Emissions of greenhouse gases (GHGs) arising from the combustion of fossil fuels have been the main cause of anthropogenic global warming. Energy use has reached a scale at which planetary boundaries are being breached for a range of essential Earth-system processes, including in terms of global warming and biodiversity loss, which is likely to lead to catastrophic environmental change (Rockstrom and others, 2009). In 2005, fossil fuels accounted for 85 per cent of the global primary energy mix, while low-carbon nuclear power accounted for 6 per cent, hydroelectricity for 3 per cent and biomass for 4 per cent.