COMMENT:

Methane's role in New Zealand's greenhouse gas emissions has been very much highlighted in recent deliberations on climate change.

This includes a recent opinion by Dr Geoff Duffy ("Methane stance way off track") an earlier note by the Parliamentary Commissioner for the Environment, Simon Upton, on methane (CH4) emissions from livestock, and the focus in Zero Carbon Bill consultations on the treatment of CH4 differently from carbon dioxide (CO2) and nitrous oxide (N2O) in the role of anthropogenic greenhouse gas increases to global warming.

It is particularly so because about half of our greenhouse gas emissions are from CH4 and agriculture.

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Duffy claims that water vapour is the most important greenhouse gas. This is incorrect. The amount of water vapour in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapour, and vice versa.

So, when something else causes a temperature increase, such as extra CO2 from fossil fuels, more water evaporates. Then, since water vapour is a greenhouse gas, this additional water vapour causes the temperature to go up even further — a positive feedback.

Another egregious claim he makes is that no global warming has occurred in the past two decades. Both global and regional warming in New Zealand's annual temperatures, apart from a cold year in 1992 because of the Mt Pinatubo volcanic eruption, has continued unabated.

This year is heading to be one of the warmest in records back to 1867.

Then there is confusion on the global warming potentials of CO2, N2O and CH4. This partly comes from a calculation used to compare the greenhouse consequences of different gases, or global warming potential, normally taken over a 100-year time scale.

Some trace gases such as CH4 have a stronger impact on the heat balance of the earth, per molecule, than CO2 does. However, to really compare them fairly, one might want to factor in the fact that CH4 only lives about 10 years before it goes away whereas CO2 has a very long lifetime.

On human time scales, CH4 is certainly an important greenhouse gas. Its global warming potential is higher on the 50-year time horizon than on the 500-year horizon.

It is well known that there are serious difficulties in defining "CO2-equivalence". Most practical policies need to look at the technical, political and cost aspects of reducing emissions of individual gases.

New Zealand's target must be primarily guided by the Paris Agreement and by any future international agreements signed by the Government. Thus, New Zealand's climate change processes and metrics must be compatible. Any other approach conflicts with an all-gases approach internationally.

Animal-sourced foods are the major source of food-system greenhouse gases, and their relative importance is likely to increase in the future. Meat production is the single most important source of CH4 from agriculture.

The three major greenhouse gases have quite different effects on climate. The warming due to CH4 is substantial and rises quickly but because of the gas' short residence time in the atmosphere, ceases growing after about two decades.

In contrast the warming due to CO2 continues to grow throughout the two centuries shown and indeed would continue to grow indefinitely so long as emissions continue. The warming due to N2O has begun to level off at the end of the two centuries and grows little in subsequent years.

Although the warming in response to a fixed CH4 emission rate levels off rather quickly, an increase in the rate of CH4 emissions, caused by an increase in livestock production, would still cause proportionate rapid increases in the CH4-induced warming.

If the climate system is allowed to reach equilibrium with these levels of greenhouse gas emissions and decay, then the earth would be 0.44C warmer.

Over the 20- to 50-year period CH4-induced warming is much more important.

The increase in water vapour is a consequence of global warming produced by the main greenhouse gases. So CH4 must be addressed together with CO2 and N2O, and agriculture involved as soon as possible because of New Zealand's role as a meat producer.

The immediate challenge for the proposed new Climate Change Commission once the bill becomes enacted, is to provide solutions for agriculture.

• Dr Jim Salinger is deputy editor of Climatic Change.