The definition of sustainability (i.e. environmental sustainability) is when all people live within the limits of the Earth’s biocapacity.
Biocapacity (or biological capacity) is the amount of biologically productive land and sea area that can supply the resources people consume, as well as assimilating associated waste. In other words, Bio-capacity is earth’s production of renewable resources. On the other hand, ecological footprint is a measure of what humans actually consume in terms of biologically productive land and sea.
Humanities’ ecological footprint is now about 1.5 times higher than the biocapacity of the earth. This is unsustainable over-consumption.
The opposite of sustainability
What is the opposite of sustainability? We often use the word ‘unsustainable’ but what does that actually mean? Sustainability is the fundamental condition of any system that lasts. Unsustainability is the condition of something that doesn’t last, meaning something that gets used-up.
Fundamentally it is all about consumption. If what you consume gets completely renewed then you can call that consumption sustainable. You could say that the net effect is no consumption at all, because you are left with what you started with. If what you consume doesn’t get completely renewed then your consumption is unsustainable. For example using firewood for fuel is sustainable if that firewood is regrown. Whereas, burning fossil fuel is unsustainable because it can’t be regrown or renewed. In other words fossil fuels can’t last because they will end up being completely consumed.
So sustainable consumption is the same as, or less than, renewable production. On the other hand over-consumption is greater than renewable production.
The problem is over-consumption
In terms of ecological footprint unsustainability is called overshoot. The only way overshoot is possible is because humans are using natural capital to meet the shortfall in renewable production. This is clearly unsustainable because it’s like using the savings and assets you have as income to live on. You will eventually use them all up and you will have nothing. It is always prudent to live on the interest, or income, from your assets. If you do this you will have an income indefinitely. This is analogous to what we described above. If humans only live on the renewable ‘income’ from nature we will be sustainable.
Overshoot is essentially over-consumption. Over-consumption means there is net consumption i.e. consumption is more than renewable production. This provides us with another definition of sustainability – zero or negative net consumption.