The story of the climate consequences of human activities – animal husbandry being responsible for methane emissions, and reforestation with pine plantations augmenting the lifetime of this potent greenhouse gas (GHG) - is uniquely a New Zealand one.

The methane emissions from livestock are well documented, but the extension of methane lifetimes by exotics is not.

The reforestation (natural or intentional restocking of existing forests that have been depleted, usually through deforestation) with exotic pine plantations is not the silver bullet believed.

They release monoterpenes into the atmosphere that rapidly mop up methane-destroying hydroxyl radicals (OH).

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When methane (CH4) is emitted from livestock belching on farms, this is eventually broken down by hydroxyl radicals – the main reason that the lifetime of methane is just over 12 years.

In the pine forests, monoterpenes are produced and this mops up extremely rapidly the OH to form carbon dioxide and carbon monoxide.

So, in the North Island atmosphere the monoterpenes remove the OH radicals preferentially before these have a chance to break down methane. This is how monoterpenes indirectly augment the lifetime of methane.

With the Zero Carbon Bill (ZCB) being introduced specifying emission reductions targets, we immediately focussed our work on modelling these interactions.

The science we did

Firstly, we determined monoterpene emissions from the North Island pine plantations and contrasted this with monoterpene emissions from the native vegetation in the North Island. The native vegetation releases much less monoterpenes than pines.

Second, changes of methane lifetime were investigated in response to the reduction of hydroxyl radicals mopped up in the oxidation of the monoterpenes.

Finally, the resultant regional warming potential of methane and heating were related to the changes in methane lifetime in the North Island regional atmosphere. We used a regional climate-air chemistry linked model to investigate these changes.

The modelling study over the North Island found that pine plantations increase in ambient monoterpene levels in the North Island.

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Further, these monoterpenes from pines extend the methane lifetime because of the reduction of its atmospheric sink by greater than a year.

This causes an increase in heating (radiative forcing). The heating effect per molecule of methane compared with a molecule of carbon dioxide in the global atmosphere is 84 to one, methane is a very potent greenhouse gas.

With an increase in methane lifetime in the North Island atmosphere from 12.4 to 15 years, the North Island heating increases by about a third, from 84 to 111.

This means that methane's share of the New Zealand GHG emissions would increase from 42 to 50 per cent.

Implications for New Zealand emissions reductions targets

The new science has consequences for New Zealand climate policy, points we made in our ZCB submission.

Given that New Zealand's methane impact on the atmosphere is higher than perceived, the reductions targets need to be increased to 20 per cent methane by 2030 and at the upper end of the proposed range in the ZCB (47 per cent) by 2050, which we recommended in our ZCB submission.

Reducing methane provides more bang for the buck than carbon dioxide because reducing one molecule of methane is 111 times more effective than one molecule of carbon dioxide.

Some farms have demonstrated reducing livestock numbers, and thus methane can be done. Owl Farm has reduced total GHG emissions by 8 per cent and lifted operating profit per hectare by 14 per cent through improving management practices over the past two years.

Plans are for changes farm management, for example reduced feed use and lowered the stocking rates, are expected to increase profitability by another 21 per cent and GHG emissions by 13 per cent – a total of 21 per cent.

The ZCB is very prescriptive on emissions reductions, especially of biogenic methane. The recent unexpected rise in atmospheric methane shows the incomplete understanding of its biogeochemistry.

We recommended the new Climate Change Commission (CCC) role, drawing on the appropriate technical expertise, determines the methane targets, rather than having these enshrined in legislation before our understanding of methane production and destruction is much improved.

Implications for New Zealand forestry

The exotic radiata pine plantations are vast monocultures of trees as business enterprises.

The one billion reforestation plans indicate that 550 to 600 million of these will be exotic plantation forestry from 2018 to 2027.

Natural-forest restoration is by far the most effective approach for removing carbon from the atmosphere and storing carbon.

Recent overseas research finds that natural forests are 12 to 40 times more effective than plantations at storing carbon.

Land clearance releases carbon followed by rapid uptake by exotic trees. But of the land cleared for replanting with exotics the carbon is released again by the decomposition of plantation waste and products.

Thus, the carbon sequestration can only be counted for the first crop, compared with natives.

Our research shows there has been collateral damage from attempts to plant pine trees en masse in order to mitigate global warming.

New Zealand forestry policy requires re-evaluation to increase the proportion of land that is being regenerated to natural forest.

Professor Jim Salinger is a Visiting Professor at the University of Florence in Italy, and Professor Jose Fuentes is at The Pennsylvania State University.