|From Global Footprint Network website|
It sounds like a straightforward calculation of income minus expenditure, but the reality of course is quite complicated. Most of the parameters, such as crop productivity and human greenhouse gas emissions, have to be estimated, so the date is not an exact one. Despite the uncertainties, the calculated date is actually be on the more generous (to humans) end of the scale.
What's scarier is that the date has been moving forward every year: from sometime in early November in 2001, to late September now, about 3 days a year. Even scarier, this ignores the cumulative effect of previous times when we've gone into debt. This crisis mirrors the financial one: in some heavily indebted countries, politicians are happy to report when they've simply slowed down the rate of debt increase.
This is a good way of getting people's attention, because of all the headlines that the poor economy has been grabbing lately. Even non-economists, like myself, have become acquainted with concepts like debt ceilings, defaults, and bailouts (though what a "tranche" is still remains opaque to me). Environmentalists and sustainability scientists are also working on figuring out how to put an economic price on ecosystems and the environment, to make the importance of preserving the environment more "real" to those who think in dollars and cents, such as policymakers and industry. Examples include factoring in the cost of ecosystem services into environmental impact assessments, or selling carbon credits, or swapping habitat protection for debt forgiveness. All of these schemes have their critics, but I like to think of them as experiments in progress, as people try to figure out what's a workable way to account for the environmental impact of our economic decisions.