation in favour of farming, and continuing growth in car numbers, we are likely emitting more CO2 than our plants can fix.

That this is occurring globally is proven by the steady increase in atmospheric CO2 levels, as well as methane levels, implying global temperature increases well beyond current targets. In other words, it will result in millions of deaths.

And because CO2 has a far longer half-life than methane, reducing CO2 emissions will take correspondingly longer to reduce GWP.

We cannot ignore the challenge of reducing CO2 emissions, by limiting the mining and burning of fossil fuels, and reducing N2O emissions, by limiting the use of nitrogenous fertilisers on our pastures, but the priority target for reducing New Zealand's GWP impact should be reducing our methane emissions.

That will require reducing our dairy herds.

Encouraging dairy farming to expand in Central Hawke's Bay would represent the opposite of a sensible climate policy.