Is a methane cloud hanging over New Zealand’s zero carbon law?

On 7th November 2019 the New Zealand parliament passed the Climate Change Response (Zero Carbon) Amendment Act. In her parliamentary speech on the subject, Prime Minister Jacinda Ardern (pictured) stated: [1]

“We have done more in 24 months than any Government in New Zealand has ever done on climate action, but we have not done it alone. There has been 170,000 New Zealanders taking to the street, calling for that action – not for hope but calling for that action – around the world. I am proud at the rate at which New Zealanders took to the street to reinforce what it is we are doing in this House today . . .

. . . we have made a choice that I am proud of, that will leave a legacy, and that, I hope, means the next generation will see that we in New Zealand were on the right side of history. “

Unfortunately, the Act’s title is a misnomer, as the legislation does not require the nation to achieve zero carbon emissions or even zero greenhouse gas emissions (noting here that not all greenhouse gases contain carbon).

Is that a problem?

Although New Zealand can be applauded for its legislative initiative, its proposed measures may be inadequate in the context of the crisis we are facing.

This article focuses on a key feature of the new legislation, which is its approach to methane (CH4) emissions.

A “two baskets” approach

The New Zealand legislation adopts what has been referred to as a “two baskets” approach.

One basket contains biogenic methane, being methane from the agriculture and waste sectors. Proponents of the two baskets approach generally regard methane as a short-lived greenhouse gas, with a lifetime of around twelve years. [Footnote 1]

The other basket contains all other greenhouse gases, including carbon dioxide (CO2) and nitrous oxide (N2O). Carbon dioxide has a lifetime of hundreds of years and nitrous oxide around 120 years. For the purpose of the legislation, they are regarded as long-lived greenhouse gases.

For the biogenic methane basket, the legislation targets emission reductions of 10 per cent by 2030 and between 24 and 47 per cent by 2050.

For the other basket, the target is zero by 2050.

All reductions targets are from the 2017 base year, and are “net accounting emissions” after allowing for atmospheric carbon removal (including from land use, land-use change, and forestry) and offshore mitigation.

Offshore mitigation represents emissions reductions and removals, or allowances from emissions trading schemes, that originate from outside New Zealand. It is a form of “offsetting”.

The targeted reductions are depicted in Figure 1, assuming steady rates of emissions reduction over the periods in question.

Figure 1: New Zealand emission reduction targets relative to 2017 base period

The two baskets approach had been proposed in a blueprint of zero carbon legislation prepared by New Zealand climate change campaign group Generation Zero in 2017. [2]

It had also been referred to as a possibility by researchers Myles Allen and Michelle Cain of the University of Oxford and David Frame and Adrian Macey of Victoria University of Wellington in a submission to the New Zealand government as part of the consultation process preceding the introduction of the legislation. [3]

Dr Harry Clark, a climate researcher and member of New Zealand’s recently-created Climate Change Commission, had also indicated that methane could be treated differently to other greenhouse gases. [4]

The Oxford and Wellington researchers had co-authored (with others) journal articles published in 2018 and 2019 challenging the conventional approach to estimating the warming impact of short-lived climate pollutants. For the purpose of their work, they focused on methane. [5] [6]

In those papers, the authors proposed new versions of two key measures, “global warming potential” (GWP) and “carbon dioxide equivalent” (CO2-e). Their findings are commented on here due to their relevance to the approach taken in New Zealand and potentially elsewhere.

Some background

Under the United Nations Framework Convention on Climate Change (UNFCCC), the emissions of different greenhouse gases are aggregated for measurement purposes by converting them to CO2-e. It is analogous to converting several different currencies to a common denomination.

The greenhouse gases are converted to CO2-e by multiplying the mass of emissions by the relevant GWP, representing the warming impact of a unit mass of the gas relative to that of CO2 over a specific period. Over a 100-year time horizon, the multiplier for methane is 28 before allowing for the effect of climate-carbon feedbacks, and 34 with those feedbacks. The figures for nitrous oxide are 265 and 298.

Figure 2 compares the effects of one-off emissions of carbon dioxide, methane and nitrous oxide, allowing for climate-carbon feedbacks. [7] [Footnote 2]

Figure 2: Global temperature effects of one-off emissions of carbon dioxide, methane and nitrous oxide released in year zero.


Limitations of GWP

The limitations of conventional GWP reporting are widely recognised. Some have been articulated by researchers from Imperial College London [8]:

  1. The selection of a single time horizon is arbitrary and means that other time horizons, often with very different results, are disregarded.
  2. GWP was designed to equate one-off “pulse” emissions rather than sustained or developing emissions. This does not generally reflect the consequences of real-world investment or policy decisions.
  3. The physical basis of GWP is the integrated radiative forcing; it does not represent the temperature or other climate impacts. Radiative forcing is a precursor to temperature change but they are not synonymous. [Footnote 3]
  4. The fact that GWP is based on an integrated measure means that it indicates the average impact over a time horizon rather than the impact at the end-point of the time horizon. Both are useful in estimating the impacts of climate change.

The recent studies

As alternatives that may help to address some of the concerns with conventional GWP, the authors of the recent studies have developed the concepts of GWP* (referred to by some as GWP star) and CO2-we (CO2 warming equivalent).

The papers focused on the “flow” quality of methane, whereby a kilogram of the gas emitted today is effectively replacing a kilogram released twelve years ago, which has since largely broken down.

A key plank of their argument is that steady rates of methane emissions will not alter temperatures from the current level, due to the replacement of an earlier pulse of methane with a new one. That is not the case with a long-lived “stock” gas such as CO2, which accumulates in the atmosphere well beyond the 100-year time horizon conventionally used for estimating GWP.

In terms of atmospheric concentrations, the authors treat a permanent increase in the rate of a short lived pollutant in the same way as a single pulse of the same amount of a long lived pollutant because each would result in an ongoing presence in the atmosphere of the relevant gas. For example, a permanent increase in the rate of methane emissions from a particular activity from 10 to 11 tonnes per year would be analogous (in the context of time frames relevant to the development of climate change policies) to a single tonne of CO2 being emitted.

The concept is illustrated in Figure 3. In the first period, one unit of each pollutant is emitted, leading to one unit of concentration. After each period, the flow pollutant decays, while the stock pollutant remains in the environment, being added to by further emissions.

Figure 3: Flow and stock pollutants over time.

Another distinction between GWP* and conventional GWP is that the former indicates temperature impacts while the latter indicates radiative forcing which (as indicated earlier) is a precursor to temperature change. The distinction is illustrated in Figure 4 (adapted from Balcombe, et al.).

Figure 4: Cause and effect chain linking greenhouse gas emissions to climate change-related damage.

The findings were fine-tuned in the second paper, which allowed for the fact that some additional long-term warming will occur from methane emissions because of the climate system adjusting in response to those emissions in order to maintain equilibrium between incoming and outgoing radiation. [Footnote 3]

The modified GWP* formula utilised in the second paper tracked closely with estimates of temperature change from methane radiative forcing derived from modeling utilised in the Intergovernmental Panel on Climate Change’s (IPCC’s) 2013 fifth assessment report (AR5).

Other alternatives to GWP

GWP* is one of many alternatives to the conventional GWP metric. Others include: global temperature change potential (GTP); sustained-flux global warming potential (SGWP); instantaneous climate impact (ICI); cumulative climate impact (CCI); technology warming potential (TWP); Integrated global temperature change potential (IGTP); temperature proxy index (TEMP); and climate change impact potential (CCIP). [9]

According to UK-based climate and energy website Carbon Brief, it is unlikely that any alternative metric will be utilised, if at all, until after relevant material has been published in the IPCC’s Sixth Assessment Report (AR6). The report is due to be released in stages between April 2021 and June 2022. [10]

Some implications of the findings

Under GWP*, the current level of methane emissions is considered to be the baseline for the purpose of determining the impact of future emissions. This means that a reduction in the rate of emissions (beyond a threshold level of around 0.3 or 0.4 per cent) is considered to have a cooling effect. Under the conventional GWP approach any level of emissions, whether increasing or decreasing over time, is considered to have a warming effect.

If methane emissions are increasing, then the impact under GWP* can be greater than under conventional GWP.

Figures 5(a) and 5(b) compare results for GWP* and GWP100 based on decreasing and increasing levels of emissions of 2 per cent per year. The calculations used for the charts also take into account changes in emissions for the preceding twenty years.

Figure 5(a): Comparative results for a decreasing rate of methane emissions

Figure 5(b): Comparative results for an increasing rate of methane emissions

Although a steady rate of methane emissions does not add to the current level of warming (assuming no feedback mechanisms are triggered), it is causing that level of warming to be higher than it would otherwise have been. [Footnote 4] Current warming is at a dangerous level and must be reduced, meaning that even a steady rate of methane emissions at the current level should be regarded as unacceptable.

Accelerating methane concentrations

Although GWP* may indicate significant benefits when methane emissions are falling, the reality is that global methane emissions appear to be increasing. Although methane concentrations were relatively stable from 2000 to 2007, they accelerated over the next six years and again, at a higher rate, over the next five.

Methane’s rate of increase for the period 2014 – 2018 inclusive was 60 per cent higher than in the preceding six years. The corresponding figure for carbon dioxide was 15 per cent. The comparisons are depicted in Figures 6(a) and 6(b). [11] [12]

Figure 6(a): Atmospheric Carbon Dioxide and Methane Concentrations 1984-2018

Figure 6(b): Atmospheric Methane Concentrations (ppb) Annual Percentage Increase

There is much uncertainty over the reasons for increasing methane concentrations. Writing in the journal Science, Sara Mikaloff Fletcher and Hinrich Schaefer have reported that changes in the ratio of carbon-12 and carbon-13 isotopes within methane molecules indicate that biogenic sources may be a major contributing factor in combination with other processes. Of the biogenic sources, ruminant livestock emissions are believed to account for about half the increase since 2007. [13]

Of major concern is the fact that global meat production is projected to double by 2050. [14]

The latest IPCC emission scenarios limiting warming to 1.5°C assume that the amount of methane in the atmosphere will fall by 35 per cent between 2010 and 2050. [15] Such a fall is very much against the current trend as depicted above. The measured warming impact of the current trend would be even more pronounced utilising GWP* than under conventional GWP.

A critical view of New Zealand’s efforts

The Climate Action Tracker (CAT) is a highly regarded initiative of climate research organisations Climate Analytics and New Climate Institute. CEO and senior scientist of Climate Analytics, Bill Hare, was a lead author for the IPCC’s Fourth Assessment Report (AR4). CAT involves an independent scientific analysis that monitors government climate action and measures it against the  Paris Agreement’s aim of “holding warming well below 2°C, and pursuing efforts to limit warming to 1.5°C”.

In relation to New Zealand’s zero carbon law, CAT has stated: [16]

“Adopting the Zero Carbon Act is a step forward, but implementation is key. The Zero Carbon Act does not introduce any policies to actually cut emissions but rather sets a framework . . . At present the Government is relying on emissions removals such as through forestry and the purchase of international units, which will still allow emissions to be released into the atmosphere in 2050 to a level rated by the CAT from insufficient to the border of highly insufficient to hold warming to below 2°C, let alone limiting it to 1.5°C as required under the Paris Agreement.”

As this writer has highlighted elsewhere, carbon offsetting mechanisms such as the purchase of international verified carbon units are of questionable merit.

The overriding problem, including offsets administered under the Kyoto Protocol’s Clean Development Mechanism, is that they excuse an ongoing carbon emitting activity when that activity itself must be addressed.

Related to that concern, the offsetting activity: (a) may have occurred independently of the emitting activity; (b) may contribute to activities that increase emissions in the longer term; and (c) to the extent it does provide longer-term benefits, should be undertaken in its own right as part of a global emergency response to the climate crisis.

In the words of Kevin Anderson of the Tyndall Centre for Climate Change Research at the University of Manchester: [17]

“Offsetting is worse than doing nothing. It is without scientific legitimacy, is dangerously misleading and almost certainly contributes to a net increase in the absolute rate of global emissions growth.”

Some views on GWP*

Although some critics of New Zealand’s approach appear not to have considered the role of GWP*, Climate Analytics has addressed it specifically. [18] [19] [20]

One of its concerns was its view that achieving net zero greenhouse gas emissions could be achieved in an accounting sense under GWP* without reaching an essential component of the Paris Agreement’s global mean temperature goals, namely net zero carbon dioxide emissions.

Co-creator of GWP* Dr Michelle Cain appears to have acknowledged that possibility by indicating that the minimum 24 per cent methane reduction target in New Zealand would compensate for the warming generated by all non-methane greenhouse gases that were still being emitted as the country approached net zero emissions of such gases. In arguing against the notion that New Zealand farmers were being handed a “free pass”, she stated that farmers would in fact provide a “free pass” to all other sectors. [21]

It is important to note that Dr Cain was not proposing that such an arrangement would continue indefinitely, as she has acknowledged the 2050 net zero emissions target for non-methane greenhouse gases.

Co-authors of the GWP* journal papers have argued the opposite of Climate Analytics in respect of carbon dioxide, stating “the new approach means countries can signal the centrality of carbon dioxide reductions in their policy mix, while limiting the warming effect of shorter-lived gases”. [22]

Although Dr Cain argues that the New Zealand government has set targets that are scientifically informed, it must be noted that a March 2019 report by the nation’s Parliamentary Commissioner for the Environment (PCE), while still suggesting a two-basket approach, recommended against grouping carbon dioxide and nitrous oxide together as long-lived gases and treating methane separately as a short-lived gas.

The report, with input from an array of prominent scientific institutions, recommended separate targets for fossil emissions (including fossil carbon dioxide and fossil methane) and biological methane, nitrous oxide and carbon dioxide emissions from forests, soils and other terrestrial ecosystems. [23]

The PCE indicated that the categorisation that was eventually adopted by the New Zealand government was “somewhat arbitrary, since whether nitrous oxide should be considered a short or long-lived gas depends on the time horizon chosen”.

Writing in the journal Environmental Research Letters (cited by Climate Analytics), researchers Joeri Rogelj and Carl-Friedrich Schleussner have raised concerns over equity and fairness in applying GWP* at a national, as opposed to global, level. Because GWP* considers future emissions in the context of past emissions, it could arguably penalise nations with historically low methane emissions that may increase in the future. On the other hand, nations with a history of high methane emissions could benefit by a reduction even if emissions remained high.

Rogelj and Schleussner provided examples of many countries, including New Zealand (the world’s highest methane emitter per person), Australia and USA, based on 2015 emissions. [24]

In terms of each metric’s version of carbon dioxide equivalent emissions, New Zealand would have reduced from 8.3 tonnes per person under conventional GWP to 0.9 under GWP*, a fall of 99 per cent.

Australia would have reduced from 4.8 to -2.2 and USA from 2.3 to -1.8, representing falls of 145 and 178 per cent respectively.

With improvements in reported emissions of that extent being a possibility of GWP* largely due to their high historic emissions, the three countries may seek to embrace the concept. However, it would backfire if methane emissions increased.

Australia’s Red Meat Advisory Council (RMAC) favours GWP*, and has written to the nation’s Climate Change Authority encouraging it to adopt the new metric. [25] RMAC is an advocacy group comprising Australian producers, lot feeders, manufacturers, retailers and livestock exporters dealing in meat from cattle, sheep and goats.

The industry has positioned perceived environmental sustainability as a key plank of its PR and marketing campaigns, and would almost certainly consider as desirable a metric that enabled it to portray a more beneficial position than at present.


While noting it is essential to end carbon dioxide emissions due to their long-term accumulation in the atmosphere, this writer and others have for many years argued that a significant reduction in anthropogenic methane emissions would provide rapid climate change benefits. That approach appears to be consistent with that of the creators of GWP*.

It should also be noted that methane’s short-term impacts can become long-term to the extent that they contribute to climate feedback mechanisms and the breaching of tipping points that may eventually lead to runaway climate change under which we would lose any ability to favourably influence the climate system.

Although GWP* enhances our ability to quantify the potential benefits of addressing emissions of methane and other short-lived climate pollutants, any utilisation of it must not lead to an acceptance of actions in relation to those emissions that contribute to us failing in our efforts to overcome the impacts of climate change, including an acceptance of ongoing emissions at the current level.

Those who focus primarily on methane emissions must also ensure they do not ignore carbon dioxide. As indicated in this article, every kilogram of new carbon dioxide emissions contributes to an increase in atmospheric concentrations, adding to the urgency of ceasing such emissions and drawing accumulated quantities from the atmosphere.

The extent of the crisis we face in the form of climate change requires us to utilise all available mitigation measures to the maximum extent possible.


Paul Mahony


  1. The lifetimes mentioned are the perturbation lifetimes, which allow for the effects of the relevant greenhouse gases arising from chemical feedbacks. Around two-thirds of methane is broken down roughly every 12 years and the same proportion of nitrous oxide roughly every 120 years. [26] [27]
  2. The quantities shown were selected by authors of the cited paper because 298 tonnes of carbon dioxide, 9 tonnes of methane and 1 tonne of nitrous oxide trap the same amount of heat over a 100-year period. These temperature response functions were estimated using the MAGICC climate model, assuming background concentrations of each gas are held constant at current levels and including the impacts of climate-carbon cycle feedbacks. Noted by this writer: The figures for carbon dioxide and methane are relative to nitrous oxide. Calculation: N2O GWP100 of 298 / CH4 GWP100 of 34 = 8.76 (rounded up to 9)
  3. For the planet’s temperature to be stable over extended periods, incoming solar radiation and outgoing thermal infra-red radiation need to be equal. This state of balance is referred to as radiative equilibrium. If the balance is upset by greenhouse gases which absorb some of the outgoing heat energy, then the planet will gradually heat up in order to restore it. The initial change in balance is known as radiative forcing.
  4. In subsequent email correspondence dated 20th May 2020 explaining certain aspects of this article, the author noted: “If we look at existing concentrations of methane, they are causing the planet to be warmer than if the gas had not been emitted. But if the gas is emitted at the same rate as existing atmospheric methane is breaking down, then the concentration is not increasing. It is maintained at a steady level, with no increase in warming from the current level. So, a single pulse of methane emitted at that rate is adding nothing to future warming beyond atmospheric methane’s current effect.”


[1] New Zealand Parliament, Climate Change Response (Zero Carbon) Amendment Bill – Third Reading, 7 November 2019,

[2] Generation Zero, Zero Carbon Act Summary, April 2017, and

[3] Allen, M.R., Cain, M., Frame, D.J., Macey, A.H., 1 Methane emissions under a Zero Carbon Act for New Zealand (undated),

[4] Interim Climate Change Committee, Why methane matters, ICCC Rural Workshop Presentation, 25 February 2019. Filmed on location at FMG Stadium, Palmerston North, New Zealand,

[5] Allen, M.R., Shine, K.P., Fuglestvedt, J.S., Millar, R.J., Cain, M., Frame, D.J., Macey, A.H., A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation. npj Clim Atmos Sci 1, 16 (2018).

[6] Cain, M., Lynch, J., Allen, M.R., K.P., Fuglestvedt, Frame, D.J., Macey, A.H., Improved calculation of warming-equivalent emissions for short-lived climate pollutants. npj Clim Atmos Sci 2, 29 (2019).

[7] Parliamentary Commissioner for the Environment, “Farms, forests and fossil fuels: The next great landscape transformation?”, March 2019, Figure 4.1, p. 100, citing Reisinger, A. 2018. The contribution of methane emissions from New Zealand livestock to global warming. Report to the Parliamentary Commissioner for the Environment. and

[8] Balcombe, P., Speirs, J.F., Brandon, N.P., Hawkes, A.D., Environ. Sci.: ProcessesImpacts, 2018, 20, 1323,!divAbstract and

[9] ibid., Table 3

[10] Carbon Brief, COP25: Key outcomes agreed at the UN climate talks in Madrid, 15 December 2019,

[11] Ed Dlugokencky, NOAA Earth System Research Laboratory,

[12] NOAA Earth System Research Laboratory, Trends in Atmospheric Carbon Dioxide,

[13] Mikaloff Fletcher, S.E., and Schaefer, H., “Rising methane: A new climate challenge”, Science  07 Jun 2019: Vol. 364, Issue 6444, pp. 932-933, DOI: 10.1126/science.aax1828, citing Wolf, J., Asrar, G.R., West, T.O., Carbon Balance Manag. 12, 16 (2017) in respect of ruminant livestock emissions.

[14] Food & Agriculture Organization of the United Nations, Meat and Meat Products, updated 15th March 2019,

[15] V. Masson-Delmotte et al., Eds., “Global Warming of 1.5°C. An IPCC special report on the impacts of global warming of 1.5°C above pre-industrial amounts and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty” (World Meteorological Organization, 2018), cited in Mikaloff Fletcher, S.E., and Schaefer, H., op. cit.

[16] Climate Action Tracker, New Zealand, update of 2 December 2019,

[17] Anderson, K., “The inconvenient truth of carbon offsets”, Nature, Vol. 484, Issue 7392, doi:10.1038/484007a, 5 April 2012, and!/menu/main/topColumns/topLeftColumn/pdf/484007a.pdf

[18] Wamsley, L., “New Zealand Commits To Being Carbon Neutral By 2050 – With A Big Loophole”, NPR, 7 November 2019,

[19] Subramanian, K., “New Zealand’s new climate change law: Inadequate and not really net zero”, Down To Earth, 8 November 2019,

[20] Climate Analytics, Greenhouse Gas Accounting Metrics Under the Paris Agreement, December 2019,

[21] Cain, M., Climate Home News, “New Zealand’s farmers have a chance to be climate leaders”, 15 May 2019,

[22] Frame, D., Macey, A.H. & Allen, M., “Why methane should be treated differently compared to long-lived greenhouse gases”, The Conversation, 12 June 2018,

[23] Parliamentary Commissioner for the Environment, op. cit., p. 105

[24] Joeri Rogelj and Carl-Friedrich Schleussner 2019 Environ. Res. Lett. 14 114039, Unintentional unfairness when applying new greenhouse gas emissions metrics at country level, Table 2,

[25] Red Meat Advisory Council, Letter to CEO, Climate Change Authority, 2nd September 2019,

[26] Parliamentary Commissioner for the Environment, op. cit., p. 59 and Table 4.1, page 98.

[27] Myhre, G., Shindell, D., Bréon, F.-M., Collins, W., Fuglestvedt, J., Huang, J., Koch, D., Lamarque, J.-F., Lee, D., Mendoza, B., Nakajima, T., Robock, A., Stephens, G., Takemura, T., and Zhang, H., 2013: “Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group 1 to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change” , Table 8.7, p. 714 [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA,


Feature image: photocosmos1, Shutterstock ID 1077305678

Figure 1: Parliamentary Commissioner for the environment, “Farms, forests and fossil fuels: The next great landscape transformation?”, March 2019, Figure 4.1, p. 100. (Copying of material was permitted provided the source was acknowledged.) and

Figure 2: Frame, D., Macey, A.H., Allen, M.R., Why methane should be treated differently compared to long-lived greenhouse gases, The Conversation, 12 June 2018,, Attribution 4.0 International (CC BY 4.0),


The writer thanks Dr Michelle Cain for supplying information to assist in calculating CO2-we under GWP*.

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