The Contribution of Non-CO₂ Greenhouse Gas Mitigation to Achieving Long-Term Temperature Goals

Abstract

This paper analyses the emissions and cost impacts of mitigation of non-CO₂ greenhouse gases (GHGs) at a global level, in scenarios aimed at meeting a range of long-term temperature goals (LTTGs). The study combines an integrated assessment model (TIAM-Grantham) representing CO₂ emissions (and their mitigation) from the fossil fuel combustion and industrial sectors, coupled with a model covering non-CO₂ emissions (GAINS), using the latest global warming potentials from the Intergovernmental Panel on Climate Change’s Fifth Assessment Report. We illustrate that in general non-CO₂ mitigation measures are less costly than CO₂ mitigation measures, with the majority of their abatement potential achievable at US2005$100/tCO₂e or less throughout the 21st century (compared to a marginal CO₂ mitigation cost which is already greater than this by 2030 in the most stringent mitigation scenario). As a result, the total cumulative discounted cost over the period 2010–2100 (at a 5% discount rate) of limiting global average temperature change to 2.5 °C by 2100 is $48 trillion (about 1.6% of cumulative discounted GDP over the period 2010–2100) if only CO₂ from the fossil fuel and industrial sectors is targeted, whereas the cost falls to $17 trillion (0.6% of GDP) by including non-CO₂ GHG mitigation in the portfolio of options—a cost reduction of about 65%. The criticality of non-CO₂ mitigation recommends further research, given its relatively less well-explored nature when compared to CO₂ mitigation.

1. Introduction

Achieving stringent mitigation of greenhouse gases (GHGs) is likely to require a multi-gas approach. As such, it is important to understand the contribution of non-CO₂ mitigation to achieving different long-term temperature goals. This requires simulations of future energy, industrial and agricultural systems to account for all GHGs together, in as consistent a manner as possible. This paper presents a multi-model approach to such a challenge, to analyse the emissions and cost impacts of mitigation of both CO₂ and non-CO₂ GHGs at a global level, in scenarios which are focused on meeting a range of long-term temperature goals (LTTGs). The objectives are threefold:

• First, to demonstrate how an integrated assessment model (TIAM-Grantham) representing CO₂ emissions (and their mitigation) from the energy and industrial sectors is coupled with a model covering non-CO₂ emissions (GAINS) in order to provide a complete picture of GHG emissions in a reference scenario in which there is no mitigation of either CO₂ or non-CO₂ gases, as well as in scenarios in which both CO₂ and non-CO₂ gases are mitigated in order to achieve different LTTGs.
• Second, to demonstrate the degree of indirect mitigation of non-CO₂ gases that results from mitigation of CO₂ sources. This principally applies to methane (CH₄) emission reductions which result from reduced extraction and distribution of fossil fuels in CO₂ mitigation scenarios which see a shift from fossil fuel energy sources to renewables and nuclear.
• Third, to analyse the costs associated with mitigating non-CO₂ GHGs to varying degrees, by considering different levels of CO₂e prices applied to the non-CO₂ GHG-emitting sectors, relative to the CO₂ prices that result from the CO₂ mitigation scenarios. This provides a picture of the marginal impact (in terms of temperature change in 2100) of varying the relative degree of effort in mitigating non-CO₂ gases when compared to CO₂ mitigation effort.

Non-CO₂ GHG emissions, at about 14 GtCO₂e in 2010 (compared to 37 for CO₂ emissions) constituted about 28% of total GHG emissions in that year, measured on a CO₂-equivalent (CO₂e) basis using the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report 100-year global warming potentials (GWP100) with climate-carbon feedback effects for each gas [1,2]. Agricultural CH₄ and nitrous oxide (N₂O) emissions, at between 5.2 and 5.8 GtCO₂e in 2010, are the largest contributor to non-CO₂ GHG emissions. Over the last three decades (comparing 1980–1989, 1990–1999 and 2000–2009) CH₄ and N₂O emissions from agriculture increased from about 4 to over 5 GtCO₂e per year, with CH₄ emissions from livestock (enteric fermentation, mainly from cattle) accounting for just under half of this level throughout this period. Emissions growth from most agricultural sources (enteric fermentation, manure and fertiliser) in Africa, Asia and the Americas has been offset to some extent by emissions reductions in Europe [3], but future demand for food from these regions could be a major driver of emissions growth over the coming decades. Waste, fossil fuel extraction, transmission and distribution, and industrial production are other significant sources of non-CO₂ GHGs, principally CH₄ and N₂O.

As well as making a significant contribution to warming of the climate, some non-CO₂ species also lead to relatively large amounts of warming per tonne emitted. CH₄ for example, by mass, has a global warming potential over 100 years (GWP100) which is 34 times larger than that of CO₂ when taking account of carbon-climate feedback effects in the atmosphere [1]. It is important to note that this value is higher than the value (25) used in the previous (fourth) IPCC assessment report [4], when such feedbacks had not been adequately considered. This comparative measure of warming—that of an equivalent mass of CO₂—is the basis for emissions accounting and allows one method of comparing the cost effectiveness of mitigation measures across different gas species for a given timeframe. The major sources and mitigation options for non-CO₂ GHGs are shown in Appendix A.

In addition to the technical supply side measures shown in Appendix A, mitigation could also come through changes in consumer preferences for meat and dairy products and reduced losses and waste of food [3,5,6] although there is in general less evidence on these demand-side emissions mitigation options [3].

There have been relatively fewer studies on the mitigation potential of non-CO₂ GHGs compared to CO₂ from the energy and industrial sectors. A number of sector specific studies were carried out in the late 1990s and early 2000s [7,8,9], many of which formed the basis of more comprehensive assessments [10,11,12]. These studies were undertaken in order to construct marginal abatement cost curves for 2010, which were then extrapolated for use in integrated assessment studies [13,14]. Further work [15] extended the marginal abatement costs (MACs) more systematically to 2100. This analysis, as well as some more recent analyses [16], has formed the basis of relatively recent estimates of long-term mitigation in for example the agricultural sector [17].

A consistent message from the multi-gas modelling studies is that the cost of mitigation to achieve a given temperature goal is less when mitigation of non-CO₂ GHGs is included amongst the mitigation options available. For example, Rao and Riahi [13] found that carbon prices associated with achieving a radiative forcing level of 4.5 W/m² by 2100 when using a multi-gas mitigation approach are about half those when using a CO₂-only set of mitigation measures. Kurosawa [18] found that a multi-gas approach (again, to achieve a 4.5 W/m² forcing level by 2100) leads to a global mitigation cost of 3.8% of GDP by 2100, compared to 8.6% of GDP with a CO₂-only approach. Lucas et al. [15] found that a multi-gas approach lowers mitigation costs between 3%–21% (by 2050) and 4%–26% (by 2100) compared to a CO₂–only approach, to achieve a 550 ppm CO₂e stabilisation concentration of GHGs. Recent analysis [19] shows that non-CO₂ mitigation to higher CO₂e prices can relax the cumulative CO₂ budget required to meet specific temperature targets, which could be a useful facet of flexibility if there are specific technology constraints which prevent the achievement of stringent CO₂ budgets.

This analysis is the first of which we are aware that examines the impact of non-CO₂ gases on overall mitigation costs when using the latest GWP100 figures produced by the IPCC [1]. In addition, this study, unlike the more recent studies on non-CO₂ gases [20,21], explicitly sets out the relative contribution of non-CO₂ gases to 2100 warming when taking into account differences between the marginal mitigation cost of CO₂ and non-CO₂. This is of particular policy relevance when considering the acceptable level of marginal mitigation effort that might be allocated to different gases. Although the “what, when and where” flexibility of mitigation advocated by Stern [22] suggests that the most cost-efficient mitigation strategies should see CO₂e prices equilibrate between sectors, gases and regions, it is clearly not going to be possible in all practical cases to achieve this. A further policy-relevant aspect of this analysis is that—in light of the Paris Agreement’s [23] goal to achieve global warming levels of “well below 2 °C”—it is instructive to consider the degree to which this might be achieved without a contraction of an already-challenging CO₂ budget, but rather with additional effort in the non-CO₂ emitting sectors. In examining scenarios in which there is greater mitigation effort in the non-CO₂ emitting sectors than the CO₂ emitting sectors, this analysis allows a consideration of the potential viability of such strategies.

2. Materials and Methods

The model used in this assessment, the Greenhouse Gas Air Pollution Interactions and Synergies (GAINS) model, has a comprehensive, multi-country and region representation of non-CO₂ GHG emissions sources, as well as the measures and costs for their mitigation [24,25]. The cost data used here is from the 2013 update of the GAINS model. It has been used in recent studies of the mitigation potential of CH₄ [26], as well as other climate forcing species such as black carbon, with a view to assessing not just climate but also air quality, health and agricultural crop yield benefits of mitigating these short-lived species [27]. As such, it has been chosen because of its relatively recent development, its state-of-the-art level of detail of mitigation options for the non-CO₂ GHGs, as well as its geographical detail which allows aggregation of countries into regions which closely match the 15 regions represented in Imperial College London’s global TIMES (The Integrated MARKAL-EFOM System) Integrated Assessment Model (TIAM-Grantham) [28,29]. This model represents the global energy and industrial system in these regions, including low-carbon technologies and their costs, and associated CO₂ emissions. It is an inter-temporal optimisation model which finds the welfare maximising solution to the objective of meeting future energy service and industrial product demands across all economic sectors within a given climate or CO₂ emissions constraint. It has been used in a model inter-comparison study as part of the AVOID 2 research programme to analyse the technologies and costs of a range of long-term temperature targets [30]. It should be noted that this analysis covers the well-mixed GHGs and does not explicitly model emissions of aerosols and precursors, for example black carbon—for each scenario these have been estimated using the methods described below.

There are in many cases interactions between measures that mitigate different GHGs. For example, mitigation of CO₂ frequently consists of substituting non-fossil energy sources for fossil fuels, which results in reduced fugitive CH₄ emissions from the extraction and distribution of these fuels [21]. In addition to accounting for such interactions, it is important to ensure a high level of consistency between the drivers of energy and industrial CO₂ emissions and those for non-CO₂ emissions sources, principally agricultural activity responsible for CH₄ and N₂O emissions.

In order to maximise consistency between the energy and industrial CO₂ mitigation modelling in the TIAM-Grantham model, and the non-CO₂ mitigation modelling in the GAINS model, a number of steps have been undertaken, as described in detail in Appendix B. In summary:

• For each LTTG (in this study 2100 temperature change levels of 2, 2.5 and 4 °C are assessed) a cumulative 2000–2100 global CO₂ budget for the fossil fuel and industrial (FFI) sectors has been estimated from a simple interpolation of the budget from the Representative Concentration Pathways (RCPs) and projections of their corresponding global temperature change when simulated with a probabilistic version of the Model for Greenhouse gas Induced Climate Change (MAGICC) (as detailed in [31]) using a distribution of equilibrium climate sensitivity from the Fifth Coupled Model inter-comparison Project (CMIP5), as detailed in [32];
• The TIAM-Grantham model has been used to produce an unmitigated reference scenario, as well as mitigation scenarios based on these estimated CO₂ budgets, using a standard set of socio-economic drivers, specifically the OECD variant of the Shared Socio-Economic Pathways 2 (SSP2), which has been used in order to represent a future world in which recent socio-economic trends continue [33];
• The GAINS model, also using SSP2 socio-economic inputs, as well as energy price and fossil fuel supply and demand outputs from the TIAM-Grantham model scenarios, has been used to produce a “baseline” level of non-CO₂ emissions for each TIAM-Grantham scenario, as well as marginal abatement cost (MAC) curves for each ten-year time point (2020, 2030, 2040, etc.) for each non-CO₂ GHG species (Methane (CH₄), Nitrous Oxide (N₂O), Fluorinated gases (F-gases, which are perfluorocarbons, PFCs, Sulphur hexafluoride, SF₆, and hydrofluorocarbons, HFCs));
• For each scenario, the 2100 temperature when mitigating non-CO₂ GHGs to different prices (on a GWP100 basis, with prices relative to the CO₂ price for each TIAM-Grantham scenario) has been calculated, using the same version of the MAGICC used to estimate the initial CO₂ budgets;
• Where the non-CO₂ and CO₂ prices are equal, if there is a major (in this case, greater than 0.1 °C) difference in the calculated 2100 temperature change relative to the initially-intended LTTG, a revision to the initial CO₂ budget has been made and the process repeated.

As indicated above, the MAC curves derived from GAINS allow analysis of non-CO₂ mitigation up to a CO₂e price equal to the CO₂ price which was output from the TIAM-Grantham model (thereby equating marginal mitigation “effort” for CO₂ and the non-CO₂ GHGs) as well as at CO₂e prices at different fractions of the TIAM-Grantham CO₂ price (thereby considering different marginal effort levels for non-CO₂ GHGs when compared to CO₂ mitigation effort). This approach allows analysis of the 2100 median temperature change and overall mitigation cost (i.e., considering both CO₂ and non-CO₂ mitigation options) when considering lower and higher levels of “effort” of non-CO₂ GHG mitigation measures compared to CO₂ mitigation measures. For each mitigation scenario, as well as the 2100 temperature change, the cumulative discounted cost (using a discount rate of 5% per year) of both CO₂ and non-CO₂ GHG mitigation is calculated, relative to the reference (unmitigated) scenario.

3. Results

3.1. Mitigation of Non-CO₂ Emissions

Figure 1 shows the emissions level for each GHG in the unmitigated reference scenario where there is no price or constraint on any of the GHGs, using the GWP100 equivalence measure (as taken from the IPCC’s fifth assessment report [1]). This unmitigated scenario follows from running the TIAM-Grantham model to produce a scenario for a least-cost energy system that meets future energy needs under the SSP2 shared socio-economic pathways assumptions [33], but with no climate constraints. Emissions rise from 50 GtCO₂e/year in 2010 to 132 GtCO₂e/year in 2100. The resulting median warming in 2100 is 4.6 °C. For both 2100 emissions and temperature change, these figures are closer to the upper end of the range for the high emissions scenarios presented in the IPCC’s 5th Assessment Report, WGIII [2], reflecting the relatively strong socio-economic growth throughout the century represented by the SSP2 input scenarios. It can be seen that CO₂ is the largest contributor to GHG emissions throughout the period (reaching 93 GtCO₂e/year by 2100), with CH₄ and N₂O continuing to remain significant. By comparison, the RCP8.5 pathway, which has the highest emissions of the RCPs, sees global GHG emissions reaching 120 GtCO₂e/year in 2100, albeit with much lower global GDP by 2100 (a seven-fold increase over the 21st century [34], compared to an 11-fold increase in this study). Of this 120 GtCO₂e/year, approximately 80 GtCO₂e/year is from CO₂ and the remainder from non-CO₂ gases (compared to 93 and 39 GtCO₂e/year respectively in this study) [35].

Hence, both RCP 8.5 and this study see non-CO₂ emissions accounting for about a third of the total GHG emissions by 2100, slightly higher than the upper end of the range (16–27%) in recent multi-gas mitigation scenarios [20]. In fact in these recent scenarios the maximum 2100 non-CO₂ emissions level (across the six models compared) is 30 GtCO₂e/year, with CH₄ emissions at 15 GtCO₂e/year. This is about 10 GtCO₂e/year below the emissions in this study, mainly because in this study CH₄ makes up 25 GtCO₂e/year in 2100. This results from three main factors: first, the relatively high-growth socio-economic assumptions driving future emissions growth in this study; second, the considerably higher GWP100 value for CH₄ (34) taken from the IPCC’s latest (i.e., fifth) assessment report compared to the lower value (25) used in the recent multi-gas mitigation scenarios [20] and also the IPCC’s earlier fourth assessment report [1], and third, the considerably higher CH₄ emissions from global oil production in GAINS’ historical emission inventories compared with others (e.g., [20,35,36]), which is a result of a more in-depth estimation method [37] and which also has implications for future emissions from this source.

Table 1. Estimates of 2000–2100 cumulative CO₂ from fossil fuel combustion and industry sectors, with associated calculated 2100 median temperature change.

Scenario	CO2 Cumulative Budget Estimate (2000–2100), GtCO2	Subsequent Calculation of 2100 Median Temperature Change in MAGICC, °C
Baseline	No budget constraint—results in cumulative CO2 of 6000 GtCO2	4.62
2 °C with delayed action to 2020	1340	2.00
2.5 °C with delayed action to 2020	2260	2.45
4 °C with delayed action to 2020	5280	3.88

shows the estimated CO₂ budgets as well as the median temperature change that results from mitigation of non-CO₂ GHGs to a CO₂e price (using GWP100) equal to the CO₂ price from the TIAM-Grantham model for each budget (taking the scenarios with delayed action to 2020). Also shown is the median 2100 temperature change resulting from the unmitigated TIAM-Grantham and GAINS scenarios (i.e., resulting from the emissions levels shown in Figure 1).

Figure 2 shows the non-CO₂ GHG emissions for a 2 °C mitigation scenario with global mitigation action starting in 2020 (and weak country/regional policy actions to 2020), after CO₂ mitigation has occurred to meet the cumulative CO₂ budget, but before any specific mitigation has occurred in the non-CO₂ sectors. Also shown is the completely unmitigated level of non-CO₂ GHG emissions that derive from the reference scenario with no mitigation action for any GHGs (which in the case of non-CO₂ means action beyond that prescribed in existing legislation as of September 2015). In other words, Figure 2 shows the indirect mitigation of the non-CO₂ GHGs that occurs as a result of changes in the energy system when transitioning to low-carbon (and in particular lower fossil fuel reliance) over the century. There is significant mitigation of CH₄ (about 9 GtCO₂e/year by 2100) resulting from reduced fossil fuel extraction and distribution, and therefore lower fugitive CH₄ emissions. The importance of accounting for this indirect mitigation effect has been highlighted in recent studies [21].

Figure 3 shows the effect of indirect mitigation for a range of long-term temperature goals. As expected, the degree of mitigation increases as the temperature goal decreases, resulting from an increasingly marked shift from a fossil fuel-based energy system to a low-carbon system in which non-fossil sources such as nuclear and renewables dominate.

Figure 4 shows the further mitigation of non-CO₂ GHGs resulting from mitigation measures targeted specifically towards these gases, for the 2 °C scenario with delayed action to 2020. Also shown are the levels of each non-CO₂ GHG for the indirectly mitigated case. The figure shows for each time step the mitigation of non-CO₂ GHGs up to the CO₂e price that is equal to the CO₂ price in the TIAM-Grantham model (i.e., the shadow price of CO₂ associated with achieving the least cost mitigation pathway to meet the specified 21st century cumulative CO₂ budget). As such, this equates a level of mitigation effort for CO₂ and non-CO₂ GHGs according to the marginal cost of abatement at any given time. In the case of Figure 4, this marginal abatement cost is calculated on a GWP100 basis.

It can be seen that there is significant abatement of all non-CO₂ GHGs up to this CO₂e price, such that by 2100 the fully mitigated level of non-CO₂ GHGs is just under 13 GtCO₂e/year, compared to 39 GtCO₂e/year in the unmitigated reference scenario. Of the 27 GtCO₂e/year reduction, 69% occurs through the direct mitigation of the non-CO₂ GHGs and 31% through the indirect mitigation (mostly of CH₄) that follows from CO₂ mitigation. Of the unmitigated reference 2100 level of each non-CO₂ GHG, 67% of CH₄, 37% of N₂O and 99% of F-gases are mitigated, leaving 7.8, 4.5 and 0.1 GtCO₂e/year of CH₄, N₂O and F-gases respectively. These reductions compare to recent modelled scenarios (focusing specifically on the issue of non-CO₂ GHG mitigation) in which by 2100 up to 71% of CH₄, 42% of N₂O and 90% of F-gases are mitigated [20], as well as the broader IPCC fifth assessment report database [38] in which, across all of the most stringent mitigation scenarios, 44–74% of CH₄, 9–42% of N₂O and 45–90% of F-gases are mitigated by 2100, compared to the relevant unmitigated baseline scenario for each model used. In this database, the range of 2100 CH₄ emissions is 12–25 GtCO₂e/year (using the most current CH₄ GWP100 value of 34) in the reference scenario and 4–11 GtCO₂e/year in the mitigation scenarios, compared to 25 GtCO₂e/year and 7.8 GtCO₂e/year in the 2 °C scenario of this study. The database’s range of 2100 N₂O emissions is 3.0–8.8 GtCO₂e/year in the reference and 2.1–8.1 GtCO₂e/year in the mitigation scenarios, compared to 7.0 and 4.5 GtCO₂e/year in this study. The database’s range of 2100 F-gases emissions is 1.2–10 GtCO₂e/year in the reference and 0.06–1.7 GtCO₂e/year in the mitigation scenarios, compared to 7.2 and 0.08 GtCO₂e/year in this study.

Hence, the reference and mitigation emissions levels in this study are within the AR5 database range, although the CH₄ reference emissions are at the higher end of the range, reflecting the relatively high socio-economic growth path and industrial output growth over the 21st century, as previously mentioned.

3.2. Costs of Mitigation Considering Non-CO₂ Gases

Figure 5 shows the time-dependent global marginal abatement cost curves for the total non-CO₂ GHGs starting from the point at which any indirect mitigation occurring as a result of CO₂ mitigation has already occurred, for the 2 °C scenario in which global mitigation action starts in 2020. Of note is that, even in 2100, there is expected to be significant abatement potential at marginal costs of $50/tCO₂e or less, with the majority of abatement in all years available at below $100/tCO₂e. The increase in mitigation potential between 2050 and 2100 is entirely driven by changes in activity levels, e.g., population, economic growth and changes in the energy-system. No effects of learning or technological development are taken into account in the assessments of future mitigation potentials. A reason is that there is a lack of empirical basis for adopting general assumptions about the rate at which non-CO₂ regulations would drive long-term technological development. Most likely, this drive is not as strong for non-CO₂ as for CO₂, where regulations reinforce already existing incentives to improve energy efficiency in order to save on energy costs. Hence, in the absence of a firm basis for assumptions on technological development of non-CO₂ mitigation measures, the estimated future potentials for non-CO₂ mitigation should be considered conservative rather than optimistic.

Also of note is the presence of some significantly negative cost mitigation measures in all years. These measures are not profitable with today’s energy prices, but expected to become profitable in the future, conditional on a rise in future energy prices. This effect is not accounted for in the reference scenario as it is defined as a scenario without further mitigation actions. Whether measures that become profitable in the future as a result of rising energy prices will be taken up automatically or not depends on more factors than pure short-run cost-effectiveness [39]. Without additional regulations in place, the presence of x-inefficiency, institutional inertia and uncertainty regarding future regulations and energy prices, are likely to discourage investments in mitigation in the reference scenario. To avoid speculation, such investment opportunities appear here as negative cost mitigation measures in the cost curves and are likely to be among the first measures to be taken up once regulations have been introduced.

Figure 6 shows the marginal abatement cost curves for 2050, for three different LTTGs (2, 2.5 and 4 °C median global warming by 2100), in scenarios with global mitigation action starting in 2020. At higher LTTGs, there is less indirect mitigation, which means that the total direct mitigation potential at a given CO₂ price is greater.

Table 2. Major non-CO₂ GHG mitigation measures in different cost ranges.

Non-CO2 GHG	≤$0/tCO2e	<$50/tCO2e	<$100/tCO2e	>$100/tCO2e
CH4	Increased recycling and energy recovery of biodegradable solid waste instead of landfill; Farm-scale anaerobic digestion on large pig farms; Recovery and use of associated waste gas from gas production; Reduced leakage from gas transmission pipelines in Russia and Eastern Europe	Oxidation of ventilation air methane from underground coal mines; Pre-mine degasification of coal mines; Recovery and use of currently vented associated waste gas from oil production; Reduced leakage from oil and gas production; Dietary feed changes for indoor-fed livestock; Intermittent aeration of rice fields	Waste optimisation; Replacing cast iron gas distribution networks	More expensive gas leakage reduction measures; More expensive waste reduction options
N2O	Best Available Technology in nitric acid production; Reduced and regulated use of N2O in anaesthetics and propellants; Optimise domestic wastewater treatment	Catalytic reduction of N2O in nitric acid production; Reduction and improved timing of fertiliser application	Nitrification inhibitors in agriculture	Precision farming; Replace N2O in anaesthetics
PFCs	-	Replace PFCs with NF3 in semiconductor industry	-	Inert anodes in primary aluminium production
SF6	Leakage control of SF6 in mid-high voltage switches	-	-	-
HFCs	End-of-life recollection of HFCs in domestic refrigeration	Replace HFCs with low-GWP alternatives and HFOs in air conditioning, refrigeration and heat pumps; Leakage control in air conditioning and refrigeration; Replace HFCs with Fluoro Ketone (i.e., FI-5-1-12) in fire extinguishers	Replace HFCs with CO2 in refrigeration in industry and transport	Replace HFCs with CO2 in ground source heat pumps, air conditioning and commercial refrigeration

Notes: All CO₂e prices calculated using GWP100 basis; many mitigation options span a range of costs, depending on region, practices and local costs—hence figures are illustrative and do not reflect all details of estimated cost curves.

sets out some significant mitigation options for each non-CO₂ GHG within different cost ranges.

Figure 7 shows, for the different scenarios explored, the total cumulative discounted cost over the period 2010–2100 (at a discount rate of 5%) associated with mitigation of CO₂ to 2100, as well as mitigation of non-CO₂ GHGs to 2100 at a range of CO₂e prices, the latter as a percentage of the CO₂ price from the TIAM-Grantham model for each time point. This cost is calculated by combining two costs: the first is the present value (using a discount rate of 5%) of the additional cost of the energy system in the TIAM-Grantham model when comparing the 2 °C scenario with the unmitigated reference scenario; the second is the present value (again at a discount rate of 5%) of the sum of annual non-CO₂ mitigation costs as calculated from the area under the marginal abatement cost curve for each year in the GAINS model. Mitigation at a zero price on non-CO₂ (thereby allowing only negative cost measures) results in a 2100 median temperature change of just under 2.5 °C. This is because the cumulative CO₂ budget for the fossil fuel and industrial sectors in order to produce a 2100 median warming level of 2 °C is appropriate only if there is also significant abatement of non-CO₂ GHGs [40] (broadly in line with the level of mitigation achieved in the RCP 2.6 scenario [41]).

Mitigation of non-CO₂ GHGs even to a small fraction (20%) of the price of CO₂ from fossil fuels and industry leads to significant abatement of non-CO₂ GHGs, and a 2100 median temperature change of much closer to 2 °C (about 2.04 °C), at an additional cumulative discounted cost of around 0.08% of 2010–2100 GDP. Even at this 20% fraction of the fossil fuel and industry CO₂ price, the non-CO₂ GHG price rises to $1170/tCO₂e by 2100. For this reason Figure 7 also shows the median warming (as well as total mitigation cost) at sustained prices of (2005) $50/tCO₂e and $100/tCO₂e throughout the century, reflecting the significant degree of mitigation potential available up to these prices, as shown in Figure 5. As expected, the scenarios with these CO₂e prices lead to median warming levels which are lower than the 2.5 °C median warming that results when a zero CO₂e price is applied to non-CO₂ GHGs.

However, the scenarios with a uniformly-applied CO₂e price are not as cost-efficient as the scenarios in which the CO₂e price is applied as a fixed fraction of the (rising) CO₂ price, which is to say that they do not achieve as low a level of 2100 median warming at the same cumulative cost as the fractional price scenarios. For example, Figure 7 shows that applying a CO₂e price of 20% of the CO₂ price throughout the mitigation period (during which the CO₂e price rises from $0/tCO₂e in 2020 to $38/tCO₂e in 2030, $62/tCO₂e in 2040 and then to $1170/tCO₂e in 2100) is actually less costly, and achieves a lower 2100 temperature change, than applying a $50/tCO₂e price uniformly from 2020 to 2100. This is because, with the uniform non-CO₂e prices, some of the mitigation effort in the early part of the century which targets short-lived CH₄ and F-gases (particularly over the decades 2020–2040, in which the uniformly applied CO₂e price is on average higher than the steadily-rising fractional CO₂e price) has no impact on the 2100 median warming level, and is in some ways therefore “wasted” effort (and cost) with regard to the 2100 median temperature change. This additional mitigation cost of the uniform non-CO₂e price, which does not achieve a benefit in terms of 2100 warming, outweighs the (discounted) cost saving of the uniform price being lower than the fractional price in later decades.

This result is, however, highly dependent on the discount rate used (with lower discount rates de-emphasising the cost of applying a uniform price in the short term, compared to the higher fractional cost in the long term). Perhaps more importantly, it is feasible that early action on mitigation of non-CO₂ gases would reap benefits in terms of learning and associated cost reductions in future mitigation measures. In addition, early mitigation of non-CO₂ GHGs may be seen as highly advantageous in order to lower near-term warming, regardless of its impact on warming in 2100. Hence, further analysis is required before any policy conclusions can be drawn on the timing and degree of effort in mitigating short-lived gases such as CH₄.

Also of note from Figure 7 is that where the non-CO₂e price is higher than the CO₂ price (the “120%” point) there is relatively little impact on 2100 median temperature change, since the vast majority of non-CO₂ abatement is already taken up at lower non-CO₂e prices.

A similar analysis is shown for the 2.5 and 4 °C scenarios, in Figure 8 and Figure 9 respectively. Of note is that the overall mitigation cost is significantly lower than the 2 °C pathway, which gives a sense of the relative degree of challenge involved in meeting the 2 °C long-term goal. In fact, in the case of the 4 °C scenario, the very low mitigation costs for CO₂ are slightly outweighed by negative cost measures for non-CO₂ gases, leading to an overall marginal negative cost of meeting the 4 °C goal. Whether this is realisable in practice depends on the realism of achieving these measures. These stem principally from recycling in developing countries, with the assumption that recycled products would be sold at international market prices—in practice the recycled products may have less economic value than this if they cannot reach these markets.

In both the 2.5 and 4 °C scenarios, there is actually over-achievement of the long-term goal (i.e., temperature change is less than 2.5 and 4 °C respectively) when the CO₂ and non-CO₂ prices are equal, indicating that the target may be achieved in a less costly way with a little less CO₂ mitigation effort. Nevertheless, the final estimated median temperature changes are sufficiently close to the desired goals to prove useful as an indicative scenario of the costs and measures associated with meeting these goals.

Figure 10 shows a subsection of the data from Figure 7, Figure 8 and Figure 9, so as to demonstrate the change in total mitigation cost and median 2100 temperature change as the non-CO₂ GHG price (in $/tCO₂e using GWP100) changes as a fraction of the CO₂ price (in $/tCO₂). This demonstrates first the significant additional cost of meeting the 2 °C target compared to the 2.5 and 4 °C targets, as well as the significant reduction in 2100 temperature change achievable by including non-CO₂ GHG options in the overall mitigation portfolio. This is most clearly illustrated with reference to the vertical dashed line around the 2.5 °C mark in Figure 10: this LTTG is achievable either at a cost of $48 trillion by focusing only on CO₂ mitigation, or alternatively at $17 trillion by including non-CO₂ GHG mitigation in the portfolio of options, a cost reduction of about 65%. This compares to the figures discussed in Section 2, in which Rao and Riahi [13] found an approximate halving of carbon price, Kurosawa [18] found an approximate 55% cost saving by 2100, and Lucas et al [15] found a 4–26% mitigation cost reduction by 2100, when achieving a 550 ppm CO₂e stabilisation concentration using a multi-gas approach compared to a CO₂-only approach. The greater percentage cost savings in this study are most likely to stem from the fact that the scenarios shown in Figure 10 are for global mitigation action starting from 2020, whereas the above-quoted cases are immediate action scenarios. As such, this makes it more challenging and costly to meet any given long-term target with CO₂ alone as a result of lock-in to CO₂-intensive infrastructure and technologies with delayed action, thereby increasing the benefit of including non-CO₂ GHG mitigation.

4. Discussion

The 21st century cumulative CO₂ budgets estimated for the 2 °C, 2.5 °C and 4 °C long-term temperature goals achieve 2100 median temperature changes of 2.00, 2.45 and 3.88 °C, once mitigation of non-CO₂ GHGs is taken into account up to a CO₂e price equal to the CO₂ price in the fossil fuel combustion and industrial process sectors.

Significant mitigation of non-CO₂ GHGs, particularly CH₄, results from the transition from a fossil fuel intensive to low-carbon energy and industrial system. This is primarily because fugitive CH₄ emissions from oil, coal and gas extraction, transmission and distribution activities decline with total primary fossil fuel demand. Further, significant mitigation of the majority of non-CO₂ GHGs is available at relatively low CO₂e prices, with the majority of options below $100/t CO₂e (when calculated on a GWP100 basis). This means that the mitigation of non-CO₂ GHGs even to CO₂e prices at a fraction of the fossil fuel and industrial CO₂ price yields significant reductions in 2100 median temperature change. The most cost-effective non-CO₂ mitigation options include:

• For CH₄, increased recycling and energy recovery of biodegradable solid waste instead of landfill, reduced leakage from gas pipelines in Russia and Eastern Europe, extended recovery of associated waste gas from gas and oil production, and farm-scale anaerobic digestion of manure on large pig farms;
• For N₂O, reduced emissions from nitric acid production through improved technologies and catalytic reduction, as well as optimised wastewater treatment practices and improved fertiliser application regimes in agriculture;
• For F-gases, reduction of leakage of HFCs from refrigeration, as well as replacement of HFCs with low-GWP alternatives in refrigeration and air conditioning.

The total cumulative discounted cost over the period 2010–2100 (at a 5% discount rate) of limiting global average temperature change to 2.5 °C by 2100 is $48 trillion (about 1.6% of cumulative discounted GDP over the period 2010–2100) if only CO₂ from the fossil fuel and industrial sectors is targeted, whereas the cost falls to $17 trillion (0.6% of GDP) by including non-CO₂ GHG mitigation in the portfolio of options—a cost reduction of about 65%.

If non-CO₂ GHGs are mitigated to the same CO₂e price level as for CO₂ from the fossil fuel and industrial emissions sectors, then there is significant abatement of all non-CO₂ GHGs up to this CO₂e price, such that in the 2 °C scenario, by 2100 the fully mitigated level of non-CO₂ GHGs is just under 13 GtCO₂e, compared to more than 39 GtCO₂e in the unmitigated reference scenario. Of this approximate 27 GtCO₂e reduction, 69% occurs through the direct mitigation of the non-CO₂ GHGs and 31% through the indirect mitigation (mostly of CH₄) that follows from CO₂ mitigation. For each non-CO₂ GHG (CH₄, N₂O, and aggregated F-gases) the absolute emissions in the reference and mitigation scenarios are within the ranges of the database of scenarios presented in the IPCC’s fifth assessment report, although the CH₄ reference emissions are at the higher end of the range, reflecting the relatively high socio-economic growth path and industrial output growth over the 21st century that underlies the scenario projections in this study. Furthermore, the percentage emissions reductions of each non-CO₂ GHG resulting from both direct and indirect mitigation are also comparable to those in the fifth assessment report database.

For the most stringent mitigation scenario explored in this study (that aimed at achieving a 2 °C median temperature change in 2100), mitigation of non-CO₂ GHGs beyond the CO₂ carbon price does not yield significant reductions in 2100 temperature. This is because the marginal abatement cost curve for non-CO₂ GHGs is relatively steep at higher CO₂e prices, so that little marginal mitigation happens as CO₂e prices increase. This indicates that mitigation to achieve the Paris Agreement’s “well below 2 °C” goal is likely to have to come through a contraction of the 21st century CO₂ cumulative budget (relative to the budget which achieves the 2 °C goal), rather than through increased action on non-CO₂ GHGs alone.

The analysis also indicates that a rising CO₂e price could result in a more cost-efficient mitigation strategy than a constant CO₂e price over time, where earlier action on short-lived gases such as CH₄ does not contribute to a significant impact on 2100 temperature change. Three factors must be accounted for in further considering this point: (i) the discount rate used and the value of early mitigation costs compared to later mitigation costs; (ii) the fact that early action on CH₄ mitigation could foster learning and cost reductions, making later mitigation cheaper, and (iii) the fact that short-term mitigation of CH₄ may be an important facet of avoiding any dangerous overshoots of temperature change.

Although the mitigation potentials and costs in the GAINS model take account of purely technical barriers to adoption on a regional basis, there are other barriers which are more difficult to account for e.g., behavioural or institutional. Such barriers may add to costs at the local level. On the other hand, the purely technical nature of the cost estimates also means not accounting for potential co-benefits of mitigation in terms of improved health and reduced agricultural damages from methane as an ozone precursor [27]. In addition, a number of mitigation options associated with demand-side measures, notably human dietary changes, are not included. These could yield significant additional non-CO₂ emissions reductions [17,42]. Finally, the analysis does not assume technological development and associated cost reductions in the non-CO₂ mitigation measures over time. Implementation of climate policies, which incentivise the wide-spread adoption of non-CO₂ abatement technology, are likely to drive the development of cheaper and more effective abatement technology as time and learning progress.

In summary, a number of major policy implications derive from this analysis. First, indirect mitigation of non-CO₂ gases from CO₂ mitigation strategies is significant, which reaffirms the policy imperative to focus on shifting from fossil-fuel intensive to low-carbon technologies. Second, direct mitigation of non-CO₂ gases is potentially very cost-effective as part of an overall mitigation strategy, with cost savings in the 2.5 °C scenario shown in this study even greater than those for similar scenarios in previous studies, owing to the increased difficulty of achieving this mitigation goal through CO₂ mitigation alone, as a result of significant global mitigation action starting from 2020 only. Each of the major cost-effective non-CO₂ GHG mitigation measures highlighted should therefore be fully incentivized through appropriate policy measures. Third, the steepness of the marginal abatement cost curve for non-CO₂ GHGs after relatively low-cost abatement (at typically below $100/tCO₂e) has been taken up implies that increasingly stringent long-term temperature goals, such as the Paris Agreement’s 1.5 °C aspiration, are likely to rely in a large part from further mitigation of CO₂. However, an important caveat is that this study does not include demand side changes such as dietary changes, which could yield further cost-effective mitigation of non-CO₂ GHGs. A fourth implication for policy makers is that it will be critical to weigh up the relative costs of early mitigation of short-lived non-CO₂ GHGs, which may not yield benefits in terms of longer-term warming (assuming they are eventually mitigated) but which could drive cost reductions in non-CO₂ mitigation technologies and measures through learning, whilst achieving important near-term reductions in warming.

This analysis, combined with the fact that non-CO₂ GHG mitigation options, particularly on the demand side, remain relatively less well explored compared to CO₂ options, highlights the importance in undertaking further research into the drivers, barriers and costs of mitigating these gases, so that policy makers can understand the trade-offs between early, gradual and delayed adoption of non-CO₂ mitigation measures.