The Modular Earth Submodel System (MESSy): lessons learned from 20+ years of continuous development

Earth System Models (ESMs) aim at replicating the essence of the Earth Climate System in numerical simulations on high performance computing (HPC) systems. The underlying software is often rather complex, comprising several source code entities (modules and libraries, sometimes combining different programming languages), and has in many cases grown over decades. ESMs are usually structured as “multi-compartment” models, i.e. disassembled into a set of different components, each of which describes a different compartment in the Earth System, such as the atmosphere, the land surface, the ocean, the cryosphere, the biosphere, etc. Each compartment model, in turn, comprises a series of algorithms (numerical solvers, parametrizations), each of which represents a specific physical, chemical or socio-economic process. The behaviour of the “system as a whole” (i.e., the development of its state over time, its response to perturbations) is characterized by non-linear interactions and feedbacks between the different compartments and processes.

The implementation of such numerical models representing these inter-compartment and inter-process connections (i.e. the coupling) poses a challenging task for the software development, in particular given the need for (scalable) continuous further development and integration of new components, aiming at keeping pace with our knowledge about the real Earth System. Common requirements to such software are maintainability, sustainability (e.g. for new HPC architectures), resource efficiency (performance at run-time), but also development scalability.

More than twenty years ago (in 2005) we proposed the Modular Earth Submodel System (MESSy) as a potential new approach to Earth System modelling. Here, we present how we started as an “atmospheric chemistry add-on” to a specific General Circulation Model, but already with a wider range of applications in mind. We further show, how we went through our 2^nd development cycle, finally arriving at our current state, the MESSy Integrated Framework that is soon to be released Open Source. Although our 4 major software design principles (will be presented!) did not change significantly from the early stage, we had to undergo several implementation revisions to reach its current state. Despite the continuous development, MESSy was always “state-of-the art” and “in operation”, i.e. used for scientific research. Thus, in retrospect, we present some of the milestones achieved by “pragmatic” software engineering in practice.