Current global oil production [is] equivalent to the daily work of billions of invisible slaves

The last two centuries of extraordinary economic growth in high-income countries are largely due to the availability of cheap fossil fuels. It makes sense when you break it down: the energy contained in a single gallon of oil is equivalent to 47 days of hard human labour, making current global oil production equivalent to the daily work of billions of invisible slaves.

K. Raworth, Doughnut Economics (2017), 263