The ECJ’s Opportunity to Address the EU’s Climate Mitigation Obligations

Different voices within the European Parliament, notably Green MEPs, are considering initiating a request for an opinion from the European Court of Justice (ECJ) on the compatibility of the EU-Mercosur Deal with EU law. This could be the ECJ’s best opportunity to speak on the EU’s climate mitigation obligations under European and international law.

After sketching how international and regional courts and tribunals have articulated mitigation obligations and explaining the implications for the EU, I argue that the ECJ should be asked to review the EU-Mercosur Deal. In particular, I argue that the agreement is incompatible with the EU’s mitigation obligations. Moreover, such a request would be an opportunity for the ECJ to assume its role as the domestic court of the EU legal order in relation to the defining global issue of climate mitigation.

An international body of climate decisions and advisory opinions

On 23 July 2025, the International Court of Justice (ICJ) issued a historic advisory opinion setting out States’ international obligations in relation to climate change. The ICJ could build on the earlier advisory opinions of the International Tribunal for the Law of the Sea (ITLOS) of 21 May 2025 and the Inter-American Court on Human Rights (IACtHR) of 3 July 2025. While the latter two spell out climate obligations under the Law of the Sea and a regional human rights regime, the ICJ is the authoritative voice on international obligations, not only under the 1994 UN Framework Convention on Climate Change (UNFCCC) and the 2015 Paris Agreement but also international human rights and customary international law. As a party to the two climate treaties and as an international actor, the EU is subject to these obligations not only under international but also EU law (Article 216 TFEU; C-286/90, Poulsen; C-162/96, Racke; C-366/10, ATAA; Opinion 1/17).

As to the EU’s regional human rights obligations, the European Court of Human Rights (ECtHR) specified on 9 April 2024 in KlimaSeniorinnen (KS) the climate-related obligations of Contracting Parties to the European Convention on Human Rights (ECHR). As I argued here, the EU is bound by these obligations under EU primary law (the Treaties and the Charter of Fundamental Rights), which is the formal legal yardstick of the ECJ’s review under the opinion procedure.

Together, the three advisory opinions and the ruling of the ECtHR shape our understanding of States’ climate obligations. The European Court of Justice (ECJ) is suspiciously absent among these judicial voices (see rejections of general emission reduction cases in Carvalho and Sabo).

The EU’s part in mitigating the climate crisis

Courts and tribunals have in different ways spelled out that a state is obliged “to do its part” (KS, 545). This also applies to the EU when it assumes the role of a climate actor by adopting nationally determined contributions (NDCs), targets, and policies for itself and the Member States. One’s “part in mitigating the climate crisis” consists of two elements: staying within one’s fair share of the global emissions budget associated with the long-term temperature limit (LTTL) of 1.5°C (KS, 558; ICJ, 224 and 189; Eckes, 2025a) and taking all feasible emission reduction measures (ICJ, 242-249, at 246).

Dividing the global capacity for polluting (global carbon budget) that keeps global warming at a specified likelihood below the agreed LTTL is the starting point for quantifying domestic carbon budgets. Fairness demands justifying the methods of division in light of international equity principles, such as common but differentiated responsibilities and respective capabilities (CBDR-RC) (Rajamani et al, 2021).

In addition, the ICJ spelled out “that each party has to do its utmost to ensure that the NDCs it puts forward represent its highest possible ambition in order to realize the objectives of the [Paris] Agreement.” (ICJ, 246) This places a high burden on States to justify why they do not do what is considered to be feasible by recognised and publicly endorsed scientific (advisory) bodies, such as the Intergovernmental Panel on Climate Change (IPCC) and, for the EU, the European Scientific Advisory Board (ESAB). The ESAB explained that 70% emission reduction in 2030 and 95% in 2040 are fairer (than the current target of 55% and the proposal of 90%) and feasible (here).

The ECtHR included GHG emissions generated abroad for products consumed on a state’s territory (“embedded emissions”) within the scope of human rights protection (KS, 283). In KlimaSeniorinnen, this was found to be approximately 70% of Swiss emissions (KS, 279). ECHR Contracting Parties, including the EU Member States, must hence also reduce import-related emissions. Via the EU Treaties, this applies to the EU, which must hence reduce export-related (generated on EU territory) and import-related emissions (of goods consumed on EU territory).

The EU’s “part in mitigating the climate crisis” consists of quantifying its own fair share carbon budget that is capable – as part of the collective effort – to keep global warming below 1.5°C and doing what the ESAB explained to be feasible.

Opinion procedure before the ECJ

The request for an opinion under Article 218(11) TFEU would allow the ECJ to review the compatibility of the EU-Mercosur Deal with EU law and spell out the EU’s part in mitigating the climate crisis. The opinion procedure with its unique features is not subject to the constraints of an individual case or the problematic interpretation of individual concern (Winter, 2023; Eckes, 2025b). The “opinion perspective” is well-suited to address climate mitigation obligations – as we have seen in ITLOS, the IACtHR, and the ICJ’s opinions.

The opinion procedure allows the ECJ to conduct a general and abstract ex ante assessment of an envisaged international agreement in order to avoid external (entrenched) commitments that run counter to EU law, including the EU’s international obligations that form part of the EU legal order (Eckes/Leino/Ghavanini, 2024, Ch 12). This “removes the pressure of subsequent costs from courts, which the ex post review of international commitments otherwise entails. It represents a workable mechanism for channeling foreign commitments back into a system of separated powers” (Möllers, 2013 at 164). The ECJ depends on the other EU institutions or Member States, as privileged applicants, to request an opinion. The Court reviews both procedural and substantive compatibility with EU law (Cremona, 2020).

From the perspective of the national jurisdictions of the Member States, the ECtHR, and international law, the ICJ is a regional court (Eckes, 2020). However, from the perspective of and within the EU legal order, the ECJ conceives of itself as a domestic court, namely the highest court of the EU legal order (ibid; Eckes, 2019). This is the logical result of conceptualizing the EU legal order as a “new legal order”, whose autonomy requires protection from international law (C-26/62, van Gend; Opinion 2/13). The effectiveness and authority of EU law depend on this conceptualisation (Eckes, 2019, Chapter 2) and give the ECJ the responsibility to act as a domestic court and review acts of the EU institutions – certainly, in an area where the EU exercises considerable external and internal competences, e.g., by submitting NDCs and dividing climate obligations among the Member States (Effort Sharing Regulation).

The EU-Mercosur Deal is incompatible with EU law

The core objective of the EU-Mercosur Trade Agreement, as is common for trade agreements, is to facilitate (increase the volume of) trade by lowering tariff rates applied to categories of products, harmonising liberalisation measures, excluding or limiting protective measures (anti-dumping), (export) subsidies, and quotas. However, trade can only and exclusively be seen as sustainable and in line with the EU’s climate mitigation obligations to the extent that it concerns “green products”, i.e., products (services, capital, or know-how) that directly contribute to the green transition or those that are produced in a less emission-intensive way.

The EU is responsible for trade-related emissions (see above). This is convincing. These emissions constitute a large share of the overall emissions of rich consumer markets, such as the EU. Consumer goods are produced to meet the demand from these markets. Trade in high-emission goods may be expected to lead to greater and cheaper availability and higher consumption, which increases emissions. This seems difficult to justify.

One striking example of an import is beef. In Mercosur countries, beef originates from animals raised for meat rather than dairy production (more common in the EU). The former is significantly more carbon-intensive than the latter. The EU-Mercosur Deal may be expected to make beef cheaper on the EU markets by significantly reducing import tariffs. Yet, the political consequences of this example may go further than the emissions from the (limited) quota of imported, more carbon-intensive beef. Fairness is a core concern of those opposing climate measures. It seems difficult to justify that, while the EU’s internal (climate) regulations raise production costs of (less carbon-intensive) beef from dairy herds, imported, more carbon-intensive beef from beef herds becomes cheaper. This is one more fairness argument against internal climate measures.

An export example is passenger cars with combustion engines. Tariff cuts would be particularly beneficial for EU car exports. Mercosur would fully open its markets (over 15 years) to passenger cars with combustion engines. Simultaneously, the sale of these polluting cars is prohibited in the EU from 2035. Again, since emissions anywhere lead to global warming everywhere, it is difficult to understand this in light of the EU’s part in mitigating the climate crisis.

Another aspect of the EU-Mercosur Deal that has been criticised for its potential to increase the EU’s emissions and undermine climate policies is the rebalancing mechanism. This mechanism offers a procedural setting for the Contracting Parties to challenge each other’s climate policies, including the implementation of the Green Deal, even where such policies do not undermine rights but only have the (vague) consequence of substantially impairing a “trade benefit” (Eckes/Krajewski, 2025).

Demanding justification

In light of these discrepancies, let’s hope that an opinion is requested from the ECJ within the window of opportunity, i.e., before the agreement is concluded, and that the ECJ seizes the opportunity to demand justification and spell out what the EU’s part is in mitigating the climate crisis. Policies that increase rather than reduce emissions and hence go against international and human rights obligations spelled out by regional and international courts require (more) justification. Growing human rights and economic costs of the climate crisis (Potsdam Institute for Climate Impact Research, 2025) make the demand for justification even more pressing. If the EU refrains from acting as the domestic court of the EU legal order, this creates a vacuum where the EU institutions can ignore their climate obligations. This should make litigants turn to the ECtHR and national courts to fill this vacuum.

The discussion often refers to “winners” and “losers”, for example in the dairy, car, meat or soja sectors. This misses the point. The climate crisis is the poster child of injustice, and the EU – for all the good reasons – is legally committed to mitigating climate-related human rights violations and injustices between and within states. How can you justify that your own scientific body recommends encouraging EU consumers to move to electric mobility and a more plant-based diet to reduce emissions (ESAB, key recommendation 9) while you export combustion engines, cheese and milk powder to Mercosur countries? It is the role of judges and lawyers to work out what these contradictions mean in light of the law, bring the discussion back to verifiable facts, and demand justification.

When political and economic (lobbying) powers or strategic geopolitical considerations pressure the EU to disregard climate mitigation, it is for the ECJ to act as the domestic judiciary within the EU’s interinstitutional decision-making process. In relation to the conclusion of trade agreements, we have, for good reasons, given the ECJ broad review powers in the opinion procedure to avoid entering into international commitments that are incompatible with the fundamental principles and values of EU law, including human rights and international treaty and customary law. In the EU’s system of separated powers, it is for the directly elected representatives within the European Parliament to enable the ECJ to exercise these review powers and demand justification.