The Global Hectare (gha) is a common unit that quantifies the biocapacity of the earth. One global hectare measures the average productivity of all biologically productive areas (measured in hectares) on earth in a given year. Examples of biologically productive areas include cropland, forests, and fishing grounds; they do not include deserts, glaciers, and the open ocean. "Global hectare per person" refers to the amount of biologically productive land and water available per person on the planet.
Ecological Debt Day, also known as “Earth Overshoot Day”, is the calendar date each year in which the total resources consumed by humanity will exceed the capacity for the Earth to generate those resources that year. It is calculated by dividing the world biocapacity, the number of natural resources generated by the earth that year, by the world Ecological Footprint, humanity’s consumption of the Earth’s natural resources for that year, and multiplied by 365, the number of days in one Gregorian calendar year; expressed as:
[ world biocapacity / world Ecological Footprint ] x 365 = Ecological Debt Day
Though humanity first went into overshoot in 1986, Ecological Debt Day was first observed on December 19, 1987. Before that date, humanity’s consumption of the Earth’s natural resources was outweighed by the Earth’s ability to regenerate its resources. To elicit discussion on the topic of natural resource consumption, the New Economics Foundation (NEF) marked Ecological Debt Day on that calendar year. Every year since then, NEF has calculated the calendar date of Ecological Debt Day for each subsequent year using the above formula. Ecological Debt Day has on average, each year fallen on an earlier date than the previous year. The authors of Ecological Debt Day argue that this fact constitutes a notorious trend in human society, in which humanity is falling deep into ecological debt.
Year Overshoot Date
1987 December 19
1990 December 7
1995 November 21
2000 November 1
2005 October 20
2007 October 26
2008 September 23
2009 September 25
2010 August 21
2011 September 27
Where do we stand now?
In 2011, Earth Overshoot Day, the approximate date our demands on nature for a given year exceeds the planet’s ability to replenish, fell on September 27. We have since been in ecological overshoot, with a projected consumption of 135 percent of the resources the Earth will create this year. We make up the deficit by depleting stocks of fish, trees and other resources, and by accumulating waste such as carbon dioxide in the atmosphere and oceans.
There were ~ 12 billion hectares of biologically productive land and water on this planet in 2008. Dividing by the number of people alive in that year (6.7 billion) gives 1.79 global hectares per person. This assumes that no land is set aside for other species that consume the same biological material as humans.
The planet has an upper bound limit, and we actually already surpassed it. In fact, we are reducing its capacity to support life, and almost every life- supporting system on the planet is in a state of decline. The math is simple: the world biocapacity is ~ 12 billion gha. Americans use 7 each. We are 7 billion. If we all lived like them we would need 4 planets. If we consumed 1 gha each, there could be 12 billion of us. Considering that the population is expected to stabilise at 9-10 billion, it is an achievable goal, but we have to re-think about the way we live. If we can manage to increase efficiency, and live with less usage of resources, we can fit even 20 or 30 billion, but the gha per capita has to decrease proportionally.