The GFN explains: “ To determine Earth Overshoot Day, one needs to calculate the ratio between the Earth’s ability to generate resources, or its biocapacity, and humanity’s demands placed upon nature, its Ecological Footprint.
“All demands on nature compete for biocapacity: sequestration capacity for CO2 from fossil fuel combustion, demand for food and fiber, energy production (from hydropower to biomass), space for roads and shelters, etc.
“Both biocapacity and Ecological Footprint can be tracked and compared against each other, based on two simple principles:
1. All the competing demands on productive surfaces, i.e., the surfaces that contain the planet’s biocapacity, can be added up
2. By scaling these areas proportional to their biological productivity, they become commensurable.
“The measurement unit used is ‘global hectare’ which is a biologically productive hectare with world-average productivity.”