Humans have typically exercised little or no restraint in their use of the earth¿s supplies and services. A study published today in the Proceedings of the National Academy of Science, however, suggests that this era of unbounded exploitation may soon be forced to a screeching halt. According to the report, people have been taking more from the planet than it can consistently restock, and have cumulatively produced so much waste that complete reassimilation is impossible.

To assess the sustainability of past and present human activity, Mathis Wackernagel and his colleagues did two things: first, they quantified the annual amount of land and water resources needed to meet the human demand for food, shelter and fuel, among other things. Then they estimated the actual productivity of the earth¿s land and oceans. When the researchers compared supply and demand, they found that in 1999 (the most recent year examined) humanity consumed 120 percent of the earth¿s sustainable—or consistently replenishable—resource capacity. In other words, the global population exhausted a supply of natural resources equivalent to that produced by 1.2 earths each year, eroding nature¿s ability to regenerate. It seems that this trend will not soon level out. In 1961 the world used 70 percent of its sustainable productivity; since the 1980s it has consistently exceeded it.

It is impossible to completely quantify something so complex as global sustainability, but the new research could provide a framework for examining the consequences of future policy recommendations. "Assessments like the one presented here allow humanity, using existing data, to monitor its performance regarding a necessary ecological condition for sustainability: the need to keep human demand within the amount that nature can supply," the authors write. The hope is that by putting the planet¿s bioproductive capacity into an economic context, better decisions regarding human use of natural resources can be made.