Footprint: our impact on Earth is getting heavier


Bjorn Lomborg's comments as reported in your recent News story (Nature 419, 656; 200210.1038/419656a) betray ignorance of ecological-footprint analysis (EFA). This is a method I proposed (W. E. Rees Popul. Environ. 24, 15–46; 2002) specifically to assess the assertions made by some economists that, because of technological advances, the human economy is “dematerializing” or “decoupling” from the natural world. If true, this implies that modern society is becoming less dependent on nature, and can be used to justify further economic growth. If false, the additional stress likely to be imposed on the natural world will be disastrous.

Certainly, no single index can represent the total human impact on the ecosphere. However, EFA is comprehensive enough to show, unambiguously, that the human eco-footprint on Earth is steadily increasing. Comparative studies (such as the Worldwide Fund for Nature's Living Planet Report 2002, criticized by Lomborg) show that the most technologically advanced nations are the most energy- and material-intensive and have the largest per-capita ecological footprints. Hence, the consumer lifestyles of their average citizens (and of the wealthy residents of the developing world) are the least ecologically sustainable on Earth, and cannot be safely extended to humans everywhere.

Lomborg's ignorance of eco-footprint science is illustrated by his claim that it ignores the potential growth in the use of renewable resources, and that if the use of such resources increases, the 2050 footprint would not be as large as projected. The fact is that EFA is already largely based on use of renewable resources, with the exception of the fossil-fuel (carbon sink) component. A shift to exclusive use of renewable energy does not necessarily imply a reduction in the eco-footprint.

Biomass fuels, for example, are likely on thermodynamic grounds alone to demand a larger growing area than the amount of land needed to provide the energetically equivalent amount of fossil energy (via carbon assimilation) today. One proposed 'renewable' resource, fuel ethanol production, is not even a net source of energy. In the United States, 70% more energy (mostly fossil fuel) is consumed in producing a litre of ethanol than is contained in the product. Even without counting the energy cost of the fermentation and distillation process, or the carbon-sink footprint of the fossil fuel used to grow the maize feedstock, the average US automobile would require 4.4 hectares of cropland to provide its annual fuel requirements. (This whopping — but only partial — fuel eco-footprint is about seven times the cropland needed annually to feed one person in the United States.) To run all US cars on ethanol would require an area of cropland equivalent to nearly the total US land area. More generally, the United States uses 85% more fossil energy each year than the total energy captured by all its plant biomass over the same period. Clearly, regardless of the efficiency of the conversion technology, there is no possibility of renewable biomass substituting, joule for joule, for fossil fuel.

The much-touted solar alternatives may not fare much better. Although the solar flux represents a vast flow of potential energy, various analysts argue, from both thermodynamic principles and empirical data, that humanity faces enormous technological obstacles in converting this flow into the energetic equivalent of contemporary fossil-fuel use.

In the light of such data and the implicit uncertainty about prospects for sustainability, Lomborg's “sceptical challenge” to the ecological footprint concept falls flat. EFA may be static, but this does not invalidate estimates of future footprints based on reasonable assumptions about technological development. Dynamic modelling is just as vulnerable to implicit error in this regard.

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Rees, W. Footprint: our impact on Earth is getting heavier. Nature 420, 267–268 (2002).

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