No additional warming, but relative to when?
Exploring the warming impacts of targets for New Zealand’s biogenic methane emissions
1) Context and aim
A couple months back, the Government announced an independent review of New Zealand’s 2050 biogenic methane target range (24 to 47% below 2017 emissions) to assess its consistency with a principle of ‘no additional warming’ (McClay et al., 2024).
At first glance, the Government’s principle appears reasonably clear. A recent study commissioned by Beef+Lamb NZ, DairyNZ and Federated Farmers (Barth et al., 2023) formulated the principle as a target to limit warming from an emissions source at some future point in time to no more than the warming from that source in a chosen reference period (e.g., no additional warming at mid-century from New Zealand’s agricultural methane above its contributed warming in 2020).1
Yet despite clear enthusiasm for the principle from parts of the agricultural sector (Beef+Lamb NZ, DairyNZ, and Federated Farmers, 2023; DairyNZ, 2023; Wallace, 2023), the Climate Change Commission’s (the Commission’s) stance on moves to modify the target range based on the principle of ‘no additional warming’ isn’t hard to guess at (Daalder, 2024a).
The Commission’s (2024c, p. 48) discussion document on New Zealand’s 2050 targets suggests that:
“such a technical analysis would obscure the more fundamental question: should Aotearoa New Zealand cause more global warming than implied by the current 2050 target?”
Yet while I sympathise with the Commission's concern for any weakening of New Zealand’s current committed efforts to reduce warming, their discussion document on the 2050 targets (2024c) included use of a simple emissions-based climate model that could be well suited to support investigation of the principle of ‘no additional warming’. Their work made use of a recent version of the Finite Amplitude Impulse Response model (FaIR) (Leach et al., 2021), calibrated in line with the IPCC Sixth Assessment Report (Smith, 2023).
So, with the Government moving ahead with its independent review, this raises the question: if the Commission had used their model to explore what’s needed for no additional warming, what results might it have come up with?
In this article, I aim to answer this seemingly simple question by using the same calibration version of FaIR as the Commission, v1.2.0 (Smith, 2023), to determine the necessary reductions in biogenic methane to achieve no additional warming at mid-century.
However, instead of looking at a rather arbitrary line in the sand premised on limiting warming to current levels – a choice that is inherently political – this article examines what would be needed to limit warming to that seen in a selection of reference years: 1984, 1990, 2005, 2017, 2020, and 2018-2022.
This set of reference years is used to offer a span of possible levels of ambition, and were selected either because they:
have been used as emissions reference years for New Zealand’s climate targets (1990, 2005, 2017),
serve as a reasonable benchmark for ‘current’ warming levels (2017, 2020, 2018-2022), or
reflect an important historical turning point for the incentives guiding farm system choices and land-use decisions, dating back to the near-overnight removal of agricultural subsidies in 1984-85 (Rae, Nixon and Lattimore, 2004; Journeaux et al., 2018).
Additionally, this article investigates key sections of the Climate Change Response Act (CCRA), and inputs and outputs of simple climate models like FaIR to identify factors that could shape the level of target/s deemed consistent with the principle of ‘no additional warming’.
A fuller version of the article with commentary on calibration of FaIR to the Commission’s advice, expanded detailing of methods, and an appendix of supplementary material can be found here (.pdf) for those particularly interested in the methods used.
2) Key takeaway messages:
nb. all targets specified as reductions below 2017 emissions
Median results from FaIR calibrated consistent with the Commission’s work (2024c), suggest that for an examined background global emissions scenario consistent with limiting temperature rise to 1.5°C (SSP1-1.9), achieving no additional warming at mid-century above 2017 warming levels requires at least a 25.5% reduction in biogenic methane by 2050.
More ambitious targets are needed to limit warming to levels seen further back in history, with a 43.5% reduction in biogenic methane needed to limit warming to 1990 levels, or 58% to limit warming to 1984 levels.
Achieving no additional warming from agricultural methane requires greater reductions than for biogenic methane (inclusive of waste sector methane) due to the relative trends for waste and agricultural methane emissions through to 2030. Agricultural methane needed to reduce 28.5% to limit its warming to 2017 levels for the background scenario consistent with 1.5°C (SSP1-1.9).
This article contends that background global emissions scenarios that do not see warming limited to 1.5°C (e.g., scenarios reflecting current global efforts) are not consistent with the purpose of the CCRA. When these background scenarios are assumed, less ambitious targets are needed to limit warming (e.g., less ambitious than the bottom of New Zealand’s current target range).
The choice for whether to premise a target on ‘no additional warming’, and if so, which reference year warming is limited to are political choices. Choosing between reference years requires engagement with (among other factors): (i) technical and feasible mitigation potential for biogenic methane; and (ii) fair-effort relative to other countries and as part of global pathways consistent with the aims of the CCRA and Paris Agreement.
Setting a target premised on ‘no additional warming’ also requires consideration of variations in: (i) plausible global emissions pathways consistent with 1.5°C; and (ii) model climate parameters representing Earth’s climate system.
Setting stricter thresholds for likelihood that a target achieves no additional warming based on the suite of modelled outcomes (with varied climate parameters) necessitates more ambitious targets. For example, a biogenic methane target of 30% was needed to limit warming to 2017 levels in 66% of modelled outcomes for the 1.5°C consistent background scenario used (SSP1-1.9), and a 53% reduction to limit warming to 1990 levels.
When New Zealand’s current target range was assessed using the 1.5°C consistent background scenario, the bottom end of the range (24% target) was about as likely as not to limit warming to 2017 levels or lower at mid-century (48.6% of modelled outcomes). The top end of the range (47%) was very likely to limit warming to 2017 levels or lower (93.8% of modelled outcomes).
When constant emissions are assumed after 2050 (consistent with the requirements of the CCRA), the current target range also saw sustained warming decrease over time after 2050, irrespective of the background SSP scenario.
Less ambitious targets (such as the lowest reported target consistent with limiting warming to 2017 levels at the median result - 14% below 2017 emissions for background scenario SSP1-2.6) see warming after 2050 increase when the background global emissions scenarios are switched to those reflecting a 1.5°C pathway (SSP1-1.9) or current global efforts (SSP2-4.5).
This suggests that while it is possible to arrive at targets based on the ‘no additional warming’ principle that are lower than New Zealand’s current target range using the median results from simple climate models and certain background global emissions scenarios; such targets can perform poorly against this principle when the stylised assumptions underpinning the target are varied. Or when higher thresholds for likelihood are applied.
3) Methods
Preparation of emissions data
For New Zealand’s biogenic methane emissions from 1850 to 2021, data was taken directly from the Commission’s dataset (2024a) used for their 2050 target advice (2024c). Estimates in this dataset drew on historical activity data for agriculture prior to 1990 (Commission, 2024b). This dataset is aligned to emissions from New Zealand’s 2023 GHG inventory for 1990-2021 (MfE, 2023a).
Emissions from 2022 to 2030 were taken from the Government’s central ‘with existing measures’ projections (MfE, 2023b). This projection sees a 10.2% reduction below 2017 emissions by 2030, consistent with meeting New Zealand’s 2030 biogenic methane target. However, the Government’s intended delay of the introduction of pricing of agricultural emissions (Kivi, 2024) clouds this picture somewhat, and means emissions through to 2030 used in this article may be slightly understated relative to current policy efforts.
For agricultural methane scenarios, waste sector emissions data from PRIMAP-hist 2.5.1 (Gütschow, Pflüger and Busch, 2024), and the Government’s 2023 inventory (MfE, 2023a) and most recent projections (MfE, 2023b) were subtracted from the time-series for biogenic methane.
Older reference years are omitted for agricultural methane due to the relatively crude approach taken for emissions prior to 1990. These reference years would be sensitive to trends in agricultural methane emissions in the 1970s and 1980s.
For background global emissions, three shared socioeconomic pathways (SSPs) (Riahi et al., 2017; Rogelj, Popp, et al., 2018) are used to reflect different levels of global action that could be deemed as consistent with aspects of the Paris Agreement’s temperature goal or reflect current global policy efforts:
SSP1-1.9: consistent with limiting warming to 1.5°C (Meinshausen et al., 2020).
SSP1-2.6: consistent with limiting warming to 2°C (Meinshausen et al., 2020).
SSP2-4.5: reflective of present-day Paris Agreement pledges (Matthews and Wynes, 2022; Hueholt et al., 2024).
This set differs from the Commission’s discussion document (2024c) which only drew on SSP1-2.6, and Barth et al. (2023) which used SSP1-1.9, SSP2-4.5 and SSP3-7.0.
Historic and future global emissions data was sourced from the reduced complexity model intercomparison project (RCMIP) (Nicholls and Lewis, 2021; Nicholls et al., 2021; Nicholls, Z. and the RCMIP project contributors, 2021).
Model parameters
The FaIR model seeks to emulate complex earth system models, and allows for users to input scenarios for greenhouse gas (GHG) emissions, with the subsequent changes in concentrations of GHGs, radiative forcing and global mean temperature over time calculated by the model (see FaIR Development Team, 2023 and Leach et al., 2021 for more on the model).
Model parameters and base code structure used to operate FaIR were taken from the ‘constrained-ssp-projections’ python file for calibration v1.2.0, available at Zenodo (Smith, 2023). In almost all cases, parameters used in this article aligned with the Commission’s workbook, with the only variation being use of a non-stochastic set-up in this article (except when noted below) to avoid randomness across multiple target-level runs.
Key relevant settings for reference:
FaIR version: 2.1.3 (run using JupyterLab with python 3.11.9 kernel).
FaIR calibration version: v1.2.0. (Smith, 2023)
Methane lifetime calculation method: derived from Skeie et al. (2020); Thornhill, Collins, Kramer, et al. (2021); Thornhill, Collins, Olivié, et al. (2021).
Forcing calculation method: derived from Meinshausen et al. (2020).
Timebounds: 1750 to 2300 in 1-year steps.
Ensemble size for varied climate parameters: 1,001-member ensemble.
Greenhouse gas species modelled: All species (per calibration v1.2.0).
Modelled global temperature rise above pre-industrial levels for SSP1-1.9 using FaIR calibration v1.2.0 is illustrated in figure 1 below for reference (with stochastic internal variability within FaIR on). Figures for SSP1-2.6 can be found here and for SSP2-4.5 here.
Model runs
Calculation of warming attributable to New Zealand’s emissions was undertaken using a comparable approach to the Commission (2024b) and Barth et al. (2023), with the model run in its relevant baseline global emissions scenarios, as well as scenarios with New Zealand’s methane emissions subtracted from global emissions (policy runs).
The difference in warming each year between the baseline and policy runs is used to estimate the warming attributable to New Zealand’s emissions.
As an ensemble of 1,001 different configuration sets of parameters representing Earth’s climate system are modelled through FaIR, reported median results reflect the median difference between temperature outcomes in the baseline and policy runs across the 1,001 configurations in each year. As each configuration will respond differently over time, the time series for median results does not necessarily reflect a single configuration set of parameters. Likewise so for percentile results presented.
To estimate the level of methane emissions needed to achieve no additional warming, a similar approach to Barth et al. (2023) was taken, with New Zealand’s 2050 target-level for methane varied, as well as a subsequent linear decline in emissions from 2030 to 2050.
To assess the warming resulting from New Zealand’s methane emissions through to and including 2050, calculated warming in the year 2051 is used. This is due to FaIR’s model structure where emissions are set at mid-year timepoints (FaIR Development Team, 2023). For example, the impact of emissions at timepoint 2050.5 within the model only presents a materialised warming impact from 2051 onwards.
Steps taken for model runs:
A range of 2050 target levels (n=21) were run through FaIR in 5 percentage point steps from 0% to 100% below 2017 emissions.
Linear regression was then used to estimate the relationship between target-levels and estimated additional warming in 2051. For each background SSP scenario, R2 values of >0.9997 were reported.
Targets estimated by the regression (rounded to the nearest 0.5 percentage point) were then re-run through FaIR, together with targets 1 and 0.5 percentage points above and below. The lowest ambition target with additional warming at or below zero (to 6dp) is presented as the final result. In the one instance in which an edge case was detected, further target steps were also tested.
Historical time series data of median results of warming attributable to New Zealand’s methane, and input emissions data can be found here [.xlsx].
4) Results
Reductions in biogenic methane emissions needed by 2050
Results are presented in figure 2 below for the reductions in biogenic methane emissions needed below 2017 emissions to limit warming at mid-century to that seen in a series of reference years.
Under global background scenario SSP1-1.9, reflective of a pathway consistent with 1.5 °C (IPCC, 2021b), these results suggest that the bottom end of New Zealand’s current target range (24%) is not quite sufficient to limit warming to the explored reference years. While the top end of the current target range (47%) is consistent with limiting warming to that seen just prior to 1990.
Previous work looking at warming from New Zealand’s methane emissions has noted that switching from background scenarios with low future global methane emissions to those with higher emissions pathways tends to reduce the level of target needed to prevent additional warming (Reisinger, 2018; Reisinger and Leahy, 2019; Barth et al., 2023).
This observation is based on methane’s inverse relationship with its atmospheric concentration, where the radiative efficiency of methane emissions decreases as concentrations increase (Reisinger, Meinshausen, and Manning, 2011; Reisinger and Leahy, 2019; Barth et al., 2023).
However, judged solely on this relationship, a comparison of the results for SSP1-2.6 and SSP2-4.5 (which features higher global methane emissions than for SSP1-2.6) looks counter-intuitive.
This result owes to the calculation of methane’s lifetime within recent calibration versions of FaIR (drawing on Thornhill, Collins, Kramer, et al., 2021), where methane’s lifetime changes over time in response to emissions/concentrations of a range of reactive gases and aerosols (e.g, methane, volatile organic compounds (VOCs), nitrogen oxides (NOx), nitrous oxide (N2O), and halocarbons), as well as temperature (Smith, 2023; Smith et al., 2024).
Global emissions and subsequent concentrations of various species affect methane’s calculated lifetime in different ways (Thornhill, Collins, Kramer, et al., 2021; Smith, 2023). Higher emissions and concentrations of some species, such as methane and VOCs, increase methane’s lifetime. Conversely, other species like NOx, N₂O, and halocarbons decrease it. VOCs and NOx, in particular, have a significant impact on the reported targets in this article. Higher VOC emissions in SSP2-4.5 in particular led to an increased lifetime for methane, necessitating more ambitious targets compared to SSP1-2.6 to limit future warming from New Zealand’s methane emissions.
Switching to a reduced form equation where methane's calculated lifetime acts as a product of its own global concentrations (detailed in Leach et al., 2021) saw results flip for SSP1-2.6 and SSP2-4.5. Using this method, SSP1-2.6 required more ambitious targets than those reported in the core results in figure 2 above (e.g., 19.5% to limit warming to 2017 levels). Targets for SSP1-1.9 remained more ambitious than the other background scenarios using both methods for calculation of methane’s lifetime.
A small dive into the effect of reactive gases and aerosols relevant to methane’s calculated lifetime in FaIR calibration v1.2.0 is appended in the full article (link) [.pdf] for those wishing to interrogate the results further.
What about agricultural methane?
This article has focused in the first instance on biogenic methane to allow for direct comparisons to New Zealand’s existing target range.
However, there is some ambiguity in the Government’s press release (McClay et al., 2024) as to whether the Government intends for no additional warming to be achieved for biogenic methane, or agricultural methane alone.
When I made attempts to model the reductions in agricultural methane needed to achieve no additional warming relative to recent reference years, the reported targets were more ambitious than for biogenic methane (inclusive of waste sector methane).
For example, for background scenario SSP1-1.9, a target of 28.5% below 2017 emissions was needed to limit warming to 2017 levels (compared with 25.5% for biogenic methane). This is shown in figure 3 below.
These results owe to the relative trends in agricultural methane and waste methane through 2030. New Zealand’s waste sector methane emissions are projected to decrease 19% from 2017 levels by 2030 and have progressively declined since 2010 (MfE, 2023a; 2023b). Whereas agricultural methane is projected to reduce by 9.3% below 2017 levels by 2030 (MfE, 2023b).
When targets to limit warming from agricultural methane to 2020 warming levels in figure 3 above were converted to a 2020 emissions reference year to compare directly to Barth et al. (2023), targets were ~1 and ~4.5 percentage points more ambitious for SSP1-1.9 and SSP2-4.5 respectively. Barth et al. reported targets of 27% below 2020 emissions for SSP1-1.9 and 15% for SSP2-4.5.
Some further commentary on the difference in results is included in the appendix with the full article (link) [.pdf].
Disaggregation of historical and future emissions
When looking to separate the impact of historical and future biogenic methane emissions, an additional scenario looking at modelled warming in the absence of any emissions from 2025 onwards saw New Zealand’s warming at mid-century returned to levels last seen in the 1920s for all SSP scenarios. This owes to methane’s relatively short lifetime in the atmosphere.
This picture reinforces the observation made by other researchers (Reisinger and Leahy, 2019) that New Zealand’s policy and target-level choices affecting future methane emissions remain important to the level of sustained warming that New Zealand contributes. The added warming contributed from New Zealand’s emissions from 2025 to 2050 for New Zealand’s current target range are illustrated in figure 4 below to highlight the impact of target-level choice on sustained warming in future.
5) Discussion
What background global scenario is reasonable to assume?
So, we’ve got results that highlight that assumptions related to global background emissions scenarios matter for the level of target needed to achieve no additional warming.
Yet how do we make sense of the target New Zealand might need to take on this basis? Would it be fair to just acknowledge the slight ambiguity of the Paris Agreement’s temperature goal, and roll results for SSP1-1.9 and SSP1-2.6 through to a target range? Or perhaps New Zealand could draw on background scenarios that resemble current efforts by countries (e.g., SSP2-4.5)?
Well on this, my suspicion is that the purpose of the CCRA is an operative one for targets premised on a principle of ‘no additional warming’. On its purpose, the CCRA is quite clear that it seeks to (2002, section 3):
“provide a framework by which New Zealand can develop and implement clear and stable climate change policies that contribute to the global efforts under the Paris Agreement to limit the global average temperature increase to 1.5° Celsius above pre-industrial levels.”
To be clear, it is entirely reasonable for governments to adopt different principles (e.g., ‘no additional warming’, ‘leadership at a global level’) to help guide their consideration of what a fair, achievable and sufficiently ambitious target is. The balance of these considerations will inevitably look different between political parties, and likewise for members of the public and different interest groups.
However, setting a target-level premised on achieving ‘no additional warming’ with the assumption held that the world follows a pathway inconsistent with 1.5°C, is, to my reading, inconsistent with the purpose of the CCRA. Particularly given such assumptions allow for less ambitious targets to be set, and a higher consequent contribution to warming.
So although on paper, global emissions pathways that reflect current international efforts (such as SSP2-4.5), or those that limit warming to 2°C (such as SSP1-2.6) can allow for less ambitious targets on paper, my belief (which readers are welcome to challenge), is that these targets and the assumptions inherent in the methodology used to determine them are not consistent with the CCRA as it stands.
If I'm right, it sure makes for a sticky wicket for the Government to play on should it wish to maintain its overt focus on ‘no additional warming’.
What reference year should New Zealand limit its warming to?
As for where the Government lands in terms of reference year for a principle of ‘no additional warming’, this remains an inherently political choice (Barth et al., 2023; Upton, 2023; Daalder, 2024b).
By opening up the scope of consideration to a range of reference years (1990, 2017 etc.), I hope these results help demonstrate that while the principle of ‘no additional warming’ presents an interesting way of looking at methane targets, science alone cannot answer what level of ambition New Zealand should choose among these.
Instead, this choice requires value judgements premised on consideration of:
mitigation pathways for biogenic methane (technically and feasibly) and the relative warming effects of these pathways;
effort and responsibility relative to other countries; and
fair-effort as part of global reductions in biogenic methane consistent with 1.5 °C.
Bringing us somewhat full circle back to the type of analysis undertaken by the Commission in their review of New Zealand’s 2050 targets (2024c).2
How should uncertainties inherent in the use of simple climate models be treated?
This article and recent research looking at warming from New Zealand’s emissions have focused attention on the central/median results from simple climate models and have assumed background emissions scenarios drawn from a single integrated assessment model (IAM) for each background SSP scenario.
Yet as previous research has highlighted (Reisinger, 2018; Reisinger and Leahy, 2019), looking beyond these selected global emissions scenarios and central results can present a spread of plausible warming outcomes from New Zealand’s emissions.
Uncertainties related to Earth’s climate system
First, let's turn to uncertainty present in results from simple climate models related to the model’s parameters representing Earth’s climate system.
In calibration version 1.2.0 of FaIR, a large ensemble of 1,001 different sets of varied parameters (e.g., ocean heat uptake, aerosol-cloud interactions, and other parameters detailed in Smith (2023) and Smith et al. (2024) are used to reflect uncertainties in our understanding of Earth’s climate system.
These parameters are derived through calibration exercises by FaIR’s development team drawing on complex earth system models and replication of IPCC assessed ranges (IPCC, 2021a; Leach et al., 2021; Smith et al., 2024).
Parameter uncertainty has been found to significantly impact estimated warming for given pathways of global methane emissions when modelled through simple climate models (Smith et al., 2020). This uncertainty has also been explored in the context of New Zealand’s methane emissions using another simple climate model, MAGICC (Reisinger, 2018).
A consequence of the large ensemble of varied parameters in FaIR is that a span of outcomes for warming attributable to New Zealand’s biogenic methane emissions above and below the median reported values are estimated. This span is shown in Figure 5 below for a 2050 target of 25.5% below 2017 emissions under background scenario SSP1-1.9. This target is consistent with limiting temperature rise to 2017 warming levels at the median result under this background scenario.
While the median results from the 1,001-member ensemble indicate this target-level would be sufficient to limit warming to that seen in 2017 or lower, about half of the reported results within the larger ensemble see warming at mid-century higher than in 2017.
It is worth asking, then, if ‘no additional warming’ is a principle to which the Government is committed and is willing to stake its political capital on with international and domestic audiences, is a target based on the median outcome appropriate? Or do we need to consider the broader range of possible outcomes more explicitly?
An illustrative example of an alternative approach would be to set a target that sees a greater likelihood (e.g., over 66% chance, or over 90% chance) that warming is limited to whichever reference year is chosen based on modelled outcomes.
Applying such thresholds for likelihood would shift up the required ambition of targets from the central results reported in this article. For example, a biogenic methane target of 30% below 2017 emissions was needed to see warming limited to 2017 levels in 66% of modelled outcomes for background scenario SSP1-1.9. Up from the 25.5% target reported for the median result in figure 2. While limiting warming to 1990 levels required a target of 53%. Up from 43.5% in figure 2.
Making use of the span of results from FaIR, we can also look at the likelihood that New Zealand’s current target range limits warming to 2017-levels or below at mid-century across different background scenarios. Figure 6 below illustrates this, together with the lowest-reported target needed to limit warming to 2017-levels from figure 2 above (a 14% target, reported for SSP1-2.6).
As illustrated in figure 6, the lower end of New Zealand’s current target range (24%) sees warming at mid-century about as likely as not to be at or below 2017 warming levels for background scenario SSP1-1.9 (48.6% of modelled outcomes). While the upper end of the target range (47%) sees a very likely chance that warming is at or below 2017 warming levels (93.8% of modelled outcomes).
In contrast to the current target range, targets based on the lowest-reported median results do not translate well to instances where the background scenario varies. For example, the 14% target examined has a very unlikely chance of limiting warming to 2017 levels or below for SSP1-1.9 (8% of modelled outcomes), and an about as likely as not chance of limiting warming for SSP2-4.5 (45.5% of modelled outcomes).
What about post-2050 warming?
If a constant rate of emissions is assumed post-2050 (consistent with the minimum requirements set out in section 5Q of the CCRA), warming in the second half of the century from the current target range continues to decline in the majority of modelled outcomes. This occurs irrespective of the background scenario assumed, and is shown for the bottom-end of the current target range (24%) in figure 7 below.
Targets premised on the lowest median results (14% below 2017 levels for SSP1-2.6) see warming at 2075 increase above 2017 levels for the central likely range (16-84th percentile results) for the other two background SSP scenarios (SSP1-1.9 and SSP2-4.5). This is shown in figure 8 below.
Again, this suggests that while a ‘no additional warming’ target lower than the current target range can be set based on median results from a specific background emissions scenario, such targets do not hold up well when uncertainties related to the background scenario context are taken into account.
Given both parameters representing Earth’s climate system and pathways for future global emissions remain uncertain, I’d suggest this emphasises the need to treat median results premised on a particular background scenario with some caution. So while it is possible to produce results that suggest relatively low targets can achieve no additional warming under certain assumptions and thresholds for likelihood, those targets are quite reliant on the stylised assumptions underpinning them.
Variations in global emissions pathways consistent with temperature goals
Variations in global emissions pathways consistent with a particular temperature goal also present challenges to landing at a single target level consistent with ‘no additional warming’.
When looking at modelled pathways that limit temperature rise to 1.5°C with limited or no overshoot incorporated within the IPCC’s 1.5 degrees special report (Rogelj, Shindell, et al., 2018) and working group 3 of the Sixth Assessment Report (IPCC, 2023b, 2023a), a spread of pathways can be observed for some of the species that affect methane’s calculated warming impact (Matthews et al., 2020; Ou et al., 2021).
As a simple illustration of this, global methane emissions at 2050 in 1.5°C consistent pathways (with limited or no overshoot) within the IPCC AR6 database vary from 143 to 204 Mt CH4 at the 16 to 84th percentile range (Byers et al., 2022).
Differences in rates of reduction of GHG species in these pathways owe to the varying input assumptions and structures of each IAM used to model the world’s economy (Weyant, 2017; Wilson et al., 2021; Dekker et al., 2023), including how the availability and cost of mitigation options for agriculture are calculated (Reisinger and Leahy, 2019).
Given methane’s calculated warming impact within FaIR is sensitive to its background concentrations and that of other reactive gases and aerosols (Thornhill, Collins, Kramer, et al., 2021; Smith et al., 2024), it follows that altering the assumed background global emissions scenario to that produced by different IAMs will see different results for calculated targets needed to limit future warming.
Further work in this space would benefit from more extensive analysis of variation in pathways for global methane, NOx and VOCs in particular given the importance of these species to methane’s calculated warming impact in this article.
Modelling of further 1.5°C and/or 2°C consistent pathways from the IPCC AR6 database would seem to be a sensible approach to this. However, use of the AR6 IAM scenario database would require use of a different calibration version of FaIR to that used by the Commission and in this article. So is a bit beyond the scope of this article, and would require a sizable time commitment to do justice to.
There are uncertainties, so what?
Many readers will be content to leave the range of choices I’ve surfaced (methodological and otherwise) to the Government’s independent review panel. And to be sure, I’m hopeful there’ll be folks on the panel well-qualified to work through these to a more rigorous standard than I can achieve.
Yet while the approach to navigating these choices is ultimately one for the Government and its independent review panel to deliberate on for the moment; I hope this article has helped to surface some of the ways in which the task of landing a target premised on ‘no additional warming' is not an easy one for which a clear and unassailable answer from science can necessarily be found.
Second, I hope that another of the core underlying messages of this article is clear, that just as with other discussions on methane (Woodford, 2024), value judgements are required. ‘No additional warming’ premised targets require value judgements on: (i) which reference year warming is limited to; (ii) which background scenario/s are used (e.g., 1.5°C consistent, or 2°C consistent); and (ii) the thresholds for likelihood relied upon (given underlying uncertainties in climate parameters and global pathways for emissions).
Hence, consideration of the political, economic, international and legal contexts in which targets are set remain an important part of the equation, and cannot be swept away from the decision making frame.
6) Final parting comments
This article and my recent commentary on the New Zealand Emissions Trading Scheme (NZ ETS) - linked here and here, are all produced in my spare time, and have not been funded or directed by any organisations or donors.
If you want to follow and support these efforts, please give me an add on LinkedIn or subscribe here on Substack. I'll be continuing to publish in-depth looks at topical areas of climate policy while my work situation allows. I’m hoping to clear some time for a piece on the Government’s consultation on NZ ETS settings next.
References:
Barth, M. et al. (2023) Agriculture emissions and warming in Aotearoa New Zealand to 2050: insights from the science. Report funded by Beef+Lamb NZ, DairyNZ and Federated Farmers, p. 33. Available at: https://beeflambnz.com/knowledge-hub/PDF/full-report-agriculture-emissions-and-warming-aotearoa-new-zealand-2050-insights (Accessed: 8 April 2024).
Beef+Lamb NZ, DairyNZ, and Federated Farmers (2023) Submission to the Climate Change Commission on the review of the 2050 target, p. 167. Available at: https://www.dairynz.co.nz/media/qu1lwtgu/joint-evidence-submission-for-target-review-final-14-sept-2023.pdf (Accessed: 14 May 2024).
Byers, E. et al. (2022) ‘AR6 scenarios database’. Zenodo: Intergovernmental Panel on Climate Change. Available at: https://doi.org/10.5281/zenodo.7197970.
Climate Change Commission (2024a) ‘Input data files for temperature modelling’. Available at: https://www.climatecommission.govt.nz/public/Uploads/Targets/supporting-docs/input-data-files-for-temperature-modelling.zip (Accessed: 10 May 2024).
Climate Change Commission (2024b) Modelling and analysis to support the draft advice on Aotearoa New Zealand’s fourth emissions budget. Technical Annex. Wellington, New Zealand: Climate Change Commission, p. 53. Available at: https://www.climatecommission.govt.nz/public/Uploads/EB4/supporting-docs/Technical-Annex-Modelling-and-analysis-9-4.pdf (Accessed: 8 April 2024).
Climate Change Commission (2024c) Review of the 2050 emissions reduction target. Discussion Document. Wellington, New Zealand: Climate Change Commission, p. 96. Available at: https://www.climatecommission.govt.nz/public/Uploads/Targets/supporting-docs/20240404-Target-Consultation.pdf (Accessed: 18 April 2024).
Climate Change Commission (2024d) ‘Temperature modelling full results’. Climate Change Commission. Available at: https://www.climatecommission.govt.nz/public/Uploads/Targets/supporting-docs/Temperature-modelling-full-results.xlsx.
Climate Change Response Act (2002). Available at: https://www.legislation.govt.nz/act/public/2002/0040/latest/DLM158584.html (Accessed: 10 May 2024).
Daalder, M. (2024a) ‘Climate Change Commission lays down wero over methane targets’, Newsroom, 9 April. Available at: https://newsroom.co.nz/2024/04/09/climate-change-commission-lays-down-wero-over-methane-targets/ (Accessed: 18 April 2024).
Daalder, M. (2024b) ‘Govt sidelines Climate Commission in seeking do-over of advice’, 8 April. Available at: https://newsroom.co.nz/2024/04/08/govt-sidelines-climate-commission-in-seeking-do-over-of-advice/ (Accessed: 18 April 2024).
DairyNZ (2023) ‘Kiwi farmers need science-led methane review’. Available at: https://www.dairynz.co.nz/news/kiwi-farmers-need-science-led-methane-review/ (Accessed: 14 May 2024).
Dekker, M.M. et al. (2023) ‘Spread in climate policy scenarios unravelled’, Nature, 624(7991), pp. 309–316. Available at: https://doi.org/10.1038/s41586-023-06738-6.
FaIR Development Team (2023) FaIR 2.1.3 documentation. Available at: https://docs.fairmodel.net/en/v2.1.3/intro.html (Accessed: 5 May 2024).
Forster, P.M. et al. (2023) ‘Indicators of Global Climate Change 2022: annual update of large-scale indicators of the state of the climate system and human influence’, Earth System Science Data, 15(6), pp. 2295–2327. Available at: https://doi.org/10.5194/essd-15-2295-2023.
Fricko, O. et al. (2017) ‘The marker quantification of the shared socioeconomic pathway 2: a middle-of-the-road scenario for the 21st century’, Global Environmental Change, 42, pp. 251–267. Available at: https://doi.org/10.1016/j.gloenvcha.2016.06.004.
Gütschow, J., Pflüger, M. and Busch, D. (2024) ‘The PRIMAP-hist national historical emissions time series (1750-2022) v2.5.1’. Zenodo. Available at: https://doi.org/10.5281/zenodo.10705513.
Hueholt, D.M. et al. (2024) ‘Speed of environmental change frames relative ecological risk in climate change and climate intervention scenarios’, Nature Communications, 15(1), p. 3332. Available at: https://doi.org/10.1038/s41467-024-47656-z.
Huppmann, D. et al. (2019) ‘The MESSAGEix Integrated Assessment Model and the ix modeling platform (ixmp): an open framework for integrated and cross-cutting analysis of energy, climate, the environment, and sustainable development’, Environmental Modelling & Software, 112, pp. 143–156. Available at: https://doi.org/10.1016/j.envsoft.2018.11.012.
IPCC (2021a) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. Available at: https://doi.org/10.1017/9781009157896.
IPCC (2021b) ‘Summary for Policymakers’, in V. Masson-Delmotte et al. (eds) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press, p. 3−32. Available at: https://doi.org/10.1017/9781009157896.001.
IPCC (ed.) (2023a) ‘Annex III: Scenarios and Modelling Methods’, in Climate Change 2022 - Mitigation of Climate Change: Working Group III Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press, pp. 1841–1908. Available at: https://doi.org/10.1017/9781009157926.022.
IPCC (ed.) (2023b) ‘Mitigation Pathways Compatible with Long-term Goals’, in Climate Change 2022 - Mitigation of Climate Change: Working Group III Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press, pp. 295–408. Available at: https://doi.org/10.1017/9781009157926.005.
Journeaux, P. et al. (2018) Literature review and analysis of farmer decision making with regard to climate change and biological gas emissions. Report prepared for the Biological Emissions Reference Group. Waikato, New Zealand: AgFirst. Available at: https://www.mpi.govt.nz/dmsdocument/32137-Farm-Behaviour-GHG-Literature-Review-Final-Dec-2018 (Accessed: 26 February 2023).
Kivi, L. (2024) ‘Govt looking at policy to restrict forestry in the ETS: Watts’, Carbon News, 7 May. Available at: https://www.carbonnews.co.nz/story.asp?storyID=31449 (Accessed: 7 May 2024).
Krey, V. et al. (2020) MESSAGEix-GLOBIOM documentation. Laxenburg, Austria: International Institute for Applied Systems Analysis (IIASA). Available at: https://doi.org/10.22022/iacc/03-2021.17115.
Leach, N.J. et al. (2021) ‘FaIRv2.0.0: a generalized impulse response model for climate uncertainty and future scenario exploration’, Geoscientific Model Development, 14(5), pp. 3007–3036. Available at: https://doi.org/10.5194/gmd-14-3007-2021.
Matthews, H.D. et al. (2020) ‘Opportunities and challenges in using remaining carbon budgets to guide climate policy’, Nature Geoscience, 13(12), pp. 769–779. Available at: https://doi.org/10.1038/s41561-020-00663-3.
Matthews, H.D. and Wynes, S. (2022) ‘Current global efforts are insufficient to limit warming to 1.5°C’, Science, 376(6600), pp. 1404–1409. Available at: https://doi.org/10.1126/science.abo3378.
McClay, T. et al. (2024) ‘Methane targets to be independently reviewed’. New Zealand Government. Available at: https://www.beehive.govt.nz/release/methane-targets-be-independently-reviewed (Accessed: 8 April 2024).
Meinshausen, M. et al. (2020) ‘The shared socio-economic pathway (SSP) greenhouse gas concentrations and their extensions to 2500’, Geoscientific Model Development, 13(8), pp. 3571–3605. Available at: https://doi.org/10.5194/gmd-13-3571-2020.
Meinshausen, M., Raper, S.C.B. and Wigley, T.M.L. (2011) ‘Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6 – Part 1: Model description and calibration’, Atmospheric Chemistry and Physics, 11(4), pp. 1417–1456. Available at: https://doi.org/10.5194/acp-11-1417-2011.
Ministry for the Environment (MfE) (2023a) New Zealand’s Greenhouse Gas Inventory 1990–2021. ME 1750. Wellington, New Zealand: Ministry for the Environment. Available at: https://environment.govt.nz/facts-and-science/climate-change/new-zealands-greenhouse-gas-inventory/previous-greenhouse-gas-inventories/ (Accessed: 1 May 2024).
Ministry for the Environment (MfE) (2023b) ‘Updated emissions projections to 2050 released’. Available at: https://environment.govt.nz/assets/what-government-is-doing/climate-change/2050-historical-and-projected-sectoral-emissions-data-November_2023-for-publishing-v01.xlsx (Accessed: 29 March 2024).
Nicholls, Z. et al. (2021) ‘Reduced complexity model intercomparison project phase 2: synthesizing earth system knowledge for probabilistic climate projections’, Earth’s Future, 9(6), p. e2020EF001900. Available at: https://doi.org/10.1029/2020EF001900.
Nicholls, Z. et al. (2022) ‘Changes in IPCC scenario assessment emulators between SR1.5 and AR6 unraveled.’, Geophysical research letters, 49(20), p. e2022GL099788. Available at: https://doi.org/10.1029/2022GL099788.
Nicholls, Z. and the Reduced complexity model intercomparison project contributors (2021) ‘Reduced complexity model intercomparison project (RCMIP)’. Available at: https://doi.org/10.5194/egusphere-egu21-3707.
Nicholls, Z. and Lewis, J. (2021) ‘Reduced complexity model intercomparison project (RCMIP) protocol’. Zenodo. Available at: https://doi.org/10.5281/zenodo.4589756.
Nisbet, E.G. et al. (2020) ‘Methane mitigation: methods to reduce emissions, on the path to the Paris Agreement’, Reviews of Geophysics, 58(1), p. e2019RG000675. Available at: https://doi.org/10.1029/2019RG000675.
Ou, Y. et al. (2021) ‘Deep mitigation of CO2 and non-CO2 greenhouse gases toward 1.5 °C and 2 °C futures’, Nature Communications, 12(1), p. 6245. Available at: https://doi.org/10.1038/s41467-021-26509-z.
PBL Netherlands Environmental Assessment Agency (2024) ‘IMAGE 3.3 documentation’. Available at: https://models.pbl.nl/image/Welcome_to_IMAGE_3.3_Documentation (Accessed: 10 May 2024).
Rae, A., Nixon, C. and Lattimore, R. (2004) Adjustment to agricultural policy reform: issues and lessons from the New Zealand experience. Working paper No. 35. Wellington, New Zealand: New Zealand Institute of Economic Research (NZIER), p. 30. Available at: https://www.econstor.eu/bitstream/10419/66075/1/49464205X.pdf (Accessed: 1 May 2024).
Reisinger, A. (2018) The contribution of methane emissions from New Zealand livestock to global warming. Report to the Parliamentary Commissioner for the Environment. Palmerston North, New Zealand: New Zealand Agricultural Greenhouse Gas Research Centre, p. 44. Available at: https://pce.parliament.nz/media/ojzbobxh/contribution-of-methane-emissions-from-nz-livestock-to-global-warming.pdf (Accessed: 16 April 2024).
Reisinger, A. and Leahy, S. (2019) Scientific aspects of New Zealand’s 2050 emission targets. Palmerston North, New Zealand: New Zealand Agricultural Greenhouse Gas Research Centre, p. 20. Available at: https://www.nzagrc.org.nz/assets/Publications/NZAGRC-Report-Scientific-aspects-of-2050-methane-targets.pdf (Accessed: 8 May 2024).
Reisinger, A., Meinshausen, M. and Manning, M. (2011) ‘Future changes in global warming potentials under representative concentration pathways’, Environmental Research Letters, 6(2), p. 024020. Available at: https://doi.org/10.1088/1748-9326/6/2/024020.
Riahi, K. et al. (2017) ‘The shared socioeconomic pathways and their energy, land use, and greenhouse gas emissions implications: An overview’, Global Environmental Change, 42, pp. 153–168. Available at: https://doi.org/10.1016/j.gloenvcha.2016.05.009.
Rogelj, J., Shindell, D., et al. (2018) ‘Mitigation pathways compatible with 1.5°C in the context of sustainable development’, in Special Report on the impacts of global warming of 1.5 °C. Geneva: Intergovernmental Panel on Climate Change. Available at: http://www.ipcc.ch/report/sr15/.
Rogelj, J., Popp, A., et al. (2018) ‘Scenarios towards limiting global mean temperature increase below 1.5 °C’, Nature Climate Change, 8(4), pp. 325–332. Available at: https://doi.org/10.1038/s41558-018-0091-3.
Skeie, R.B. et al. (2020) ‘Historical total ozone radiative forcing derived from CMIP6 simulations’, npj Climate and Atmospheric Science, 3(1), p. 32. Available at: https://doi.org/10.1038/s41612-020-00131-0.
Smith, C. (2023) ‘FaIR calibration data version 1.2.0’. Zenodo. Available at: https://doi.org/10.5281/zenodo.7112539.
Smith, C. et al. (2024) ‘fair-calibrate v1.4.1: calibration, constraining and validation of the FaIR simple climate model for reliable future climate projections’, EGUsphere, 2024, pp. 1–36. Available at: https://doi.org/10.5194/egusphere-2024-708.
Smith, C. (2024) Review of use of FaIR for the New Zealand Climate Change Commission, p. 2. Available at: https://www.climatecommission.govt.nz/public/Uploads/Targets/supporting-docs/TR-02-Review-of-NZCCC-analysis-March-2024.pdf (Accessed: 9 April 2024).
Smith, S.J. et al. (2020) ‘Impact of methane and black carbon mitigation on forcing and temperature: a multi-model scenario analysis’, Climatic Change, 163(3), pp. 1427–1442. Available at: https://doi.org/10.1007/s10584-020-02794-3.
Thornhill, G., Collins, W., Olivié, D., et al. (2021) ‘Climate-driven chemistry and aerosol feedbacks in CMIP6 Earth system models’, Atmospheric Chemistry and Physics, 21(2), pp. 1105–1126. Available at: https://doi.org/10.5194/acp-21-1105-2021.
Thornhill, G., Collins, W., Kramer, R.J., et al. (2021) ‘Effective radiative forcing from emissions of reactive gases and aerosols – a multi-model comparison’, Atmospheric Chemistry and Physics, 21(2), pp. 853–874. Available at: https://doi.org/10.5194/acp-21-853-2021.
Upton, S. (2023) ‘Parliamentary Commissioner for the Environment letter to Jim van der Poel, Kate Acland and Wayne Langford on joint submission to the Climate Change Commission’. Available at: https://pce.parliament.nz/media/pywiuzzs/letter-to-dairynz-beefpluslamb-nz-and-federated-farmers-oct-2023.pdf (Accessed: 8 May 2024).
Wallace, N. (2023) ‘Methane metrics under fire’, Farmers Weekly. Vol 21 No 36, 18 September, pp. 1–3.
Weyant, J. (2017) ‘Some contributions of integrated assessment models of global climate change’, Review of Environmental Economics and Policy, 11(1), pp. 115–137. Available at: https://doi.org/10.1093/reep/rew018.
Wilson, C. et al. (2021) ‘Evaluating process-based integrated assessment models of climate change mitigation’, Climatic Change, 166(1), p. 3. Available at: https://doi.org/10.1007/s10584-021-03099-9.
Woodford, K. (2024) ‘Calculating methane emissions requires a mix of science and value judgements, writes Keith Woodford’, Interest.co.nz, 16 January. Available at: https://www.interest.co.nz/rural-news/125912/calculating-methane-emissions-requires-mix-science-and-value-judgements-writes (Accessed: 22 May 2024).
I’ve chosen to adapt the interpretation of the principle used by Barth et al. (2023) premised on achieving ‘no additional warming’ at mid-century relative to some current/recent/historical reference period. Other formulations of the principle are possible, but would require fairly extensive discussion to cover adequately.
This statement shouldn’t be read as a wholesale endorsement of the Commission’s analysis of these factors, as I recognise readers will have different views on the Commission’s analysis from a technical or reasoning standpoint.