COMMENT:

In the past month there has been much discussion around methane, involving baskets, agricultural emissions and forestry offsets.

However, the bottom lines are that climate policy must be based on sound science and the two-basket idea of separating out methane from the other greenhouse gases, is not based on sound science and it should be rejected.

Pine forestry reforestation is not the silver bullet – because conifers absorb more heat from the sun than most types of vegetation.

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They heat up the land, and release compounds into the atmosphere that mop up methane-destroying chemicals.

Methane

Now for some facts: Recently, Professor Martin Manning and colleagues noted that atmospheric methane (CH4), a potent greenhouse gas, is increasing rapidly since 2014 from 1800 to 1850 parts per billion (ppb).

Atmospheric methane changes in parts per billion (ppb) 2000 - 2018 (circles), and various projections (IPCC). The solid green line is that used by the Intergovernmental Panel on Climate Change for keeping temperature change at 2C. Climate models run in 2013 assumed that methane (CH4) would remain constant and did not allow for changes in removal rate. Source: Professor Martin Manning, Victoria University of Wellington.
Atmospheric methane changes in parts per billion (ppb) 2000 - 2018 (circles), and various projections (IPCC). The solid green line is that used by the Intergovernmental Panel on Climate Change for keeping temperature change at 2C. Climate models run in 2013 assumed that methane (CH4) would remain constant and did not allow for changes in removal rate. Source: Professor Martin Manning, Victoria University of Wellington.

The question at hand is why have these increases occurred?

There are three possible reasons:

• Biological emissions mainly from wetlands and ruminant livestock have increased;
• Removal of methane molecules by hydroxyl radicals (OH-) is likely to be limited by the presence of volatile organic carbon from conifers; or both.
• The second mechanism may well be the dominant reason as afforestation (planting of forests) and reforestation (grow-back of forests) occurs.

This alarming rise in methane in the atmosphere means Aotearoa New Zealand should not be complacent in waiting to reduce methane emissions.

Two baskets

There has been much said about separating greenhouse gas reductions into two baskets: one for carbon dioxide and nitrous oxide, and the other for methane, given their different lifetimes in the atmosphere.

As Professor Tom Wigely of the National Center for Atmospheric Research in Boulder, Colorado, has pointed out that the key point that has been forgotten is that the response in mean temperatures from changes in greenhouse gases at any point in time is almost totally a result of the history of that specific gas, like methane, heating up to that date.

The climate system does not care how the heating history arose. The climate system only cares about what the heating by that gas is to that date.

Therefore, the two-basket approach is completely irrelevant in the setting of atmospheric heating changes in response to increases in the various greenhouse gases.

Pine forests

Those who have lived among the pine forests of the volcanic plateau in the North Island will be familiar with a blue haze on calm days with clear skies.

That is produced by volatile organic compounds.

One of us has measured the emissions of isoprene in the air above forests in Canada using aircraft-based technology.

Blue ridge Mountains in the United States by John Hardwick. These and the Smoky Mountains are famous for their Blue Haze. Photo / Supplied
Blue ridge Mountains in the United States by John Hardwick. These and the Smoky Mountains are famous for their Blue Haze. Photo / Supplied

The haze is a result of sunlight being scattered by small airborne particles and aerosols.

These scatter the light and so the haze appears bluish. The pine forests are significant emitters of volatile organic compounds which give the pines their characteristic smell.

One of these groups of compounds, isoprene, forms as unstable liquids on the leaves which are emitted into the atmosphere.

The isoprene then binds up the hydroxyl radicals and could prolong the life of methane in the atmosphere.

This will be the subject of our investigations at Penn State University, where there has been afforestation in the southeastern US with the forests emitting the isoprene compounds into the atmosphere.

Professor Jose Fuentes, at Penn State University, has calculated that the current area of 1.2 million hectares of conifers with an extra 800 million radiata pine trees would emit a maximum of 2.4 million tonnes of monoterpenes in summer into the New Zealand atmosphere and inhibit the destruction of methane.

This would prolong the life of methane.

We are also going to calculate the effects of planting hundreds of millions of pines on methane destruction by release of isoprene into the atmosphere.

The change from grassland to a darker surface results in the landscape absorbing more heat from the sun.

Measurements of reflectivity comparing the pine forests with grasslands over Canada show that the coniferous forests absorb about 10 per cent more heat.

As well, widespread planting of pine forests in the drier climates of Canterbury and Otago increases the forest fire risk especially during hot dry nor'westers.

The effects of pine plantations on the soils need also to be well thought-out.

Adding it all up

Our first conclusion is that the recent unexpected ramping up of methane concentration in the atmosphere means that New Zealand and other countries must tackle reductions in methane emissions.

Countries need to show they are serious and have at least curbed any methane increase by 2030.

We have shown here that the two-basket idea is not based on sound science and should be rejected.

The IPCC notes that mitigation options are available for every major sector.

It notes that mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of all end-use sectors, decarbonise energy supply, reduce net emissions including agricultural sources.

The recent proposal to ring-fence off forestry plantings to agriculture amounts to geo-engineering the landscape.

The idea is giving New Zealand farming industries another free pass in tackling methane reductions, which they have had ever since the introduction of the Emissions Trading Scheme.

As the industry, transport and urban household sectors will be bearing the brunt of carbon dioxide reductions, in all fairness the farming sectors needs to make their contribution as well, as the IPCC recommends.

Finally, planting of vast pine plantations may not be the great silver bullet: pines particularly in the South Island will make the land surface darker, leading to absorption of heat from the sun.

These forests produce isoprene which mops up the methane-destroying hydroxyl radicals in the air, thus prolonging the life of any methane produced.

Such an unintended consequence is the subject of further research here in North America.

Finally, pine trees in dry areas will increase the forest fire risk.

Professor Jim Salinger is a New Zealand climate scientist and a visiting scholar at Pennsylvania State University. Dr Raymond Desjardins is a senior scientist at Agriculture and Agri-Food Canada.