" In 2009, physicist Robert Ayres and ecological economist Benjamin Warr decided to construct a new model of economic growth. To the classic duo of labour and capital they added a third factor of production: energy (or, more precisely, exergy), the proportion of total energy that can be harnessed for useful work, instead of being lost as waste heat. And when they applied this three-factor model to data on twentieth-century growth in the United States, UK, Japan and Austria, they found that it could explain the vast majority of economic growth in each of the four countries: Solow’s mystery residual, long assumed to reflect technological progress, turned out to reflect the increasing efficiency with which energy is converted into useful work.36 The implication? The last two centuries of extraordinary economic growth in high-income countries are largely due to the availability of cheap fossil fuels. "

― Kate Raworth , Doughnut Economics: Seven Ways to Think Like a 21st-Century Economist