Methane's (CH4) role in New Zealand's greenhouse gas (GHG) emissions has been very much highlighted in recent deliberations on climate change.

This includes a recent opinion by Dr Geoff Duffy (New Zealand Herald, October 2)
"Methane stance way off track", an earlier note by Parliamentary Commissioner for the Environment Simon Upton on 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) on the role of anthropogenic GHG increases to global warming.

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

Duffy claims that water vapour is the most important GHG.

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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 annual temperatures, apart from a cold year in 1992 because of the Mt Pinatubo volcanic eruption, has continued unabated.

2018 is heading to be one of the warmest years in records back to 1867.

Blue represents the global temperature record from the Climatic Research Unit. The orange is that for New Zealand from the author's and NIWAs research. Dashed lines are trends. Image / Supplied
Blue represents the global temperature record from the Climatic Research Unit. The orange is that for New Zealand from the author's and NIWAs research. Dashed lines are trends. Image / Supplied

Then there is confusion on the global warming potentials (GWP) of CO2, N2O and CH4.

Another source of short-lifetime bias in the community probably comes from a calculation used to compare the greenhouse consequences of different gases, (the GWP) 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.

For CH4 the GWP is higher on the 50-year time horizon than on the 500-year time horizon.

CO2 has a GWP of 1 regardless of the time period used, because it is the reference and remains in the climate system for a very long time.

CH4 is estimated to have a GWP of 28–36 over 100 years, and 84-87 over 20 years.

N2O has a GWP 265–298 times that of CO2 for both time scales.

GWP20 is sometimes used as an alternative to GWP100.

The latter is based on the energy absorbed by a gas over 100 years, and the former over 20 years.

There is also misperception on the standing of GWP100.

Although the IPCC has defined the metrics the IPCC is unconnected to the issue.

It is the United Nations Framework Convention on Climate Change (UNFCCC).

The Paris Agreement is a proviso to the UNFCCC, which is the legally binding document.

The issue of CO2-equivalence is related to this and GWP100 was agreed soon after the Kyoto Protocol was legislated.

Thus, all international agreements have used CO2-equivalent as a key metric with GWP100.

This is a one-basket approach.

It is well-known that there are serious difficulties in defining "CO2-equiv".

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.

Therefore, the two-basket or any other approach conflicts with an all-GHG approach internationally.

Finally, the diagram below from recent article in Science from Goffray and others shows the potency of CH4 as a GHG.

(A) shows GHG emissions from the production of different food types in 2005–2007 and projections for 2050 (assuming an emissions pathway that would keep global temperatures below 2C).

The vertical axis is the percentage of total GHG emissions.

Animal-sourced foods are the major source of food-system GHGs, 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 GHGs shown in (B) have quite different effects on climate.

At the current rate produced by livestock operations were introduced in Year 0 and thereafter held fixed, the warming due to CH4 is substantial and rises quickly but because of the gas's 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 GHG emissions and decay, then the earth would be 0.44C warmer.

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

The increase in water vapour is a consequence of global warming produced by the main GHGs.

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.

This is an immediate challenge for the proposed new Climate Change Commission when it is appointed, once the Bill becomes enacted, to provide solutions for.

Dr Jim Salinger is a New Zealand climate scientist and deputy editor of Climatic Change.