Tuesday, July 10, 2007

Reliability of Growth Rate Projections

The decreasing world population growth rate extrapolated from 1961-2004 data anticipates zero population by 2049, only one year later than my consumption model. If growth rate was the controlling factor, then we would expect population to be independent of ecological footprint.

Backward extrapolation (to years before 1961) of both population and per capita ecological footprint turns out to be unreliable (for example, the population would have grown 339 percent in 1900), indicating that curve fits to growth rates are only marginally useful, and implying that the growth rates themselves may not be independent of each other. This last point is one of the fundamental assumptions of the consumption model, which assumes that population is a function of cumulative consumption (with annual consumption proportional to the footprint).

What I believe we are left with is a bit more confidence in the population projection of the consumption model, which I had hoped to throw doubt on. If our behavior continues as it has over last 45 years, then in ten years we should know which of the projections of footprint are correct: If the footprint peaks by then, the growth rate is the deciding variable; if it continues to climb, then the consumption model is right. Unfortunately, we would then be beyond the point of forestalling population loss.

There are signs that behavior is changing, and may change enough to make a difference. Leaders around the world are talking about thwarting global warming (one sure contributor to population loss, both in our species and others). Countries such as Great Britain are promoting “green” technologies and renewable energy. The major oil companies have stopped building new oil refineries, an indication that they no longer expect to get most of their business from petroleum. In my consumption model, this change in behavior corresponds to a decrease in consumption, and translates into direct changing of the annual growth of the footprint, which has a lesser impact on the growth of the population.

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