From Licensing to Open Access: Designing a Sustainable Transition in Operational Weather Data

TL;DR

This paper details ECMWF's transition from restricted licensing to open access for weather data, highlighting the design, implementation, and early outcomes of a tiered service model aligned with EU policies.

Contribution

It presents a practical case study of a large-scale, sustainable transition to open data in operational weather forecasting, including policy, operational, and economic considerations.

Findings

Over 93% of paying organizations retained agreements post-transition.

Open download volumes increased substantially within six months.

The tiered model balances open access with operational sustainability.

Abstract

This translational article documents the European Centre for Medium-Range Weather Forecasts (ECMWF) transition from a restricted data licensing model to open access under CC BY 4.0, completed in October 2025. The policy context included EU open data requirements and alignment with international data exchange frameworks. The transition was implemented through a tiered service model that kept core forecast data open while offering operationally supported delivery as a cost-recovered service. Between 2020 and 2025, ECMWF executed an iterative planning cycle: setting an annual target for revenue reduction, specifying additions to the open tier under that target, provisioning infrastructure, and assessing outcomes to update assumptions. Drawing on internal administrative records (2014 - 2025), we describe design choices, operational constraints, and early outcomes. In the six months following the end of the transition, more than 93% of previously paying organisations retained a Service Agreement, while open endpoint download volumes increased substantially. We discuss trade-offs in defining the open tier (resolution, parameters, schedule), the reduction of compliance overheads formerly associated with redistribution restrictions, and the scalability implications of global distribution. We note an emerging sustainability question as AI-based forecast products become freely available. The early evidence is consistent with the view that a tiered service model can be designed to reconcile open-access obligations with operational sustainability, subject to monitoring over longer contract renewal cycles (typically annual).