IN THE discussions surrounding last month's United Nations World Summit on Sustainable Development, a particular measure of environmental stress was often cited—mankind's “ecological footprint”. The idea is to translate human demands on the environment into a common unit, so they can be added up. The footprint is the physical area required to meet man's needs and wants. This can be compared with the productive area available, to give a sense of how sustainable those needs and wants may be.

In its Living Planet Report 2002*, WWF, a conservation group, says that mankind's ecological footprint is already 1.2 Earths. “Humans are currently running a huge deficit...using over 20% more natural resources each year than can be regenerated, and this figure is growing each year.” The group predicts that by 2050 “humans will consume between 180% and 220% of Earth's biological capacity.”

Running this enormous “overdraft” with the planet is going to have bad consequences, the group says. “Unless governments take urgent action, by 2030, human welfare, as measured by average life expectancy, educational level and world economic product will go into decline.” The Observer, a British newspaper, summed it all up neatly in a headline: “Wanted: New Earth by 2050”.

Are these extraordinarily pessimistic forecasts at all plausible? No, says an analysis by Denmark's Environmental Assessment Institute, headed by Bjorn Lomborg, author of “The Skeptical Environmentalist”. Two main things are wrong with WWF's approach. First, the footprint is a difficult measure to make sense of. Second, even if assorted environmental pressures could be aggregated in the way footprinters advocate, WWF's predictions would still look mysteriously gloomy.

In principle, there is nothing wrong with trying to come up with a common unit of environmental pressure, provided the simplification is useful rather than misleading. The footprint idea combines the area required for cropland, grazing land, forest products, fishing grounds, sustainable use of energy, and built-up land (making adjustments for the productivity of different sorts of space). In 1999, according to WWF, the total came to 13.7 billion hectares. But the group says that only about a quarter of the planet's surface, 11.4 billion hectares, is available to supply these services—hence the overdraft.

Why is this misleading? To begin with, the institute points out, the approach builds in questionable assumptions. Crucial is an implicit, and very strict, idea of sustainability, which in effect denies that natural resources can often be replaced or augmented by man-made ones.

This aside, the main drawback of the analysis is the way it treats energy. WWF defines the footprint for fossil fuels as the area of forest required to absorb emissions of CO₂ (excluding those absorbed by the oceans). Growth in the energy footprint then drives almost everything else. The energy footprint increased from 2.5 billion hectares in 1961 to 6.7 billion in 1999, the fastest-growing component of the overall footprint—and, by the end of the period, very much the biggest.

Exclude energy from the total footprint, and it is harder to get alarmed. Growth in the ecological footprint excluding energy was much smaller than growth in global population between 1960 and 2000: that is, the ecological footprint per person, excluding energy, has been shrinking rapidly. Hardly a sign of reckless unsustainability.

So a lot turns on whether it is right to include the energy footprint in the aggregate measure—and on whether it is right to estimate this as the forest area required to absorb CO₂. On the first question, it is not obvious why sustainable development should require no change whatever in CO₂ concentrations, as the energy footprint implicitly demands. And even if sustainability did require this, there would be other ways of doing it—such as by increasing the use of renewable sources. Some renewable sources (such as windmills in deserts) make no demands on biologically productive space, which incidentally implies that the limit of 11.4 billion hectares for the area available to provide environmental services is wrong.

Framing the right policy on energy and CO₂ is complicated, to be sure. The point is, the footprint approach muddles the issue rather than simplifying it, apparently for the sake of yielding an alarming overall ecological deficit.

However, supposing everything WWF says about footprints is correct, how does the group come up with the catastrophe they have pencilled in for 2030? Between then and 2060 their projections appear to show average life expectancy across the world falling to less than 25 years. (The United Nations population division predicts that life expectancy will rise to nearly 80 years by 2060.) Living Planet Report 2002 does not say how this cataclysm comes about. The underlying analysis has not yet been published, and Mr Lomborg's institute is at a loss. The answer cannot be global warming. By 2030, according to the Intergovernmental Panel on Climate Change, global warming will have added at worst one degree to global temperatures. How could this cause the global catastrophe WWF says it fears?

WWF is using an updated version of the model which the Club of Rome used in the 1970s to predict, among other things, that the world would run out of oil by 1992. Maybe that explains it.

Living Planet Report 2002 is available on WWF's website, www.panda.org. The Environmental Assessment Institute's paper can be found at www.imv.dk

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