Ecological Limits

The evidence that humanity is reaching the limits of what the Earth can sustain

One of the articles in Toward A Sustainable World Order (IC#36)
Originally published in Fall 1993 on page 12
Copyright (c)1993, 1996 by Context Institute

What’s wrong with continuing business as usual? There’s increasing evidence that the human species is close to filling the ecological space available to it and other land-based species. It’s hard to dispute that there is a limit; the question is, how close are we to reaching that limit and how much can we gain by arguing the fine points rather than getting on with making a shift to sustainability?

Robert Goodland, an advisor to the Environment Department of the World Bank and a writer on tropical ecology, summarized some of the most compelling evidence of ecological limits as part of his contribution, excerpted here, to Population, Technology, and Lifestyle: The Transition to Sustainability, published by Island Press in 1992.

  • Human Biomass Appropriation: The best evidence that there are imminent limits is the calculation by Peter Vitousek et al.1 that the human economy today uses – directly or indirectly – about 40% of the net primary product of terrestrial photosynthesis. (This figure drops to 25% if the oceans and other aquatic ecosystems are included.) After only a single doubling of the world’s population (say, in 35 years) we will use 80%, and 100% shortly thereafter.

  • Global Warming: Scientists now practically universally agree that global warming will occur, although differences remain on the rates. Greenhouse warming is a compelling argument that ecological limits have been exceeded because it is globally pervasive, rather than disrupting the atmosphere only in the regions where the CO2 was produced. The nearly 7 billion tons of carbon released into the atmosphere each year by human activity (from fossil fuel consumption and deforestation) accumulate in the atmosphere, which suggests that the ecosystem’s sinks capable of absorbing carbon have been exceeded, and carbon accumulation appears for all practical purposes irreversible on any relevant time frame.

  • Ozone Shield Rupture: Although 85% of CFCs are released in the industrial North, the main ozone hole has appeared over Antarctica, showing the damage to be widespread and truly global. The global ozone layer is thinning far faster than models had predicted. A second hole has been discovered over the Arctic, and ozone-shield thinning recently was detected over both north and south temperate latitudes, including northern Europe and North America.

    Even if CFC emissions cease today, the world still will be gripped in an unavoidable commitment to 10 years of increased damage. This would then gradually return to pre-damage levels over the next 100 to 150 years.

  • Land Degradation: Land degradation – decreased productivity such as caused by accelerated soil erosion, salination, and desertification – is not new; land degraded thousands of years ago (for example, in the Tigris-Euphrates Valley) remains unproductive today. But the scale has mushroomed.

    Pimentel et al.2 found that soil erosion is serious in most of the world’s agricultural areas and that this problem is worsening as more marginal land is brought into production. Soil loss rates, generally ranging from 10 to 100 tons per hectare per year, exceed soil formation rates by at least tenfold. Today’s agricultural practices are leading to erosion, salination, or waterlogging of possibly 6 million hectares per year.

    As a result of widespread deficiencies in fuel wood, crop residues and dung are being diverted from agriculture to fuel. This diversion coupled with fuel wood overharvesting intensify land degradation, hunger, and poverty. As 35 percent of the Earth’s land already is degraded, and because this figure is increasing and is largely irreversible in any time scale of concern to society, such degradation is a sign that we have exceeded the regenerative capacity of the Earth’s soil source.

  • Decrease in Biodiversity: The scale of the human economy has grown so large that there is no longer room for all species in the ark. The rates of takeover of wildlife habitat and of species extinctions are the fastest they have been in recorded history and are accelerating. The world’s richest species habitat, tropical forest, has already been 55 percent destroyed; the current rate exceeds 168,000 square kilometers per year.

    Conservative estimates put the rate of extinction at more than 5,000 species each year. This is about 10,000 times as fast as prehuman extinction rates. Less conservative estimates put the rate at 150,000 species per year.3


1 Peter Vitousek, et al. "Human Appropriation of the Products of Photosynthesis." BioScience 34(6) 1986.

2 D. Pimentel, et al. "World Agriculture and Soil Erosion." BioScience 37(4) 1987.

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