David MacKay, Professor of Physics, University of Cambridge: How can we power a modern lifestyle without fossil fuels? Individual actions saving 10% here and 40% there will not get us off fossil fuels. To eliminate fossil fuel use, we will surely also need to increase the amount of energy we get from non-fossil-fuel sources. Even if we imagine strong efficiency measures and smart technology switches, halving our energy consumption from 125 kWh per day per person to 60 kWh per day, we should not kid ourselves about the scale of the energy challenge which would remain. If Britain and the United States were to "get off" fossil fuels, what would the effect be on Earth's climate? Most of the rest of the world can not afford to "get off" fossil fuels or do not have the right governments, economies, education systems, industrial capacity to do so.