The history of climate science is closely tied to the history of computing. From the first computational weather forecasts developed by von Neumann and Charney to run on ENIAC, to the earth system models used to produce projections of future climate change for the most recent IPCC reports, climate scientists have always pushed the limits of computational modelling. Along the way, climate scientists have developed a sophisticated set of software development practices tailored to the needs of a science in which virtual experiments are essential for understanding the relationships between human activity and the global climate system. In this talk, I will first explain what climate models do, via a quick tour of the history of climate modelling. I will then show how a core set of software development practices are used to support a culture of scientific experimentation which provides robust answers to societally important questions. I will end the talk with an overview of the current generation of climate model experiments. These address critically important questions such as whether there are still viable pathways to deliver the UN’s commitment to constrain global warming to no more than +2°C, and whether geo-engineering can buy us more time to address the underlying causes of climate change.