Policy Update: Multicomponent climate policy: why do emission metrics matter?

Figure 1.  Non-exhaustive list of elements to consider in the design and use of an emission metric.

The elements used for the Global Warming Potential in the Kyoto Protocol are checked. The criteria below are neither mutually exclusive nor independent. Items in the left column are important for the applications of metrics in climate policies and assessments. Those in the right column are more relevant to the development of metrics.

Figure 2.  Issues relevant to the elements of metrics in Figure 1.

The United Nations Framework Convention on Climate Change (UNFCCC) requires that “policies and measures should […] be comprehensive […] and […] cover all relevant sources, sinks and reservoirs.” Many studies have shown the advantages of a multicomponent agreement [1,2], but challenges remain regarding how to establish a common scale to compare the emissions of different greenhouse gases and aerosols (components) in a way that is supported scientifically, economically and politically [3].

An ‘emission metric’ designates relative weights of the emissions of various components according to their climatic effects. Owing to a large span of atmospheric adjustment times (i.e., lifetimes) and radiative efficiencies of the components (Table 2.14 of [4]), a particular metric can give different relative weights to different components [5], which will change the mitigation priorities between components and hence potentially affect climate policy negotiation.

The most common metric is the Global Warming Potential (GWP) [4,6], which is defined as the ratio of the integrated radiative forcing of a component in question over a time horizon relative to that of CO₂ after their instantaneous pulse releases to the atmosphere in amounts of 1 kg. While the GWP is widely accepted for applications such as climate policies (e.g., the Kyoto Protocol) and assessments (e.g., life cycle assessment [LCA] [7] and carbon footprint analysis [8,9]), it has received a number of criticisms in the climate research community [3,5,10–12] and, as a result, alternative metrics such as the Global Temperature Change Potential (GTP) have been proposed (e.g., [13–18]). The issues at stake are complex and require multidisciplinary perspectives (i.e., from climate physics, atmospheric chemistry, biogeochemistry, environmental economics and political science) in addition to value judgments [3,12,19].

This paper provides an overview of the discussions related to emission metrics (Figures 1 & 2) to narrow the gap between the science of emission metrics and the policy applications of metrics. More comprehensive and detailed overviews of the metrics can be found elsewhere [3,5,12]. In March 2009, the Intergovernmental Panel on Climate Change (IPCC) Expert Meeting on the Science of Alternative Metrics [19] was held in Oslo, Norway, which brought attention to metric research activities. This paper touches upon recent advances and new challenges in the field, as well as outstanding issues that still need to be addressed.

Back to the fundamentals

Given all the complex issues surrounding the metrics, it is useful to begin with the fundamental question: why do we need emission metrics? The comprehensive multicomponent agreement embedded in UNFCCC can be realized by a single-basket approach, a multibasket approach or a set of single gas protocols [20]. The single-basket approach, which is currently used in the Kyoto Protocol, treats all the components within a single agreement, while a multibasket approach comprises agreements of several component groups (e.g., a two-target approach treating the short-term rate of warming and the long-term warming separately [21,22]). In the case of the single-basket approach, a metric is required to compare the emissions of different components. By contrast, in the instances of a multibasket approach and a set of single gas protocols, a metric would not be required by the emitting entities [23], but it would still be needed to compare the total reductions that the individual agreements require. The use of metrics would be shifted from the implementation level to the design level in climate policy [5].

The single-basket approach, which was made possible through the existence of GWP [12], has some advantages over the multibasket approach. First, it has been claimed that a single-basket approach simplifies negotiations [20,24]. Second, parties retain more flexibility on the actual mixture of the components to abate. However, the disadvantage lies in the very metric, namely the difficulty in arriving at a metric that can be justified, as the IPCC stated at the onset: “there is no universally accepted methodology for combining all the relevant factors into a single [metric]” [6]. Surprising to many is that the GWP was never intended to be used in policy: a “simple approach (i.e., the GWP) has been adopted [in the IPCC report] to illustrate the difficulties inherent in the concept” [6]. In the single basket approach, the intricate balance between the abatement options involving a number of components is predominantly based on the metric. This makes the metric of crucial importance to policy makers. In the following, we unravel the complexity step by step.

Types of metrics

Emissions of different components could be compared in many ways, such as GWP [4,6] and alternative metrics (e.g., [13–18]). Among the alternative metrics, the GTP [15,16] has been investigated and applied the most (e.g., [5]). The main difference between the GWP and the GTP lies in the choices of the indicator (Figure 1). While the GWP integrates the radiative forcing along the time path up to the chosen time horizon, and puts equal weights on all times between the time of emission and the time horizon (i.e., no discounting over time up to the time horizon), the GTP focuses on one particular chosen point in time and gives the temperature effect at that time (relative to that of CO₂). The relationships among the GWP and alternative metrics can be interpreted in an economic framework [25].

Hybrids of the GWP and the GTP are the two other alternative metrics that have recently been proposed: the Temperature Proxy Index (TEMP) [17] and the Mean Global Temperature Change Potential (MGTP) [18]. These metrics refer to the temperature change (as with the GTP) but integrate it as the indicator (as with the GWP). While the MGTP is more transparent since it is simply calculated based on pulse emissions, the TEMP has an attractive attribute of deriving climatic equivalence [12] based on an inverse estimation using a simple climate and carbon cycle model [26].

While the preceding four metrics (i.e., GWP, GTP, TEMP, and MGTP) are natural science-oriented metrics, the price ratio [14] (or the Global Cost Potential [GCP] [19]) and the Global Damage Potential (GDP) [13], among others, are economy-oriented metrics (or more comprehensive metrics that include economics [19]). The GCP fits into the cost–effectiveness framework consistent with UNFCCC. By contrast, the GDP falls under the cost–benefit framework [25]. The GCP is the most comprehensive and complex metric that uses the inter-temporal optimization approach to minimize the cost of emission abatement of all components simultaneously [2]. The complexity of the computational derivation of the GCP makes it unsuitable for actual policy applications, but it gives useful insights into what other simpler and more transparent metrics mean economically [1].

Elements of metrics

Any metric involves choices, as illustrated in Figure 1. Issues relevant to such choices are discussed in Figure 2. Figure 1 is an updated attempt to map out the issues associated with metrics [3]. It should be noted that no single figure, such as Figure 1, fits all purposes – relevant categories and entities depend upon the users of the figure.

Some of the choices in Figure 1 can be supported by the scientific basis since previous studies have led to a better understanding of what these choices imply [3,5,10–12]. However, many of the choices cannot be rigorously made from science alone and require value judgments [3,12,19]. The time horizon of the metric is probably one of the most contentious issues to have been debated in the field of emission metrics to date (Figure 1; e.g., [3,17]). A choice of a time horizon implies a particular set of dangerous anthropogenic interferences with the climate system. Science elucidates the time scales and the magnitudes of anthropogenic climate change, but the choice of a time horizon involves value judgments of which anthropogenic interferences the metric should address (see Figure 2).

Applications of metrics

With the increasing interest in comparing short- and long-lived components [27], there is a particular need to develop metrics that are capable of comparing components with vastly different time scales (Table 1). Short-lived components are more uncertain and have a variety of additional properties owing to chemistry [28]. A variety of indirect effects could be included in a metric, such as changes in snow albedo due to black carbon deposition [29].

The applications of metrics are not confined to climate science. The GWP is widely adopted in LCA [7] and carbon footprint analysis [8,9]. Despite the ongoing debate on metrics as previously discussed, the LCA has largely taken the GWP as a scientifically accepted metric. Owing to the lack of coordinated research efforts between the climate community and the LCA community, the latter community occassionally uses the GWP differently to the way in which it was originally intended to be used by the former community (e.g., [30]).

Concluding remarks

Metric design has a variety of issues spanning over diverse disciplines (Figures 1 & 2). There are ample research challenges in the field of emission metrics as discussed above – the 2009 IPCC Expert Meeting [19] renewed the interest in metrics, a new stimulus for future endeavors. While more sophisticated metrics can be created and more aspects of metrics can be delved into further, we believe that the very reason for conducting metric research is to contribute to climate assessments and policies. From this perspective, various inputs from many angles must eventually come down to a simple and transparent metric concept that can be politically feasible [20,24]. To be useful for the recent political efforts to agree on a new policy target (or multitargets [21,22]) that follows the commitment period of the Kyoto Protocol (2008–2012), we argue that metric research should prepare alternatives that fit into possible new climate targets [31].

From the policy and assessment point of view, which metric to use may give rise to a substantial difference in the CO₂-equivalent emissions for certain countries and industries (Table 1)[12,24]. It would be beneficial if future climate policies and assessments provided some flexibility to allow emission metrics to be revised and updated. It has been shown that a time-dependent metric has advantages (over a fixed metric such as the GWP employed in the Kyoto Protocol) to capture the time scale of a climate policy or an assessment [14,16,17,31,32]. Such flexibility in the policy and assessment side would allow new scientific knowledge to be incorporated into the metric. Conversely, a concept of metrics that can be dynamically adjusted is put forward to reflect revisions of climate policy targets in the future [31].

Value judgments are inherent in the choice of a metric [3,12,19] since the climate policy that the metric serves chooses to address certain aspects of climate change over a selected time perspective. For this reason, we suggest that the choice of metrics for climate agreements should not be made by natural scientists and economists alone – it should involve dialog with policy makers. It must reflect the perception of which anthropogenic interference with the climate system should be avoided. Importantly, an interdisciplinary approach may reinvigorate debate into metric design, which will ultimately lead to a more efficient policy outcome.

Table 1.  GWP and GTP values of key components for three IPCC time horizons.

Common name (chemical formula)	Lifetime (years)	GWP				GTP
		20-year	100-year	500-year		20-year	100-year	500-year
Carbon dioxide (CO2)	150^†	1	1	1		1	1	1
Methane (CH4)	12	72	25	7.6		57	12	4
Nitrous oxide (N2O)	114	289	298	153		303	322	265
Sulphur hexafluoride (SF6)	3200	16,300	22,800	32,600		17,500	23,400	28,000
Black carbon	0.020	1600	460	140		470	77	64

Lifetimes and metric values are taken from Table 2.14 of [4], and [5].

^†CO₂ lifetime is representative and cannot be expressed by a single estimate because of the multiple timescales on which CO₂ is removed. (e.g., [26]).

GTP: Global Temperature Change Potential; GWP: Global Warming Potential; IPCC: Intergovernmental Panel on Climate Change.

Acknowledgements

The authors acknowledge Terje Berntsen whose comments and suggestions were useful in improving this paper.

Financial & competing interests disclosure

This study was funded by the Norwegian Research Council under project 184681/S30 (Terra C – Terrestrial C sequestration potential in Norway under present and future climate) and project 172521/S30 (LIBRO – Linked issues as a way to broaden participation in the climate regime: exploring the cases of air quality and energy technology). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.