I had an email exchange earlier this week, an exchange about a column I wrote over a year ago titled "Induced to carrying the latently obvious" (available, as always, on the Bloomington Alternative's Web page).

The premise was that all systems, man-made and natural, have an inherent quality called "carrying capacity." That is, the capacity at which the system can sustainably keep on keepin' on.

For instance, the carrying capacity of a lake is the number of people who can drink from it without affecting the lake's water quality or level.

Inherent in the concept of carrying capacity is the concept of overshoot. That is, although all systems have inherently sustainable carrying capacities, it is possible to draw down those systems at a rate beyond their carrying capacity. When that's done, the system is said to be in overshoot.

In the example above, far more people could drink from the lake than it was capable of sustainably supporting. And they might be able to do that for some time. But during that time, the carrying capacity of the lake would be said to be in overshoot.

A wonderful exposition on the concept of overshoot can be had, by the way, in William Catton's Overshoot: The Ecological Basis of Revolutionary Change.

Death's prerequisite: Growth

Sustainable systems have no endgame. That is, an inherently sustainable system can be expected to do whatever it is doing: feeding a given population, providing energy to heat their homes, endowing them with water, etc., forever.

Systems in overshoot have an endgame. At some point in the future they will stop doing whatever it is they do. Whether that's providing drinking water or the clothes on your back or whatever. At some point, all systems in overshoot run abruptly out of whatever it is they're overshooting.

And that endgame has a name. It's called collapse.

A wonderful exposition on the concept of collapse can be had, by the way, in Jared Diamond's bestseller Collapse.

(More, more, more) How do you like it? How do you like it?

All systems that expand exponentially, that is systems that are expected to deliver an increased percentage of whatever it is they provide year after year, have an inherent quality.

Something called "doubling time."

Doubling time is simply the amount of time that the system will either need, or provide, twice as much as it currently does. It's easy to figure out, just divide the percentage increase into 70.

For example, the number of housing units in Monroe County is increasing at more than 5 percent a year. Divide 70 by 5 and it gives you 14 — meaning that in less than 14 years there will be twice as many houses in Monroe County as there are now.

Those houses don't exist in a vacuum. As they double, so will the things they consume and produce. Their need for water, their need for food, their need for roads, the sewage they produce, the garbage they make and the traffic they generate, each doubling every 14 years.

But the problem is that the underlying natural resources supporting that doubling aren't themselves increasing. Nature isn't exponentially producing more water, more land, more air and more energy every year. Natural endowments are either permanently fixed, as they are for water and land, or they increase in linear amounts — like energy, all of which comes in the same amount, more or less, every year from the Sun.

We feel we're entitled to an economic system hard-wired for exponential growth. It's an unfortunate and inconvenient truth, then, that nature ultimately serves up all exponentially-growing systems with collapse.

We're special, this time

Despite the evidence, historic and mathematical, detailing the inherent un-sustainability of exponential growth we nevertheless insist on denying that it will happen to us this time.

Each age always considers itself incapable of the failures of the ages that came before.

That denial has a name. It's called faith. Faith in limitlessness, a cargo-cult of infinite substitutability. When we run out of oil, we'll just switch to another cheaper and more abundant source of energy. When we run out of land, we'll just blast off and build another planet.

When we run out of water, we'll just switch to beer.

That was the theme of the past couple weeks. The Herald-Times started by chiding those who questioned facilitating exponential growth in Bloomington's automotive traffic as "truly retro thinking and a history-ignoring lack of confidence in an ever-more technologically sophisticated society."

Maybe. But it's much more likely that those who do not learn from history are doomed to repeat it.

Meanwhile, Indianapolis' Dr. No suggested that the continuance of that city's exponential suburban growth was threatened by the collapse of the city's water resources. And that a solution to that collapse would be to drive our own Monroe Reservoir into overshoot.

Maybe. But it's much more likely that those who do not learn from history are doomed to repeat it.

And, finally, our county Plan Commission voted 5-4 to move forward with publicly funding developer Bud Bernitt's private suburban stripmall along Fullerton Pike, on the premise that doing so would facilitate a new five-lane automotive gauntlet. Because, as the county's Commissioners opined, we need to plan for exponential growth.

Maybe. But it's much more likely that those who do not learn from history are doomed to repeat it.

Gregory Travis can be reached at greg@littlebear.com.