Presenting MESMER v1 - Integrating Multiple Climate Emulator Modules Into One Sustainable Research Software Package

Earth system models are able to simulate the physical processes that govern the Earth's climate system and are essential to understand and predict climate change. However, these models come at a significant computational cost since they need to simulate a multitude of variables at a high temporal and spatial resolution to adequately represent the climate system. Climate model emulators are statistical models that are trained to reproduce (emulate) selected variables of full-fledged physical climate models at a much lower computational cost and higher speed. Such emulators are especially interesting in the context of climate change mitigation policies, which often deal with a limited number of relevant variables (e.g. annual mean temperature, number of hot days, annual maximum precipitation, etc.), but require several scenarios of how these variables may evolve under different policy choices. The “Modular Earth System Model Emulator with spatially Resolved output”, in short MESMER, is a climate model emulator that can emulate large ensembles of several climate variables (see list of modules below) for any future climate change scenario conditional on global mean temperature.

Four MESMER modules have been developed over the last five years by several researchers from different institutions: (1) MESMER: module for annual mean temperature, (2) MESMER-M: module for monthly mean temperature, (3) MESMER-M-TP: module for monthly mean temperature and precipitation, and (4) MESMER-X: module for conditional distributions with focus on climate extremes. These modules were developed largely independent of each other and grew organically to meet the needs of the individual researchers and the analyses they performed without following consistent coding standards or software architecture.

Here we present how we unified the MESMER code base, integrating all modules into a single repository and rewriting them to adhere to sustainable software standards. We redesigned MESMER with respect to (1) maintainability, (2) extensibility, (3) flexibility, (4) adherence to a defined software architecture and, (5) accessibility. The result is an open source software tool that anyone can use and/or extend. Moreover, the software is easily available and understandable to users who are interested in emulating variables for their own scenarios without being proficient in climate modelling, for example policy makers. In addition, MESMER output from the revised modules is stable and reproducible. Here we present the unified MESMER version 1.0.0 and provide insights into the achievements, challenges and lessons learned during this process. This includes insights into the chosen architecture, our testing and code review framework, stability and performance enhancements and recommendations for the scientific programming community.