The critical feature of sustainability isn’t how many people can be supported by the planet at any given moment in time. Rather, it is the number of humans that could live here without irreparably damaging the biosphere we depend on for survival.

A sustainable species never damages the biosphere irreparably. That’s a pretty tall order.

Humans damage the biosphere in many ways.

One is by shifting resources in space. We usurp the habitat and resources needed by other species, and sequester them for human use. Resources obtained in regions that are unimportant to humans are moved to wherever humans need them, at the expense of indigenous species in the original location.

We also shift resources in time, by stealing resources from the past and the future and using them in the present. An example of this is using fossil fuel energy to pump water out of aquifers for agriculture, thereby using historical fossil fuel resources to diminish future water resources, in favour of growing crops today.

We usurp habitat from other species simply by moving humans to that location, and in the process making it inhospitable to indigenous life (the affected indigenous life doesn’t even need to be non-human…) The sequestering of habitat and resources for human use often go hand in hand.

The unsustainability of our species at any time can be roughly gauged by the degree to which we have concentrated the the spatial and temporal distribution of resources into the here and now, and the extent to which humans have displaced wild life of all sorts.

In contrast, being a fully sustainable presence would require us to do no damage to the planet that could not be repaired by natural biophysical processes in real time.

Given such constrained behaviour, the human species could survive for a very long time indeed (perhaps tens of millions of years) alongside all other sustainable species. Of course, any damage that can’t be repaired invokes the concept of overshoot, which will shorten our species’ period of survivability by some (unknown, perhaps unknowable) amount.

It should be obvious to everyone here that our species’ current way of life is “quite unsustainable” by these criteria.

Is it possible to return our species to sustainability? To answer that question it helps to have a benchmark. When was the last time Homo sapiens might have qualified as a sustainable species using these criteria?

In my opinion, the timestamp has to be placed at least prior to the invention of agriculture, since it was agricultural technology that kicked off the population and cultural growth that got us here.

Before the development of agriculture (as distinct from the horticulture practiced by many hunter forager societies), the global human population is estimated to have been about 6 million people, with an annual growth rate around 0.02%

Such a population of 6 million hunter foragers could perhaps be considered sustainable, except for a couple of caveats.

One caveat is population growth. With a climbing net birth rate it didn’t take long for a population of 6 million to turn into 6 billion. We managed it in just over 12,000 years, at an average growth rate of a measly 0.06%. Our current growth rate is over 1%, 50 times higher than the 0.02% of “Homo sustainabilensis”.

The other caveat is per-capita consumption growth, as well as the growth in technology that is required to sustain both growing population and consumption levels.

Per-capita consumption can be loosely approximated by energy consumption, since all material goods require energy to produce. A hunter-forager consumed about 150 watts in food and fuel. A modern human uses more than twenty times that amount. This energy use amplifies the damage done to the biosphere by the growing number of humans.

So, 6 million humans all living as hunter-foragers might be considered sustainable. But only if they were to maintain a permanently static population capped at 6 million, and a static level of per-capita consumption capped at the equivalent of 150 watts of energy use.

By this estimate, compared to our nominally sustainable forebears we are already in overshoot by a factor of about 25,000. And it’s climbing with every new mouth and every increase in energy consumption.

(Sarcasm generator on)

Humanity could of course move back toward sustainability. Easy-peasy. All we’d have to do is: reduce our population by almost 7.5 billion; stop population growth completely; reduce our energy consumption and the activity that it drives – say by 90%); and eliminate all technological development that results in greater energy consumption (I’m looking at you, William Stanley Jevons.)

(Sarcasm off)

What? We can’t/won’t do that? I know that. This isn’t an exercise in goal-setting. It’s an exercise in measuring the width of the Atlantic Ocean in case we’re ever inclined to try swimming across it.