Thomas' perceptive analysis of the decision is below. The decision (in German) can be found here, and the English press release is available here.
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The present decision concerns the classic problem of fundamental rights protection against acts of EU law that gave rise to the Solange I+II decisions. There had been rumors about an impeding “Solange III” decision for some time, but a closer look reveals that the German Constitutional Court uses a different tool out of its how-to-check-EU-law-toolbox: Identitätskontrolle. The Court held that human dignity -- as the paramount value under German constitutional law, and thus part of its constitutional identity -- needs to be safeguarded by precluding the enforcement of a European arrest warrant against an American citizen who was sentenced to 30 years in prison in absentia in Italy.
Clearly, this resembles the Melloni case. One should recall that AG Bot had indicated in his Opinion at the time that there might be room for protecting constitutional identity under Article 4(2) TEU, but the Spanish government had reassured the ECJ that there was no such problem (cf. para 78 of the present decision, referencing the reaction by the Spanish Constitutional Tribunal). The Bundesverfassungsgericht makes it clear that it will not back off when human dignity is at risk. It also uses the case for a lengthy exploration of its long held view that the primacy of EU law has (national) constitutional limits.
The case is a departure from Solange II (as refined by Bananenmarkt) in terms of admissibility criteria. To challenge an act of EU law in Karlsruhe on the ground that it infringes the fundamental rights enshrined in the German constitution, one would need to show that the level of fundamental rights protection on the European level is generally deficient. The Court now departs from this jurisprudence on the ground that human dignity (as part of Germany’s own constitutional identity) was invoked by the applicant (cf. para 34).
Finally, it's interesting to note that the Court avoids a preliminary reference procedure by invoking the acte clair exemption. This is obviously a misuse of the exception, because if anything is clair, it’s that Melloni points in a different direction. However, a preliminary reference procedure was indeed not necessary (from the standpoint of German constitutional law) because the outcome of the case would never depend on it (if one buys into the theory that constitutional identity is an absolute limit). Against this backdrop, it might even be an amicable gesture by Karlsruhe to claim that Luxembourg would clearly share its view on the outcome of the case without giving it the opportunity to prove the opposite. (In theory, the Oberlandesgericht Düsseldorf to which the case is sent back could still send the case to Luxembourg, but this seems unlikely given the directions from the Bundesverfassungsgericht).
In two weeks, the German Constitutional Court will hold yet another hearing in the OMT case to figure out how to deal with the ECJ’s response to its first ever preliminary reference. In the present case, it declined to invite that kind of dialogue—and perhaps open conflict—with Luxembourg. But whether that conflict has been truly avoided, or merely deferred, remains to be seen.