Tracking the Ecological Overshoot of the Human Economy

“The human economy depends on the planet’s natural capital, which provides all ecological services and natural resources. Drawing on natural capital beyond its regenerative capacity results in depletion of the capital stock. Through comprehensive resource accounting that compares human demand to the biological capacity of the globe, it should be possible to detect this depletion to help prepare a path toward sustainability. Sustainability requires living within the regenerative capacity of the biosphere. Our accounts indicate that human demand may well have exceeded the biosphere’s regenerative capacity since the 1980s. According to this preliminary and exploratory assessment, humanity’s load corresponded to 70% of the capacity of the global biosphere in 1961, and grew to 120% in 1999.

Thus, the ecological impact of humanity is measured as the area of biologically productive land and water required to produce the resources consumed and to assimilate the wastes generated by humanity, under the predominant management and production practices in any given year.

Our accounts include six human activities that require biologically productive space. They are (i) growing crops for food, animal feed, fiber, oil, and rubber; (ii) grazing animals for meat, hides, wool, and milk; (iii) harvesting timber for wood, fiber, and fuel; (iv) marine and freshwater fishing; (v) accommodating infrastructure for housing, transportation, industrial production, and hydro-electric power; and (vi) burning fossil fuel. In each category and for each year of the 40-year time series, we calculate both human demand and existing capacity. Our calculations rely on publicly available government data sources, and use conservative estimates where uncertainties exist.

Agriculture

(i) Growing crops requires the most productive land of all. The Food and Agriculture Organization (FAO) estimates that today about 1.5 billion hectares of cropland exist worldwide - 1.3 billion hectares of cultivated crops and 0.2 billion hectares of unharvested land that supports temporary pastures and fallow land, failed plantings, and shoulders, shelterbelts, and other uncultivated patches.

Livestock

(ii) Grazing animals requires pasture. The FAO defines permanent pasture, which currently amounts to 3.5 billion hectares, as ‘‘land used permanently (five years or more) for herbaceous forage crops, either cultivated or growing wild (wild prairie or grazing land)’’. We calculate the demand for pasture for each year by estimating the metabolic requirement of populations of five major classes of livestock: cattle, sheep, goats, equines, and camels. We then subtract dietary needs met from cultivated feeds and crop residues from the total dietary requirement to obtain the amount supplied from grazing.

Deforestation

(iii) Harvesting timber requires natural forests or plantations. According to the FAO’s Forest Resource Assessment (FRA) 2000, there are 3.8 billion hectares of such forest worldwide, which experienced an annual deforestation rate of 0.2% between 1990 and 2000. Before 1990, we estimate past forest area from the FRA baseline with annual deforestation rates. We estimate productivities by using tropical growth rates published by the Intergovernmental Panel on Climate Change (IPCC) and temperate and boreal growth rates from the United Nations Economic Commission for Europe’s (UNECE) Temperate and Boreal Forest Resource Assessment 2000.

Fisheries

(iv) Fishing requires productive fishing grounds. Of the total ocean area, the 6% concentrated along the world’s continental shelves provides over 95% of the marine catch. Assuming that these numbers reflect productivity distribution, this translates into 2.0 billion biologically productive hectares of the earth’s 36.3 billion hectares of ocean area. Inland waters make up an additional 0.3 billion hectares. We use FAO fish catch figures, including by-catch, and compare them to FAO’s ‘‘maximum sustainable yield’’ figure of 93 million tons per year. The 93 million tons are then expressed as their primary production requirement (PPR) per hectare, according to the 1996 mean trophic level and bycatch composition of 35 categories of fish, mollusks, crustaceans, and other aquatic animals. Annual landings are calculated by deducting aquaculture from production in these 35 categories, yielding the wild harvest.

Infrastructure

(v) Accommodating infrastructure for housing, transportation, industry, and hydroelectric power results in built-up land. The space occupied by this infrastructure is the least well documented, because low-resolution satellite images are not able to capture dispersed infrastructure and roads. We use an estimate of 0.3 billion hectares, a minimum estimate of the extent of infrastructure worldwide today, and assume that built-up land replaces arable land, as has been documented for the United States. We estimate built-up area by consulting data from Tellus PoleStar and the European Union.

Climate Change

(vi) Burning fossil fuel adds CO2 to the atmosphere. We calculate the area requirement by estimating the biologically productive area needed to sequester enough carbon emissions to avoid an increase in atmospheric CO2. Because the world’s oceans absorb about 35% of the CO2 emissions from fossil fuel combustion, we account only for the remaining 65%, based on each year’s capacity of world-average forests to sequester carbon. This capacity is estimated by taking a weighted average across 26 forest biomes as reported by the IPCC and the FAO. The sequestration capacity will not remain constant in the future. For instance, changed atmospheric CO2 concentrations and global temperature may increase the eventual saturation biomass level and the rate at which that is approached. Some sequestration and oceanic absorption may even be reversed. Also, CO2 sequestration rates may decrease as more and more forest ecosystems reach maturity. Eventually afforestation will saturate so that the net rate of CO2 uptake goes to zero.”