This talk describes some results from a collaboration between Computer Science, Physics, and Climate Impact Research on theories and tools for performance optimisation of strongly coupled physical systems with a large parameter space. The first part of the talk discusses computing optimal policies; we have used these techniques for climate decisions and for fusion energy designs. The second part of the talk will focus on one particularly important concept: the Pareto-front, which mathematically captures the trade-offs between two (or more) conflicting objectives. This talk is aimed at a broad technical audience, both computing and sustainability.