Tackling climate change will require reducing anthropogenic carbon dioxide emissions not just in electricity, but also in heating, transport and industry. This is challenging, because the most abundant fossil-free energy sources come from wind and solar energy, which vary

over time and space. Their variability is currently pushing existing network and storage facilities to their limits. In this talk we look at the methodologies required to determine a robust system design for weather-driven energy systems that can reduce emissions in all energy sectors, while still remaining affordable. We take a tour through the techniques of data reduction, complex network theory and economic optimisation that help system planners to manage the transition to the sustainable energy system of tomorrow. We present some initial results for the European energy system that demonstrate the importance of understanding the different temporal and spatial scales of renewable variability.