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European environmental Research Infrastructures (RIs) collectively produce some of the most valuable in-situ observations for Earth System Science. Yet, despite widespread adoption of FAIR principles, the ability to actually combine, reuse, and operationalise these data across infrastructures remains limited. Differences in mandates, standards, and access mechanisms continue to translate into practical barriers for transnational and cross-domain research, particularly for scientific questions that span atmosphere, ocean, land, biodiversity, and solid Earth processes.This contribution argues that the bottleneck in Earth System Science is no longer data availability, but federation capability. Within the European Open Science Cloud (EOSC), the ENVRI Node positions itself as a response to this gap by acting as a thematic federation layer for in-situ environmental research infrastructures. Central to this approach is the ENVRI-hub, which provides a shared integration environment enabling coordinated discovery, access, and interoperability across multiple RIs without centralising control or diluting infrastructure mandates.We present concrete examples where integrating services from multiple providers through the ENVRI-hub enables new federation-level products, such as cross-domain catalogues and semantic discovery services, that cannot be delivered by single infrastructures alone. These examples highlight how interoperability, rather than new data production, becomes the key enabler for scientific progress.Using selected Earth System Science use cases, we deliberately expose where current infrastructure boundaries fail to meet research needs, including limitations in harmonising in-situ observations, aligning access policies, and supporting machine-actionable, AI-ready data. These gaps point to use cases that cannot be solved by incremental improvements within individual infrastructures, but require coordinated action across them.The ENVRI Node is presented as a practical, and intentionally opinionated, experiment in how international research infrastructures can move beyond coexistence towards federation, raising the question of whether future Earth System Science can afford not to.