Sir

Your News report “Ecological footprint forecasts face sceptical challenge” (Nature 419, 656; 200210.1038/419656a), stating that Bjorn Lomborg's environmental-policy group disputes the usefulness of the 'ecological footprint' concept, comes as no surprise to those familiar with Lomborg's recent work. He argued in The Skeptical Environmentalist: Measuring the Real State of the World (Cambridge University Press, New York, 2001) that societies become “greener” and presumably more sustainable as they accumulate wealth and purchasing power. To support this 'wealth hypothesis' he correlates his preferred measure of environmental impact with gross domestic product (GDP) by country. However, his preferred measure is the Environmental Sustainability Index (ESI) generated by the World Economic Forum (WEF).

This index, plagued with methodological problems, offers no useful information about our long-term affairs and prospects. It considers 67 variables divided among 22 environmental indicators, most of which have little to do with a region's ability to provide ecosystem goods and services. For example, high petrol prices and the memberships of environmental organizations are tabulated, as are the number of scientific articles a country publishes and its record of regulatory innovation — but these probably do not relate to the general workings of the biosphere.

Additionally, none of the variables is weighted appropriately, so ecologically tenable variables such as soil degradation, water availability, ecological deficit and fertility rate are considered equal to petrol prices and measures of political corruption. To its credit, the WEF report recognizes these limitations, some of which may be addressed in future versions of the index.

The ecological-footprint concept offers a snapshot of our resource demand independent of GDP — unlike the ESI. Thus, by recognizing the reality of biophysical limits, a country's ecological footprint offers a conservative measure of the total productive land, water and resources required to support a given population and cope with its waste materials. Such measures are necessary before we can hope to deal with more difficult concepts such as regional and global sustainability. Lomborg's group misses the point by stating “there is no 'correct' ecological footprint”. No one has claimed that an ecological footprint calculation is “correct” and should be accepted uncritically.

Estimating anything will always be subject to error — a fact that a statistician such as Lomborg surely recognizes. But given the urgent need to understand fully our global predicament and true prospects for sustainability, should we abandon conceptual tools if they rely on estimates?

I am aware of no measure of ecological footprint that supports Lomborg's wealth hypothesis, which may explain why he dismisses the value of the ecological-footprint concept. It is not clear to me how Lomborg's view will contribute to sustaining a growing human population over the long term, given the challenges of becoming regionally sustainable.