1. The fair-share approach

Due diligence obligations on climate change mitigation have frequently been interpreted and applied in the light of collective objectives, in particular the temperature goals of the Paris Agreement. ScholarsFootnote ⁴⁶ and non-governmental organizations (NGOs)Footnote ⁴⁷ have sought to develop methodologies to determine what precisely each State—or even each corporation—should do as part of global efforts that would achieve these goals; these methodologies could be used to interpret due diligence obligations under domestic or international law. The Supreme Court of the Netherlands has already interpreted the European Convention on Human Rights, in the light of customary international law, as requiring the State to act consistently with the 2°C goal, which the Court construed as requiring a 25 per cent reduction in national GHG emissions by 2020, compared with 1990.Footnote ⁴⁸ Further, the District Court of the Hague has applied Dutch tort law in the light of the temperature goals of the Paris Agreement and ordered a corporation, Royal Dutch Shell, to decrease its carbon dioxide emissions by 45 per cent by 2030, compared with 2019.Footnote ⁴⁹ Outside the Netherlands, however, multiple courts have dismissed similar cases, in particular on the ground that defining such targets exceeded the scope of judicial functions.Footnote ⁵⁰

The fair-share approach faces important practical difficulties. For one, the temperature goals are poorly defined (eg 1.5 or 2°C, on what time horizon, compared with what ‘pre-industrial’ baseline, with what level of certainty?).Footnote ⁵¹ Furthermore, States have not agreed on a formula to determine how global efforts should be distributed among themselves—let alone how they should be imposed onto multinational corporations.Footnote ⁵² Principles such as ‘common but differentiated responsibilities and respective capabilities’ (CBDRRC) and ‘equity’ do not settle protracted political disagreement on the relevance and weight of various criteria related to historical responsibility, financial capacity and geographical circumstances, among other things.Footnote ⁵³ As a result, any assessment of a State's fair share is bound to be highly contentious, as it would depend almost entirely on the assessor's value-based judgments.Footnote ⁵⁴

Yet a more fundamental issue with the fair-share approach is its premise that States have accepted an obligation to act consistently with global mitigation objectives.Footnote ⁵⁵ Nothing in the text of the Paris Agreement indicates that NDCs are required or even expected to be consistent with the temperature goals or any other global mitigation objective. The drafters of the Paris Agreement considered—but rejected—an alternative treaty architecture whereby a ‘global emission budget’ consistent with the temperature goals would have been ‘divided among all Parties’ in accordance with rules that the treaty would have defined.Footnote ⁵⁶

Objections could invoke the object and purpose of the treaty or its context, including subsequent practice, to suggest that NDCs must reflect the Party's fair share despite the absence of any explicit treaty provision. Yet it is uncertain that this interpretation would be consistent with the law on treaty interpretation. In the plain meaning of Article 31 of the Vienna Convention on the Law of Treaties, neither the context of a treaty nor its object and purpose can justify an interpretation of the treaty at odds with its textual provisions.Footnote ⁵⁷ Consistently, Richard Gardiner notes that interpreting a treaty in the light of its object and purpose does not normally ‘allow … the general purpose of the treaty to override its text’.Footnote ⁵⁸ No textual provision can be interpreted as requiring ‘nationally determined’ contributions as being, in fact, determined by the object and purpose of the treaty. States do not have a legal obligation to achieve every treaty objective that they agree upon.

It may also be suggested that an obligation to act consistently with the temperature goals emerged subsequently to the adoption of the Paris Agreement. Yet there does not appear to be any evidence of a subsequent agreement between the Parties requiring them to implement their fair share in global efforts. Admittedly, the Meeting of the Parties to the Paris Agreement decided that each NDC would contain information on how the Party considers it to be ‘fair and ambitious’ and to ‘contribute … towards’ the temperature goals,Footnote ⁵⁹ and the International Law Commission found that such ‘decisions adopted within the framework of a Conference of States Parties’ may embody a subsequent agreement.Footnote ⁶⁰ However, this agreement on a procedural obligation for Parties to justify the fairness of an NDC provides scant evidence of the existence of a substantive obligation for NDCs to represent the Party's fair share.

Alternatively, a fair-share obligation might conceivably arise from State practice, whether as subsequent treaty practice, customary law, or in application of general norms of international or domestic law (eg tort or human rights law).Footnote ⁶¹ Here again, however, there appears to be no clear evidence of such general practice. To the contrary, the existence of a wide ‘ambition gap’ seems to indicate that States do not generally act consistently with the temperature goals.Footnote ⁶²

2. The HPA approach

A more classical approach to interpreting a State's due diligence obligations starts by assessing the State's circumstances—what this State could realistically do—thus focusing primarily on what is possible rather than what is desirable.Footnote ⁶³ Such a bottom-up approach to due diligence is especially appropriate when an obligation relates to an open-ended objective, such as climate change mitigation, where the priority should be to do as much as possible rather than to achieve any predetermined outcome. Despite an oft-heard argument, temperature goals are a political goal rather than a safe limit determined by scientists;Footnote ⁶⁴ and any incremental GHG emission exacerbates climate harms.Footnote ⁶⁵

Consistently, climate treaties have promoted this bottom-up approach to the determination of national mitigation action, either by requiring each Party to prepare its own mitigation ‘programme’ (UNFCCC)Footnote ⁶⁶ and ‘contribution’ (Paris Agreement),Footnote ⁶⁷ or by allowing each Party to decide its own treaty commitment in the course of international negotiations (Kyoto Protocol).Footnote ⁶⁸ The inclusion of the HPA standard in the Paris Agreement confirms that a State's climate action should be assessed primarily by reference to national circumstances, with global mitigation goals playing at most a secondary role.Footnote ⁶⁹

The HPA standard has received scant attention in the climate-law literature.Footnote ⁷⁰ Lavanya Rajamani and colleagues remarked that HPA ‘shape[s] the due diligence standard’ applicable to climate change mitigation; yet, they did not elaborate further.Footnote ⁷¹ Christina Voigt and Felipe Ferreira praised HPA as ‘a potent and powerful tool’,Footnote ⁷² but they suggested that this standard requires Parties to take ‘adequate … measures … to … achieve the objective of the Paris Agreement’.Footnote ⁷³ Thus, in effect, they assimilated the HPA standard with the fair-share approach. Similarly, when the authors of a communication to the UN Human Rights Committee suggested that several States had ‘fail[ed] to prevent foreseeable human rights harms caused by climate change by reducing [their] emissions at the “highest possible ambition”’,Footnote ⁷⁴ they were referring to an analysis of national ‘fair shares’.Footnote ⁷⁵ However, assimilating HPA with the fair-share approach betrays the ordinary meaning of the terms: HPA points first and foremost to what the State can do, not to what needs to be done to achieve the objective of the Paris Agreement.

Other authors, critical of a top-down approach to HPA, do not see any viable alternative. Alexander Zahar, for instance, proclaimed that ‘whichever ambition a State decides for itself is its highest possible ambition’.Footnote ⁷⁶ Others offered more nuanced views. In particular, Duvic-Paoli noted that HPA is initially ‘self-determined’,Footnote ⁷⁷ and Eckard Rehbinder suggested that there is ‘a lack of clear ideas about [HPA's] meaning’.Footnote ⁷⁸ Yet, as Rehbinder added, the fact that HPA is initially determined by States when preparing their NDC does not necessarily mean that HPA rests ‘exclusively … on free self-judgment by State Parties’.Footnote ⁷⁹ The Intergovernmental Panel on Climate Change noted that HPA ‘could be read to imply a due diligence standard’.Footnote ⁸⁰

Voigt justly noted that HPA ‘can be seen as a standard of behaviour, a reflection of due diligence’.Footnote ⁸¹ Political and judicial processes taking place on both the domestic and international planes could conceivably influence or control the national determination of NDCs, in particular based on interpretations of the HPA standard.

As a matter of treaty interpretation, the HPA standard is to be construed, under the principle of effectiveness, with ‘preference for an interpretation which gives a term some meaning rather than none’.Footnote ⁸² Article 4(3) does not create an obligation on its own (as it is introduced with the auxiliary ‘will’), but it does reflect and reinforce an expectation—or standard—that can inform the interpretation of other norms.Footnote ⁸³ To the extent that HPA can be understood as implying a relatively effective standard, this interpretation should therefore be preferred to one leaving HPA's determination entirely to each State's discretion. Consequently, as Rehbinder points out, HPA should be approached as far as possible as ‘an objective yardstick for assessing and justifying the quality of NDCs’, with the implication that NDCs could be ‘open to comments and criticism for lack of sufficient ambition’.Footnote ⁸⁴

Two difficulties present themselves when relying on HPA to interpret a State's mitigation obligation, but neither of these difficulties is greater than the obstacles to a fair-share approach. First, HPA is undeniably ill defined. Applying the HPA standard does require far-reaching value-based judgments, although no more so than applying the fair-share approach.Footnote ⁸⁵ Second, HPA is not expressly defined as a legal obligation in the Paris Agreement. Yet, by using the auxiliary ‘will’, Article 4(3) of the Paris Agreement reflects a ‘strong expectation’Footnote ⁸⁶ that NDCs reflect the Party's HPA. By contrast, the Paris Agreement does not indicate any similar expectation that NDCs would reflect Parties’ fair shares. The expectation that NDCs reflect the Party's HPA, to the extent that it is upheld by State practice, can be a benchmark to assess States’ compliance with their diligence obligations on climate change mitigation.

1. The ‘possible’

Due diligence obligations are frequently described in hyperbolic terms: as requirements to do one's ‘best effort’, one's ‘utmost’Footnote ⁸⁷ or ‘all in one's power’Footnote ⁸⁸—to ‘take all measures at [one's] disposal’.Footnote ⁸⁹ Of course, no due diligence obligation requires a person to use literally all its resources to pursue a unique goal. A State has many due diligence obligations relating to different goals: not only climate change mitigation, but also human rights protection and ecological preservation, among other things. Inevitably, each of these goals needs to be weighed with other priorities.Footnote ⁹⁰ Due diligence only requires the implementation of the measures that are appropriate in the circumstances. As the International Law Association (ILA) Study Group on Due Diligence in International Law concluded, it is ‘a standard of reasonableness … that seeks to take account of the consequences of wrongful conduct and the extent to which such consequences could feasibly have been avoided’.Footnote ⁹¹

It is in this context that HPA's condition of ‘possibility’ has to be interpreted. An NDC's highest ‘possible’ ambition cannot be understood as requiring the highest level of ambition ‘[t]hat is capable of being’.Footnote ⁹² Rather, the word ‘possible’, in this phrase, refers to the most that ‘can … be done … in given or assumed conditions or circumstances’Footnote ⁹³—the most that a Party can reasonably intend to achieve in a given context. To make sense of the HPA standard, these conditions must be determined.

To do so, a useful analytical framework can be found in the ‘six dimensions of feasibility’ identified by the Intergovernmental Panel on Climate Change when assessing the feasibility of mitigation action.Footnote ⁹⁴ Conditions of ‘geophysical’ and ‘technological’ feasibility are obviously relevant, but they are not sufficient to ensure that States balance their due diligence obligation on climate change mitigation with due diligence obligations relating to other priorities. Of greater relevance are ‘environmental–ecological’, ‘economic’, ‘socio-cultural’ and ‘institutional’ conditions of feasibility, which reflect the need to balance climate change mitigation with other legitimate priorities, some of which are themselves the object of due diligence obligations (eg human rights and environmental protection). States have agreed that financial, technology and capacity-building support would facilitate ‘higher ambition’ in developing countries,Footnote ⁹⁵ precisely because such support would enhance these countries’ technological, economic and institutional capabilities.

Yet, some conditions of socio-cultural and institutional feasibility raise particular issues, especially those relating to ‘public acceptance’ and ‘political acceptance’.Footnote ⁹⁶ Admittedly, measures fulfilling all other feasibility conditions may still face genuine social and political obstacles. However, recognizing political or public acceptance as relevant conditions of ‘possibility’ would allow self-fulfilling prophecies: it would allow a government to assert that its NDC represents the most that this government ‘can’ accept, or a protest movement (eg the French ‘gilets jaunes’) to decide that proposed measures ‘cannot’ be accepted.

Ultimately, whether public and political acceptance are to be considered as conditions under the HPA standard relates to broader questions about the relation between climate action and democracy. There could be concern that voters might prioritize their own interests (eg national interests) over other policy objectives (eg interests of foreigners), thus downplaying the global benefits of a State's mitigation action. Swiss voters have rejected by referendum additional mitigation objectives that the federal government had communicated in an NDC update,Footnote ⁹⁷ forcing the government to renounce these additional objectives.Footnote ⁹⁸

Finding ways to ease the tension between climate action and democracy is beyond the scope of this article. Suffice it to note that deferring entirely to political and social acceptance in the interpretation of the HPA standard would make the HPA standard ineffective—HPA would, by definition, be whatever a national government or a State as a whole declares as the highest ambition it can accept. By analogy, a State cannot rely on the provisions of its internal law as justification for its failure to comply with international law,Footnote ⁹⁹ since internal law—in much the same way as political acceptance—is ultimately what the State decides that it should be. Yet this does not necessarily exclude a limited role for conditions of public and political acceptance in assessing HPA, if only as a way to maintain well-functioning democratic institutions.

2. ‘Ambition’

Duvic-Paoli notes that, despite its prevalence in climate law, the concept of ambition remains ‘poorly understood’.Footnote ¹⁰⁰ At the most basic level, a distinction needs to be drawn between two alternative approaches to ambition. On the one hand, ambition can be conceptualized in terms of expected outcomes, reflected for instance in a rate of reduction in national emissions (whether in absolute terms, per capita, or in economic intensity). On the other hand, ambition can be approached as a level of effort, relating to the stringency of the measures taken and the significance of the resulting economic and social burden.

These two conceptions of ambition on climate change mitigation have different practical implications. This is because the outcome of a State's climate change mitigation policy depends not only on that State's level of effort, but also on a range of extraneous factors that influence the evolution of the State's GHG emissions. At times, a Party could anticipate a greater mitigation outcome thanks to technology development or far-reaching social-distancing measures, among other things. At other times, the Party's mitigation policy may face headwinds, for instance if an increase in the price of natural gas hinders a national policy to cut coal consumption, or if revised accounting methodologies make it more difficult for the Party to achieve an emission-reduction target expressed by reference to national emissions in a base year.Footnote ¹⁰¹

Ambition for the purpose of Article 4(3) is more convincingly understood as referring to a level of effort—as a due diligence (or ‘best efforts’) standard. Yet, it is difficult to measure climate ambition as a level of effort for lack of relevant quantitative indicators. Carbon prices may appear as an attractive metric of ambition, yet these prices can rarely be identified: few national mitigation measures impose a direct price on GHG emissions,Footnote ¹⁰² indirect taxes may be justified in part by other political objectivesFootnote ¹⁰³ and many States rely on more traditional command-and-control regulation that does not set any explicit price at all. On the other hand, ‘implicit’ carbon prices are difficult to estimate,Footnote ¹⁰⁴ if only because they require a determination of the role that climate change mitigation has played in the adoption of policies that may be justified in the light of co-benefits. Moreover, a carbon price does not adequately reflect what an economic incentive to avoiding GHG emissions really means in the economic, social, political or cultural circumstances of a particular State. For instance, the same nominal carbon price would impose a higher social burden on populations with a lower purchase power.

Due to these practical difficulties, HPA is more likely to be expressed in terms of expected outcome, for instance as an emission reduction target, rather than as a level of effort. Expected outcome is a relatively convenient way to assess national action on climate change mitigation, building in particular on national GHG emission data. But while reliance on expected outcome may be inevitable when discussing a State's HPA, it should be kept in mind that this is only an imperfect proxy for assessing what really matters: namely, the level of effort that the State is making.

1. HPA as a cost–benefit analysis

Due diligence, ITLOS noted, ‘is a variable concept’.Footnote ¹⁰⁵ What a due diligence obligation involves depends on the risk that this obligation seeks to reduce and the costs of reducing this risk.Footnote ¹⁰⁶ In other words, what diligence is due to reduce a risk depends on the cost and the benefit of diligence (where ‘cost’ and ‘benefit’ are used in a broad, figurative sense: many costs and benefits are not economic in nature).Footnote ¹⁰⁷ These observations also apply to climate change mitigation: whether a State is doing enough to mitigate climate change cannot be assessed in abstraction from the constraints and opportunities facing mitigation action in that State. Consistently, what constitutes a State's HPA probably depends on both the costs and the benefits of mitigation action in that State.

First, applying the HPA standard involves an assessment of the costs of the State's mitigation action. This is because if the cost of mitigation action is unbearable it could not be deemed to be ‘possible’. Consistently, Parties have recognized that international support could allow developing countries to enhance their ambition.Footnote ¹⁰⁸ However, even the most developed countries with the greatest technological and economic capabilities face constraints: under the conditions of feasibility implied by the HPA standard, they may be able to achieve rapid cuts in their GHG emissions, but not an immediate cessation. An interpretation of the HPA standard failing to take such costs into consideration would conflict with the States’ other due diligence obligations.

Second, applying the HPA standard also involves an assessment of the benefits of the State's mitigation action. This proposition is perhaps less obvious. Admittedly, it does not flow clearly from the text of Article 4(3) of the Paris Agreement: ‘HPA’ seems to require every Party to do its utmost to mitigate climate change notwithstanding the benefits of such action. Yet this provision is to be interpreted in the light of the object and purpose of the treaty—in particular its mitigation objective—and in its context.Footnote ¹⁰⁹

In particular, this latter proposition is supported by the fact that the Paris Agreement contains no sunset clause: each Party must communicate an NDC every five years, in principle, forever.Footnote ¹¹⁰ Yet States would not be expected to keep making the same level of effort in a future in which anthropogenic climate change was well under control. Inevitably, what represents a Party's HPA depends on the urgency of mitigation action at the relevant time, as assessed by States themselves. Thus, less would be expected from each State if States were collectively on course to achieving or exceeding the temperature goals, or if States had agreed on a less stringent temperature goal (eg 2.5°C). On the other hand, more is expected from them at present, as a consequence of their decision to include a 1.5°C goal in the Paris Agreement, their repeated expression of a ‘serious concern’ with the ambition gap, and their emphasis of the urgency of enhancing mitigation action.

An implication of this analysis is that the global temperature goals are relevant to interpreting Parties’ due diligence obligations. By contrast to the fair-share approach, however, the temperature goals are only one part of the equation: these goals influence but do not determine a State's HPA. In the present analysis, the full implementation of the HPA standard by all States would not necessarily ensure the achievement of the temperature goals, and it could possibly lead States to exceed these goals.Footnote ¹¹¹

Overall, this analysis suggests that the HPA standard needs to be interpreted as expressing, in hyperbolic terms, the need for an adequate level of ambition, as determined by taking into account both the costs and the benefits of mitigation action. It remains, however, that the comparison between these costs and benefits is challenging as they differ in scope and nature. The costs of a State's mitigation action unfold primarily within the State's own territory: they include, for instance, the economic cost of mitigation action for individuals and corporations subject to the State's jurisdiction and generally within its territory; the environmental impacts of various measures (eg hydroelectric or nuclear energy) taken by the State, which mostly unfold within the State's own territory; and the cost of opportunity for other policy objectives, which is more likely borne by individuals within the State's territory.Footnote ¹¹² The benefits of a State's mitigation action, by contrast, are global in scope and often more abstract in nature. Among these benefits, the Paris Agreement highlights utilitarian goals such as food security,Footnote ¹¹³ ecological preservationFootnote ¹¹⁴ and sustainable development.Footnote ¹¹⁵

More specifically, a cost–benefit analysis of mitigation action faces three difficulties. First, many costs and benefits concern social and ecological interests that have no inherent economic value (eg no market price), and yet would need to be compared among themselves and with economic costs and benefits. Second, the costs and benefits of mitigation action unfold on different time scales. While the costs of mitigation action are often incurred rather quickly, the benefits (ie avoided climate impacts) extend over centuries or millennia. Yet there is no obvious or consensual way to compare long-term benefits with present costs. It is generally agreed that future costs and benefits are to be discounted when they are compared with present ones, but there is no agreement on the value of the discounting rate.Footnote ¹¹⁶ Third, while the costs of mitigation action tend to be relatively foreseeable, its benefits include the reduction of very small risks of cataclysmic outcomes (eg runaway climate scenarios and civilizational collapse). The valuation of such benefits is impeded by deep uncertainties about the likelihood of such cataclysmic outcomes and possible disagreements on the degree of aversion to such structural risks.Footnote ¹¹⁷ As a whole, the comparison that the HPA standard implies between the costs and benefits of mitigation action relies heavily on value-based decisions.

2. HPA as a standard of differentiation

States have long agreed that each Party's action on climate change mitigation may differ, in particular based on the CBDRRC principle.Footnote ¹¹⁸ Yet, as noted above, they have not agreed on all differentiation criteria, or on their respective weight.Footnote ¹¹⁹ To the extent that the HPA standard can assist in assessing each State's requisite level of mitigation action, it inevitably implies a particular approach to differentiation among States.

In this regard, the text of Article 4(3) of the Paris Agreement hints at seemingly contradictory demands. It suggests that each Party's successive NDCs are to reflect not only the Party's HPA, but also its CBDRRC and ‘national circumstances’. CBDRRC could be understood as involving both responsibility-based criteria (eg historical emissions, per capita emissions and emission intensity) and capacity-based criteria (eg financial and technical capacities). By contrast, the HPA standard suggests a narrower conception of differentiation, focusing exclusively on capacity-related criteria: it requires every Party to do the best it can to mitigate climate change notwithstanding its responsibility for causing it.

There is no easy way to reconcile these two conceptions of differentiation. One could argue that NDCs should reflect both the Party's HPA and its CBDRRC, including responsibility-based criteria. Yet there are two issues with this interpretation. First, it seems inconsistent with the text of Article 4(3), which indicates that CBDRRC is to be reflected in the Party's HPA, rather than directly in its NDC. Second, this interpretation would lead to the absurd conclusion that, when the CBDRRC principle indicates a higher threshold than the HPA, the Party would be required to do the impossible—it would have to aim for more than its HPA. Alternatively, one could suggest that NDCs need to reflect either the Party's HPA or its CBDRRC. However, this would imply that a Party with little responsibility for causing climate change, but significant capability to mitigate it, would not, in fact, be expected to reflect its HPA in its NDC—a conclusion that is entirely irreconcilable with the text of Article 4(3).

In practice, the difference between the HPA standard and the broader conception of differentiation suggested by the CBDRRC principle can be significant. For instance, a developing State with little historical or current emissions may be capable of taking effective measures to reduce future emissions, for instance if it benefits from strong economic growth.Footnote ¹²⁰ An extreme case is Bhutan, a country with reportedly net negative GHG emissions (ie with more sinks of GHGs, such as trees, than emissions by sources, such as cars), which may nonetheless be able to take effective measures on climate change mitigation by further decreasing its emissions by sources and by further enhancing its sinks of GHGs.Footnote ¹²¹ HPA as a differentiation criterion is in principle more effective (as it requires every State to do as much as it can) but it could result in unfair outcomes by ignoring differentiated responsibilities.

State practice in relation to the implementation of the Paris Agreement largely confirms that HPA implies capacity-based differentiation. When prompted to justify how their NDCs reflected their HPA, Parties have predominantly referred to capacity-based criteria, for instance their ‘mitigation opportunities’,Footnote ¹²² ‘level of development’Footnote ¹²³ and ‘needs for poverty reduction’.Footnote ¹²⁴ For instance, Bhutan suggested that, because its NDC reflected its HPA, it represented ‘more than its fair share of action and burden’Footnote ¹²⁵—that is, more than its responsibility.

Thus, the HPA standard seems to reflect a refinement of differentiation in climate law, including under the CBDRRC principle. Regarding mitigation action, this refinement involves a clearer focus on capacity rather than responsibility. This evolution is in line with the emphasis of developed States on capacity, rather than responsibility, when they differentiated among themselves (eg under the Kyoto Protocol or within the European Union).Footnote ¹²⁶ It is also consistent with a pragmatic attempt at maximizing global efforts on climate change mitigation by making the best use of all capacities, consistently with the urgency of mitigating climate change acknowledged by States. On the other hand, responsibility-based criteria remain relevant to other aspects of climate action, in particular the provision of support.Footnote ¹²⁷ Even though the States most responsible for causing climate change may have a limited capacity to mitigate climate change by themselves, they should support capacity to do so in other countries in line with responsibility-based differentiation criteria.

3. HPA and progression

Besides establishing the HPA standard, Article 4(3) of the Paris Agreement suggests that a Party's new NDC ‘will represent a progression beyond the Party's then current’ NDC.Footnote ¹²⁸ This norm of progression is also reflected in Article 4(11), which allows a Party to adjust its NDC ‘with a view to enhancing its level of ambition’,Footnote ¹²⁹ thus suggesting that regressive NDC updates may not ordinarily be permitted.Footnote ¹³⁰

It is difficult to make sense of the relation between the HPA standard and the standard of progression. At times, States have referred to progression as an indication that their NDCs fulfilled the HPA standard.Footnote ¹³¹ Yet, progression does little to demonstrate that the new NDC has reached the HPA standard. If anything, a Party's ability to progress seems to reveal that the Party's previous NDC did not truly reflect (or no longer reflected) its ‘highest possible’ ambition. Evolving national circumstances might lead a State to reassess its HPA, but there is no reason to assume that this reassessment will necessarily lead to progression: adverse changes in national circumstances might equally compel a State to reduce its HPA.Footnote ¹³²

This apparent contradiction can be avoided by interpreting the HPA standard in terms of effort and the progression standard in terms of cumulative outcomes. By continuously doing its best (HPA), a Party will come increasingly close to a model of development consistent with the mitigation objective of the Paris Agreement (progression). A necessary caveat is that progression may not be an entirely linear process—a Party may experience setbacks due to extraneous circumstances, say if a cold winter leads to an increase in energy demand, though such setbacks should be temporary. To account for this caveat, the progression standard should be interpreted in abstraction from the ebbs and flows of national circumstances, based perhaps on a multi-year trend rather than on annual emissions, and on the departure from a business-as-usual scenario rather than on national emissions data.Footnote ¹³³

1. State declarations

When communicating their NDCs, Parties have committed to provide ‘the information necessary for clarity, transparency and understanding’.Footnote ¹³⁴ The Meeting of the Parties to the Paris Agreement decided that, for Parties’ second and subsequent NDCs, this would include information on ‘[h]ow the Party considers that its [NDC] is fair and ambitious in the light of its national circumstances’ and how it ‘has addressed Article 4, paragraph 3, of the Paris Agreement’.Footnote ¹³⁵ Even before this decision, some Parties had provided such information in their intended or first NDC.Footnote ¹³⁶ Others have included similar information in national communications, biennial reports and biennial update reports communicated under the UNFCCC, as part of a broader set of information on the measures and policies that they were implementing on the mitigation of climate change.Footnote ¹³⁷

In principle, an obligation to justify compliance with the HPA standard could pressure States to ensure such compliance. Moreover, it could lead each Party to explain how it interprets its own HPA. In turn, such interpretative statements could be used to gauge the ambition of the Party's subsequent NDCs.

At present, however, the potential of State declarations as a touchstone to assess compliance with the HPA standard remains largely untapped. A review of NDCs, national communications, biennial reports and biennial update reports shows, perhaps unsurprisingly, that no Party had any difficulty finding that its NDC represented its HPA.Footnote ¹³⁸ Further, this review confirms Duvic-Paoli's observation that ‘states tend to self-proclaim their NDCs to be “ambitious”’ without necessarily engaging in any systematic demonstration.Footnote ¹³⁹ On the other hand, the review reveals that some Parties do seek to provide at least some rudimentary justification for their ambition. They allude, for instance, to ‘national circumstances’Footnote ¹⁴⁰ or, more specifically, to ‘the context of [their] national analysis of mitigation potential’,Footnote ¹⁴¹ their ‘multiple initiatives’Footnote ¹⁴² or the fact that ‘the mitigation actions will be undertaken domestically’.Footnote ¹⁴³

One could question whether a Party's mere assertion that its NDC reflects its HPA is enough to ensure compliance with the obligation to explain how the NDC reflects the Party's HPA.Footnote ¹⁴⁴ This treaty requirement and, more broadly, the increasing domestic and international pressure on national governments to justify their level of ambition may induce States to develop more sophisticated justifications in their successive NDCs and NDC updates. This could lead States not only to elaborate increasingly sophisticated interpretations of the HPA standard, but also to specify how this standard applies in the light of their national circumstances. In turn, these statements could provide fodder for various political and even legal processes.

2. Multilateral processes

Parties have largely opposed any role for multilateral processes under the Paris Agreement in assessing national ambition, including compliance with the HPA standard. For instance, the idea of an ex ante review of NDCs ‘in the light of the ambition required’, which was floated in the negotiations leading to the adoption of the Paris Agreement,Footnote ¹⁴⁵ was dismissed in favour of an approach resolutely more deferential to national sovereigntyFootnote ¹⁴⁶ and to the ‘nationally determined nature of [NDCs]’.Footnote ¹⁴⁷ Yet the multilateral processes that Parties did agree to set up will be implemented in a changing political context. Civil society and perhaps even some States are increasingly willing to challenge individual ambition, and they may seek to exploit any potential opportunity to do so—even if this means stretching the scope of existing processes.

A process that might appear relevant, in this regard, is the global stocktake established under Article 14 of the Paris Agreement.Footnote ¹⁴⁸ Taking place every five years, it aims at assessing progression towards the realization of the mitigation objective of the Paris Agreement.Footnote ¹⁴⁹ However, there is no obvious opportunity in this process for questions to be raised about an individual Party's compliance with the HPA standard, if only because the process does not involve a review of Parties’ individual action. Indeed, the Meeting of the Parties to the Paris Agreement decided that the global stocktake would concentrate on ‘collective ambition’Footnote ¹⁵⁰ and have ‘no individual Party focus’.Footnote ¹⁵¹ And while the Paris Agreement calls for the outcome of the global stocktake to ‘inform Parties in updating and enhancing’ their NDCs, they are to do so ‘in a nationally determined manner’.Footnote ¹⁵²

Another potentially relevant process is conducted by the Implementation and Compliance Committee under Article 15 of the Agreement. The Committee can consider ‘issues’ relating to the failure of a Party to comply with procedural obligations, including the obligation of communicating an NDC.Footnote ¹⁵³ Based on the recommendation of the technical expert review team and with the consent of the Party concerned, the Committee may also consider ‘cases of significant and persistent inconsistencies’ with treaty requirements relating to national reporting.Footnote ¹⁵⁴ However, the Parties agreed that the Committee would not ‘address the content of the contributions, communications, information and reports’ themselves.Footnote ¹⁵⁵ As such, while the Committee can play an indirect role in ensuring that Parties communicate an NDC and report on its implementation, it cannot directly review compliance with the HPA standard, or even with the procedural requirement that Parties justify ‘how’ their NDCs comply with the HPA standard.

The ‘enhanced transparency framework’ established under Article 13 of the Paris Agreement could play a more instrumental role in promoting compliance with the HPA standard. Under this framework, each Party must communicate biennial transparency reports that are to undergo a ‘technical expert review’ followed by a ‘facilitative, multilateral consideration of progress’ (FMCP).Footnote ¹⁵⁶ Biennial transparency reports will provide relevant information that could be used to raise questions about the State's ambition during the FMCP.

In particular, a Party's biennial transparency report should include ‘a description of its NDC’.Footnote ¹⁵⁷ Although a justification of the NDC's compliance with the HPA standard is not specifically required, States have been tempted to include such justification.Footnote ¹⁵⁸ At any rate, the report should include a description of the Party's ‘national circumstances relevant to progress made in implementing and achieving its NDC’Footnote ¹⁵⁹ and ‘information on how its national circumstances affect GHG emissions and removals over time’.Footnote ¹⁶⁰ Such circumstances and information are likely to be relevant to a State's justification of how its NDC complies with the HPA standard.

The subsequent technical expert review is not intended to assess the Party's ambition. In fact, the technical expert review team is specifically precluded from engaging in a ‘[r]eview of the adequacy or appropriateness of a Party's NDC’ or ‘of its associated description’ in the biennial transparency report.Footnote ¹⁶¹ Nonetheless, the technical expert review will scrutinize the information reported by the Party, including the circumstances that the Party presents as relevant to its mitigation action. A technical expert review team could conceivably question whether the circumstances that the Party presents as relevant to the implementation of its NDC are, indeed, relevant, and whether these circumstances could really affect the Party's GHG emissions and removals in the way that the Party suggests.

In conclusion, a discussion of a Party's compliance with its HPA could take place under the FMCP. The FMCP is the only multilateral process under the Paris Agreement in which Parties discuss their individual action. Moreover, in contrast to the global stocktake, it is a loosely defined process. During this process, any Party will have the opportunity to ask questions in writing and orally to the Party whose report is under consideration.Footnote ¹⁶² A Party willing to question another Party's ambition could take advantage of this opportunity, building in particular on the description of NDC and of the national circumstances in the Party's report as reviewed by the technical expert review team.Footnote ¹⁶³ The Party concerned might object to a question that it considers to fall beyond the scope of the procedure,Footnote ¹⁶⁴ but it might also be tempted to seek to address any such challenge to its mitigation action. The record of each Party's FMCP, which will be publicly available,Footnote ¹⁶⁵ could then be used to assess whether the Party remains consistent with its own interpretation of its HPA.

3. Adjudication

Adjudication provides another potential pathway for applying the HPA standard. In 2019, the Dutch Supreme Court in Urgenda v the Netherlands upheld a judicial determination that the national government had to achieve a 25 per cent reduction in national GHG emissions by 2020, compared with 1990, in order to comply with its due diligence obligations on climate change mitigation.Footnote ¹⁶⁶ Similarly, in neighbouring Belgium, a court of first instance found that a national policy on climate change mitigation was unlawful because it lacked ambition;Footnote ¹⁶⁷ the case is currently under appeal. Yet, courts in other countries have dismissed multiple cases seeking to transplant Urgenda, at times merely on procedural grounds,Footnote ¹⁶⁸ but also at times on the ground that determining a State's requisite level of ambition would call for policy judgments exceeding the scope of judicial functions.Footnote ¹⁶⁹

More recently, challenges to States’ mitigation ambition have been brought to international courts and treaty bodies. Here again, some of the first cases were dismissed on procedural grounds such as the failure to exhaust national remediesFootnote ¹⁷⁰ or the standing of the applicant.Footnote ¹⁷¹ Further, the UN Human Rights Committee summarily dismissed a claim that Australia was failing to implement enough ambition on climate change mitigation, merely by noting that the government had an emission-reduction target.Footnote ¹⁷² In a separate opinion, Gentian Zyberi argued that the Committee should ensure that States ‘set their national climate mitigation targets at the level of their highest possible ambition’,Footnote ¹⁷³ although he did not explain how the Committee could do this.

As of February 2024, at least nine contentious cases remain pending before the European Court of Human Rights,Footnote ¹⁷⁴ and three other international courts—the ICJ, ITLOS and the Inter-American Court of Human Rights—have received requests for advisory opinions.Footnote ¹⁷⁵ It is unclear how decisions in these cases could help foster ambitious action on climate change mitigation. Some might expect that these courts could devise and apply a formula to determine what constitutes a State's HPA. Doing so, however, would expose international judges to criticism of judicial overreach. Further, powerful States are unlikely to comply with such judicial pronouncements, a fortiori if they are taken in the form of non-binding advisory opinions.Footnote ¹⁷⁶

To avoid extensive reliance on policy considerations, many courts have preferred to turn to a State's own assessment of its HPA. For instance, in Grande-Synthe v France, France's State Council noted that a statutory emission-reduction target aimed to ensure the effective implementation of the State's due diligence obligation on climate change mitigation under the UNFCCC and the Paris Agreement.Footnote ¹⁷⁷ Having found that the State had not adopted adequate measures to achieve this statutory target, the Court ordered it to devise additional measures.Footnote ¹⁷⁸

Like Grande-Synthe v France, many successful climate litigations focused on the consistency between the State's definition of its HPA and the measures it implements, thus avoiding reliance on the court's own assessment of the State's HPA.Footnote ¹⁷⁹ For instance, the Federal Constitutional Court of Germany in Neubauer found that the State's medium-term policy on climate change mitigation was inconsistent with the State's long-term aims.Footnote ¹⁸⁰ Tellingly, when courts outside of the Netherlands have referred to the decision in Urgenda, they have often misconstrued it as a decision relying on a previous national policy on climate change mitigationFootnote ¹⁸¹—that is, a decision that would have required the national government to remain consistent with its own mitigation policy. Such cases can complement one another to create a relatively comprehensive and effective patchwork of (mainly procedural) obligations. For instance, courts can ensure that each national government establishes an appropriate statutory or regulatory frameworkFootnote ¹⁸² and implements it;Footnote ¹⁸³ adopts clear and coherent long-term policiesFootnote ¹⁸⁴ that do not unduly defer to future efforts;Footnote ¹⁸⁵ updates these documents if need be;Footnote ¹⁸⁶ and mainstreams consistent considerations in planning and project approval processes.Footnote ¹⁸⁷

4. Domestic political processes

Finally, the HPA standard can be implemented through political processes within each State. Relevant processes, which differ between countries, may include public consultations (eg in relation to the preparation of new and updated NDCs), referendums, elections, parliamentary debates, and protests. Some States have set up dedicated forums or institutions to conduct and contain political deliberations, ranging from citizens’ assembliesFootnote ¹⁸⁸ to independent expert bodies.Footnote ¹⁸⁹ NGOs might also play a role in debates about what a State's HPA is or how to identify it.Footnote ¹⁹⁰

It is noteworthy that, while domestic political processes can push for enhanced action on climate change mitigation, they can also oppose the adoption or implementation of relevant measures. For instance, a referendum initiated by Swiss citizens led to the rejection of a governmental proposal to enhance the national ambition on climate change mitigation, due in large part to concerns about the economic cost of doing so.Footnote ¹⁹¹ Likewise, the French ‘gilets jaunes’ opposed the adoption of certain measures aimed at reducing national GHG emissions in the transportation sector.Footnote ¹⁹² Nonetheless, the inclusion of the HPA standard in the Paris Agreement provides fodder to advocates for ambitious climate action.

1. Extraneous assessment

Since the Urgenda decision, many scholars and advocates have sought to devise tools and methods to assess States’ requisite level of mitigation action. The emphasis has generally been on a top-down assessment of States’ ‘fair shares’ in global mitigation pathways consistent with global temperature goals. However, extraneous assessments could also be conducted through a bottom-up approach relying on States’ HPA. Economic, social and political studies, for instance, could seek to determine the level of effort that a State could implement, in the light of national circumstances.

However, the robustness of the conclusions drawn from such studies is likely to be limited, primarily due to their reliance on many, far-reaching assumptions. Some of these assumptions would relate to the standard itself, for instance the conditions under which what is ‘possible’ can be assessed (eg the relevance of the condition of public and political acceptance, discussed above).Footnote ¹⁹³ Other assumptions would regard the application of these conditions based on national circumstances—for instance, the determination of what level of effort a society would find to be ‘acceptable’. Based on such far-reaching assumptions, any two extraneous assessments of a State's HPA could reach very different conclusions. For virtually any extraneous assessment finding that an NDC falls short of Party's HPA, another extraneous assessment, relying on different assumptions, could find that the NDC complies with this HPA standard.

These observations imply that there are limited prospects for an extraneous assessment of HPA standards in the context of judicial proceedings.Footnote ¹⁹⁴ HPA is a legal standard that could inform the judicial control of governmental action and State conduct, but it is a standard so vague that judges would probably grant extensive deference to policy judgments. Except perhaps in extreme situations—for instance, where a State fails to show any ambition whatsoever—judges are unlikely to be confident enough in the robustness of an extraneous assessment to rely on it as evidence to decide that a State's NDC falls short of the HPA standard.

Nonetheless, extraneous assessments have a role to play in political processes. Voters, after all, do not need to defer to the political judgments of their representatives. They are not bound to comply with legal standards either, but they may be guided by such standards in assessing how their government ought to respond to climate change. Likewise, government advisors, independent expert authorities, and even citizens’ assemblies can be tasked with identifying what they consider as the most convincing application of the HPA standard based on their reading of national circumstances and their opinion on what is reasonably ‘possible’. The HPA standard may only provide very loose guidance to political deliberation, but no more so than the temperature goals.Footnote ¹⁹⁵

2. Assessment of internal consistency

The difficulties facing an extraneous assessment of a State's requisite level of mitigation action have led some scholars to exclude the possibility that HPA could be a justiciable legal standard.Footnote ¹⁹⁶ Yet, an extraneous assessment is not necessary to demonstrate that a Party's NDC falls short of the Party's HPA. An alternative methodology would be to rely on the Party's own determination of what represents its HPA. A State appears to be in breach of the HPA standard when its NDC is less ambitious than what the State itself has identified as its HPA. Moreover, a Party also appears to be in breach of the HPA standard if its new NDC fails to reflect an increase in ambition commensurate with the Party's own determination that its HPA has increased, for instance due to favourable changes in national circumstances.

The potential of this methodology can be illustrated with a simple hypothetical scenario. Consider a State, called Carbonland. Before ratifying the Paris Agreement, Carbonland established the Climate Change Committee, an independent expert agency in charge of advising the government on its climate policy in line with Article 4(3) of the Paris Agreement. The Committee's initial study on the feasibility of various policy options recommended the imposition of a carbon tax whose rate would be set at Car$50 per tonne of carbon dioxide emission in 2020, and would increase by Car$5 each year, up to Car$200 in 2050. Consider that the government has endorsed and implemented this recommendation, and in its first NDC, Carbonland announced the imposition of a carbon tax at a rate reaching Car$75 by 2025.

Consider, however, that the government changes its policy: it decides that, from 2025 onward, the rate of the carbon tax will only increase by Car$1 per year. This new policy is reflected in Carbonland's second NDC, according to which the national carbon tax would reach Car$80 by 2030. The Committee does not support this policy reform—on the contrary, its successive reports purport to demonstrate that Carbonland could take advantage of favourable national circumstances to enhance its mitigation action, including with a more stringent carbon tax. It is submitted that, in such a scenario, there are strong reasons for a court to consider that the Carbonland's second NDC does not reflect its HPA and, therefore, that Carbonland probably does not comply with its due diligence obligations on climate change mitigation.

This methodology has obvious limitations. First, it can only be deployed when there is an actual inconsistency between the Party's NDC and its own HPA assessment. As such, this approach would not flag scenarios where a Party has consistently underestimated its HPA. Yet the methodology will progressively become more relevant as States have more opportunities to interpret their HPA. These interpretations will be developed on the international plane, through government-led processes under the Paris Agreement, including the communication of NDCs and long-term mitigation strategies.Footnote ¹⁹⁷ However, national interpretations of a State's HPA will also be developed by national institutions such as expert authorities or citizens’ assemblies.

Second, relying on internal consistency to apply the HPA standard requires evidence that the Party is not acting consistently with its own interpretation of its HPA. Yet, as noted above, there is no simple metric to measure ambition, especially when it is approached in terms of a level of effort rather than as an expected mitigation outcome.Footnote ¹⁹⁸ The simplistic hypothetical scenario above assumes a unique and simple mitigation policy (a carbon tax). In reality, States generally rely on a far more complex toolkit of measures, some of which are not exclusively aimed at the mitigation of climate change. Moreover, this hypothetical case relies on the simplistic premise that a tax rate indicates a level of ambition of a carbon tax, whereas a tax rate says little about the economic, social and political costs of mitigation action. Rather than the straightforward quantitative analysis suggested in relation to Carbonland, the internal consistency methodology would frequently rely on a more qualitative comparison of the level of ambition reflected in the State's assessment of its HPA with that reflected in its NDC.