A case for open communication of bugs in climate models

Climate models are not only numerical representations of scientific understanding but also human-written software, inherently subject to coding errors. While these errors may appear minor, they can have significant and unforeseen effects on the outcomes of complex, coupled models. Despite existing robust testing and documentation practices in many modeling centers, bugs’ broader implications are underexplored in the climate science literature.

We investigate a sea ice bug in the coupled atmosphere-ocean-sea ice model ICON, tracing its origin, effects, and implications. The bug stemmed from an incorrectly set logical flag, which caused the ocean to bypass friction from sea ice, leading to unrealistic surface velocities, especially in the presence of ocean eddies. We introduce a concise and visual approach to communicating bugs and conceptualize this case as part of a novel class of resolution-dependent bugs - long-standing bugs that emerge during the transition to high-resolution models, where kilometer-scale features are resolved.

By documenting this case, we highlight the broader relevance of addressing bugs and advocate for universal adoption of transparent bug documentation practices. This documentation complements the robust workflows already employed by many modeling centers and ensures lessons from individual cases benefit the wider climate modeling community.