When the first overshoot calculation was announced in 2006, it found that Earth used a year’s worth of resources by Oct. 9. The Global Footprint Network determines the date by drawing data from the United Nations, the International Energy Agency and the Intergovernmental Panel on Climate Change, among others. These estimates of productive land and sea area, grazing land, cropland and fishing grounds are expressed in so-called global hectares. This measurement (roughly 2.5 acres) is meant to be a standard unit, projecting average productivity, that can be tallied to represent the Earth’s total “biocapacity.”
The researchers then examine the demand side: mankind’s need for crops, livestock and fish, timber and space for urban development, along with a calculation of the forests’ capacity to absorb carbon dioxide emissions from the burning of fossil fuels. The difference between this “ecological footprint” and the Earth’s biocapacity represents the overshoot.
The Aug. 1 date declared this year means that, for the final five months of the year, mankind is overdrawing natural resources. Framed another way, it would take 1.7 Earths to supply the resources needed to feed, clothe and sustain Earth’s 7.6 billion people for a year.