The HEF is normalized to be 1 in 1970, and the resulting index varies from 0.5 in the year 1900, through 1.76 in year 2000, to highly unsustainable values above 3 for short periods in scenarios showing overshoot and collapse. In the most successful scenarios it proves possible to keep the HEF below 2 for most of the twenty-first century. The sustainable level of HEF is probably around 1.1, a level that was passed around 1980.
