Impact of short-lived non-CO₂ mitigation on carbon budgets for stabilizing global warming

A near-linear relationship between cumulative emissions of carbon dioxide (CO₂) and peak global-mean temperature increase is seen in many climate models, and the ratio between these two quantities is referred to as the transient climate response to cumulative emissions of carbon (TCRE). TCRE is defined as the global-mean surface temperature increase for an emission of 1000 PgC to the atmosphere, and applies for cumulative emissions up to about 2000 PgC until the time temperatures peak (Collins et al 2013). The Working Group I (WGI) contribution to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) assesses TCRE to fall with greater than 66% probability—'likely' in the calibrated uncertainty language of the IPCC (Mastrandrea et al 2010)—within the range of 0.8–2.5 °C (Collins et al 2013, Technical Summary in IPCC 2013).

As a consequence of the near-linear relationship between cumulative carbon emissions and peak temperature, a CO₂ budget can be computed that defines the emissions compatible with limiting peak warming to below a given temperature limit with a given probability. IPCC AR5 WGI computed that to limit CO₂-induced warming to below 2°C with at least 33, 50, and 66% probability, the corresponding compatible carbon budgets would be 1570, 1210 and 1000 PgC, respectively⁸ (Summary for Policymakers in IPCC 2013). With historical CO₂ emissions amounting to about 515 PgC ('likely' range 445–585 PgC) by 2011 (Friedlingstein et al 2014, Summary for Policymakers in IPCC 2013), this suggests that we have already emitted about half of the CO₂ emissions compatible with limiting warming to below 2°C with a greater than 66% chance. This interpretation would only be correct for the hypothetical case that all the warming is caused only by CO₂.

Many other, both cooling and warming, species influence the radiative balance of the Earth (Myhre et al 2013). As their resulting effect at the time of zero CO₂ emissions is projected to be net positive (i.e. to be a net warming effect), compatible CO₂ emissions budgets are smaller when taking into account all radiatively active species (Collins et al 2013, Clarke et al 2014, Knutti and Rogelj 2015, Technical Summary in IPCC 2013). For instance, IPCC AR5 WGI estimates that compatible CO₂ emissions from 2011 onward to limit peak global-mean temperature increase to below 2°C would be reduced to about 410, 355, and 275 PgC for having at least 33, 50, and 66% chance, respectively. Likewise, based on the Working Group III (WGIII) contribution to the IPCC AR5 (Clarke et al 2014, Summary for Policymakers in IPCC 2014), scenarios that have a 'likely' chance of limiting warming to below 2°C have a range of about 170–320 PgC for CO₂ emissions from 2011 to 2100. This is much lower than what would be estimated for a world in which only CO₂-induced warming would play a role. In other words, half of the CO₂ emissions compatible with limiting warming to below 2°C (with a greater than 66% chance) have been emitted if we consider only CO₂—taking also the influence of non-CO₂ species into account, however, about two thirds of 2°C-compatible CO₂ emissions have been emitted to date.

The level of non-CO₂ emissions could thus play an important role in determining the size of the CO₂ budget. Here we explore if, and by how much, the targeted mitigation of various, both cooling and warming, non-CO₂ species would influence CO₂ emissions budgets during the 21st century consistent with limiting warming to below specific temperature thresholds. We explore the potential impact of methane (CH₄), soot (or black carbon—BC), sulphate (SO₂), and hydrofluorocarbons (HFCs), and do so by accounting for possible linkages between sources of emissions of CO₂ and non-CO₂ species (see section 2).

Emission mitigation actions discussed in the framework of the United Nations Framework Convention on Climate Change (UNFCCC) focus on the so-called Kyoto-basket of greenhouse gases (GHGs). This basket contains CO₂, as well as CH₄, N₂O, HFCs, perfluorocarbons, sulphur-hexafluoride (SF₆), and nitrogen trifluoride (NF₃) (UNFCCC 1998, 2012). Actions to reduce BC and sulphates are thus not explicitly pledged under the UNFCCC. Recently, however, initiatives have been launched by other forums that focus on limiting so-called short-lived climate pollutants (SLCPs). These SLCPs consist of CH₄ and HFCs, (both controlled under the UNFCCC), the tropospheric ozone arising from CH₄, NMVOC, CO and NO_x emissions, and BC (UNEP 2011). The cooling sulphates, although also being air and climate pollutants, are not considered under the group of SLCPs.

It is well-established that, unlike for CO₂, the annual rate rather than the cumulative emissions of SLCPs have the strongest effect on peak warming (Smith et al 2012, Bowerman et al 2013, Pierrehumbert 2014, Rogelj et al 2014c). Moreover, despite not being directly covered by actions undertaken under the UNFCCC, also some SLCPs (including BC) will be strongly reduced by mitigation measures to limit cumulative CO₂ emissions (figure 1; Rogelj et al 2014b, Rogelj et al 2014c). This is because CO₂ and some SLCPs are emitted by common sources. For example, many combustion processes, like diesel engines, release both CO₂ and BC. If diesel engines are phased out because CO₂ emissions are limited, the BC emissions that originally originated from these engines disappear.

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Based on an initial scenario set of almost 200 scenarios (Rogelj et al 2011), we construct a set of cases that allow an assessment of the influence of SLCP mitigation on CO₂ budgets. Each of the original scenarios contains an internally consistent set of both CO₂ and non-CO₂ emission trajectories over the 21st century. We here use a set of earlier published methods (Rogelj et al 2014b, Rogelj et al 2014c) to recalculate new, internally consistent baseline emissions for CH₄, BC, co-emitted species like organic carbon (OC), and SO₂, and compare these to stringent emission mitigation pathways. These new baselines are required to allow an assessment of the maximum effect of non-CO₂ mitigation on CO₂ budgets: the difference between a mitigation path and a baseline in absence of mitigation targeting a specific non-CO₂ forcer.

CH₄, BC, and SO₂ are affected differently by CO₂ mitigation (figure 1). For BC, OC, SO₂ and other co-emitted pollutant emissions, we use a tool provided by Rogelj et al (2014b) that allows to calculate consistent global relationships between CO₂ and these air-pollution species. Default baseline emissions for these air pollutants assume that current air-pollution legislation is fully implemented and that global air-quality standards converge in line with regional economic development. This baseline also assumes that with increasing economic development an increasingly larger share of the population will get access to clean sources of energy (Pachauri et al 2012, Rogelj et al 2014c). The mitigation path for BC and its co-emitted species mimics the 'all measures' case developed by the UNEP SLCP report (UNEP 2011) until 2030 and is further projected throughout the century as described by Rogelj et al (2014c). The mitigation path for SO₂ assumes that stringent air-pollution controls are implemented over the 21st century (see Rogelj et al 2014b).

Also three alternative baselines are constructed for a sensitivity analysis: two with air-pollution legislation frozen at its 2005 levels throughout the 21st century for BC and SO₂/NO_x, respectively. In these two baselines, no improvements for BC or SO₂/NO_x air-pollution legislation are assumed to have occurred over the last decade and the stringency of air-pollution control is frozen at its 2005 levels throughout the 21st century for BC and SO₂/NO_x, respectively. These assumptions represent a counterfactual evolution of air-pollution control over the past decade, and a failure to effectively implement any additional air-pollution measures in the future. A third alternative baseline includes no targeted energy access policies (see table 1 and Rogelj et al 2014b for details). Under the latter assumption, large shares of the global population remain without access to clean energy until the end of the century. The use of traditional biomass for cooking and heating is currently a major source of anthropogenic BC emissions globally (Pachauri et al 2012).

For CH₄, we follow the method presented in Rogelj et al (2014c) to create CH₄ baselines consistent with each respective CO₂ emission trajectory. This method applies no common carbon price to CO₂ and CH₄, but only to CO₂. Because CO₂ and CH₄ have few common sources, CH₄ baseline emissions do not vary much across a wide range of CO₂ emission pathways (figure 1). These CH₄ baseline emissions are then compared to a very stringent CH₄ mitigation path, derived from a model with particularly strong CH₄ reduction response to, for example, increasing carbon prices (van Vuuren et al 2011). Also a case assuming a 20-year delay of these stringent CH₄ measures is constructed.

For HFCs, we use updated HFC baseline estimates (Velders et al 2009) and assess their influence relative to earlier estimates (Nakicenovic and Swart 2000) as described in Rogelj et al (2014c). These estimates are the high end of the literature (Gschrey et al 2011).

The 'reference case' of our study includes internally consistent baseline evolutions for air pollutants that all follow an extrapolation of current policies, as well as the original CH₄ and HFC pathways from Rogelj et al (2011). Note that carbon-price or policy effects are typically included in the CH₄ and HFC 'reference case' pathways which are found in the literature. To explore the maximum range, these carbon-price effects are excluded in our CH₄ and HFC baselines. However, CH₄ baselines still change as a function of CO₂ mitigation, not due to a carbon price but because some sources of CH₄ are phased out together with CO₂ mitigation (figure 1).

Comparing the various baseline, sensitivity and reference cases will provide us with an estimate of the potential effects of a set of policy interventions, which are, however, rarely fully independent from CO₂ mitigation. While BC and SO₂ emissions are coupled to CO₂ mitigation by co-emission from economic activities and technologies, CH₄ and HFCs are coupled to CO₂ mitigation by multi-gas carbon-price/policy effects, resulting from multilateral agreements under the UNFCCC and/or national policies.

The temperature outcome of each scenario variation is assessed with the reduced-complexity carbon-cycle and climate model MAGICC, version 6 (Meinshausen et al 2011a), in a probabilistic setup that is consistent with the IPCC AR5 WGI climate sensitivity assessment (Meinshausen et al 2009, Rogelj et al 2012, Rogelj et al 2014a). Temperature increase is computed relative to preindustrial levels (1850–1875). Global-mean temperature projections and assessing associated uncertainties are a key application for which MAGICC has been extensively vetted (Meinshausen et al 2011b). For each scenario, the 50%, 66% or other percentiles are computed out of a sample of 600 climate model runs; a cubic smoothing spline (smoothing parameter: 5 × 10⁻⁹) is computed to show the general dependence of maximal target temperature versus cumulative CO₂ emissions (figure 2(A)). Alternative appropriate values for either smoothing parameter or fit type do not change our main conclusions.

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We here look at how CO₂ budgets consistent with limiting warming to below a specific temperature limit by 2100 are influenced by the mitigation of short-lived non-CO₂ species. As introduced earlier, to first order, annual emissions of short-lived non-CO₂ species leading up to the time of the peak play the most important role for peak warming (Smith et al 2012). Therefore mitigation actions on these species are only important insofar as they effectively reduce the annual emission burden of short-lived non-CO₂ species around the time of peak (or maximum) warming during the 21st century. In most temperature stabilisation scenarios, peak warming is reached by the last quarter of the 21st century. Scenarios with little CO₂ mitigation reach their maximum warming during the 21st century only by 2100, and are still increasing afterwards. Any scenario that does not reach zero or lower annual CO₂ emissions by 2100, will exhibit further warming after the 21st century.

Results are reported as relative changes to the 'reference case' (tables 1 and 2). Absolute values are reported in table S1.

Earlier literature has shown that mitigation measures of BC and its co-emitted species only have a limited effect on maximum 21st century temperatures (≪0.1°C) when compared to a baseline which already includes current and planned legislation (Rogelj et al 2014c). In the context of this study, this limited effect on maximum 21st century temperatures translates into virtually no effect (<2.5%) of 'BC measures' on CO₂ budgets (figure 2(B) and table 2, relative shifts of CO₂ budgets are rounded to the nearest 5%). This limited impact is robust across all temperature levels assessed here.

These findings are sensitive to the air-pollutant baseline evolution. Therefore sensitivity cases were created (table 1). First, when assuming a baseline without explicit energy access policies, CO₂ budgets decrease by about 5% for 2°C-compatible budgets (table 2). For higher temperature levels, this relative effect becomes smaller. Second, we assume a frozen BC baseline, representing a roll-back of future air-pollution controls over the 21st century from today's levels. In this case, maximum 21st century temperature is affected by 0–0.4°C, depending on the concurrent CO₂ mitigation. This translates in a 0–5% smaller CO₂ budget compatible with 2°C and 3°C, and 5–10% smaller CO₂ budgets compatible with 4°C by 2100 (figure 2(B)). The lower end of these ranges corresponds to higher probabilities of limiting warming to below these temperature levels. This is a logical result as to achieve higher probabilities of staying below a given temperature limit, increasingly lower CO₂ pathways are required. A discussion of the robustness and adequacy of these values is provided in the following section.

The 'BC measures' assumed in the previous paragraph only tackle sources of air pollutants that have a net warming effect. However, other air pollutants (like SO₂) exist which have a net cooling effect. SO₂ emissions are strongly linked to emissions of CO₂, and in addition, technology shifts lead to a projected steady phase-out of these emissions over the 21st century. Therefore, stringent emission reductions of SO₂ are projected to influence maximum 21st century warming to a very limited degree compared to our reference case which includes a 'current legislation' baseline, and their influence on CO₂ budgets consistent with particular temperature limits is therefore assessed to be virtually zero. Assuming that SO₂ controls are frozen at their 2005 levels, however, would increase the CO₂ budget for limiting median global-mean temperature to below 2, 3, and 4°C, by 10, 15, and 25%, respectively. For higher probabilities, this effect is smaller (table 2).

In the CH₄ pathways of our 'reference case' typically a multi-gas mitigation approach was pursued (e.g., Clarke et al 2009). This means that CH₄, and other gases of the Kyoto-GHG basket, are targeted together with CO₂ by means of a common carbon price and a metric translating non-CO₂ emissions into CO₂-equivalent emissions (for example, 100-year Global-Warming-Potentials, as currently under the UNFCCC). By contrast, in our baselines for CH₄, we decoupled CH₄ from CO₂ mitigation by assuming that the carbon price is only applied to CO₂. When switching CH₄ pathways in all scenarios to this hypothetical baseline, CO₂ budgets for limiting warming to specific temperature levels during the 21st century are reduced across all scenarios, but with important variations. For limiting warming to below 2°C, CO₂ budgets consistent with a 50, 66, and 75% chance of success are reduced by 35, 40, and 50% respectively (table 2). For higher temperature limits, this change is smaller (figure 3(A), table 2), because for higher CO₂ budgets the 'reference case' includes higher reference CH₄ emissions due to a lower (implied) common carbon-price pressure in the original scenarios.

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Alternatively, if CH₄ pathways are all switched to a very stringent mitigation path (van Vuuren et al 2011) CO₂ budgets for limiting warming to below 2°C are increased by 20, 25, and 35%, for achieving a probability of 50, 66, and 75%, respectively. These relative shifts are similar for higher temperature limits, yet become increasingly less plausible (see discussion). When delaying this shift towards a stringent CH₄ mitigation path by 20 years, CO₂ budgets for 2°C are increased by about 5% less.

Finally, the high end of recent projections of HFCs are significantly higher than earlier estimates. Not tackling this projected increase strongly reduces the CO₂ budgets consistent with 50% probability of keeping warming to below 2, 3, and 4°C, by 20–45%, 10–15%, and 5–10%, respectively (table 2, figure 3(B)). In some cases, the highest updated HFC projections would push the achievability of staying below low temperature levels with high probability (66 or 75%) beyond the here assessed scenario literature. Given that our HFC assumptions are based on the highest available literature estimates and also lower—equally plausible—estimates are available, our results should also be read as upper-limit estimates.

4.1. Comparison to IPCC AR5 ranges

4.2. Applicability and limitations

In conclusion, we have quantified the possible influence of short-lived non-CO₂ mitigation on CO₂ budgets consistent with limiting warming to various levels during the 21st century. The most meaningful findings are for 2°C, as for this level temperatures are actually stabilized by 2100, and peak warming has thus been reached. Our study looked at warming during the 21st century. If 21st-century reductions of short-lived climate forcers are traded with reductions in CO₂, warming during the 21st century might be the same, but the multi-century warming commitment would still be larger due to larger cumulative CO₂ emissions.

We find that mitigation of air-pollution species (both warming and cooling) barely affects consistent CO₂ budgets. That is largely because a phase-out of CO₂ emissions would lead to reductions of co-emitted emissions as their sources disappear.

Very stringent CH₄ mitigation can markedly influence 2°C-consistent budgets, at the expense of a higher post-2100 CO₂ warming commitment. However, this influence depends strongly on the realism of the very low CH₄ scenario that was used here, or on the hypothetical case in which no CH₄ mitigation would be undertaken at all. Delaying stringent CH₄ mitigation by 20 year still yields a comparably large benefit as immediate stringent CH₄ mitigation. Similarly, updated HFC baseline projections could trigger substantial extra warming that would constrain the remaining CO₂ budget. However, for both CH₄ and HFCs, mitigation options and policy instruments are readily available.

Although our results indicate relatively large hypothetical variations of the remaining CO₂ budget, the real-world effects of more or less stringent mitigation action on CH₄ are likely much more limited. Thus, our results can be seen as an exercise to sketch the boundaries of the sensitivity. Even in case of very stringent CH₄ mitigation, the CO₂ budget's increase would not in any way change the fundamental necessity to limit the cumulative amount of CO₂ emissions and hence to phase-out unabated fossil-fuel emissions to net zero or below, likely earlier rather than later in the second half of the 21st century.