“Carrying capacity” and whether humanity can attain it in terms of its population on Earth, may determine if the species survives or becomes extinct. The concept itself is not that difficult a concept to grok – and makes perfect sense when one thinks about it.
For example, a spacecraft with 3 astronauts sent to the Moon has enough food & oxygen supply for a five day round trip. For those specific conditions, ‘3’ is the carrying capacity of the craft. ‘Three” is the maximal population that the craft can sustain given its food, water, fuel supply.
Double the number of astronauts and you must double the resource supply for the craft, or you have to reduce the time of the journey by a factor two. There is NO other way! (And bear in mind for a spacecraft redundancy of resources is built in just in case of an emergency - e.g. a micro-meteorite puncturing the shell of the craft and causing oxygen loss).
In many ways, Earth is like a self-contained “space craft”. It is isolated in space, not infinite in extent, and its resources (fuel, water, food supply) are also finite and set for a certain maximum consumption – unless the inhabiting species radically alter.
In determining this, one must bear in mind that a planetary population’s impact extends vastly beyond immediate space, living habitat. Recently some studies (e.g. Millennium Ecosystem Assessment Project) have translated impact into hectares consumed over a lifetime. The avg. – when all humans are factored in – amounts to nearly 2.3 hectares per person reckoning in crops and meats-cattle etc. consumed over a life not to mention water, other resources. No high population density scheme that doesn’t factor acreage impact in can be taken seriously.
Americans consume an exorbitant amount in planetary hectares – around 10 hc per American – which is exactly why the planet can’t afford “two US of As”. (Not that China is paying much attention as they ramp up their economy!) It also means the distribution is that much less for other nations, peoples. In general then – the base relationship defining carrying capacity for Earth is:
Carrying capacity = (usable land-water resource base providing water + food + fuel) / (individual food, fuel + water requirement)
Now, if the numerator is ~ 11.4 x 10^9 hectares of usable aggregate equivalent land-water resource base and if 6 hectares is the ideal "mean individual requirement" over a lifetime (e.g. meet all basic needs and have a few private luxuries) , that means:
CC = (11.4 x 10^9 hectares) / 6 hectares/person ~ 2 billion
Obviously, this can be increased if the numerator can be increased or the denominator (each individual's ecological footprint) decreased. The problem is how to achieve it? (Especially if the total population continues to increase at 2-3% per year)
As we know, all three land base contributors to the numerator (land base for food supply, land base for water supply, land base for fuel supply) are decreasing. By 2025, for example, non-contaminated potable water hectares is expected to be roughly halved.
This means, according to ‘The State of the World’ (2000) report notes (and at present rates of declining watersheds, and increased population) the number of people living in water-stressed countries will rise from 470 million to 3 billion by 2025. This is more than a sixfold increase. Add in projected new climate change data and the expanding drought, desertification regions - and likely effects (see. eg. recent issues of Eos) and the stressed populations increase nine or tenfold.
Similarly, arable land supply hectares are diminishing owing to drought from climate change and encroaching development on previous farmland (here in Colorado, for example, farmers have sold nearly one-fourth of their lands to developers). True, people could make do with less – especially Americans- but WILL THEY? Short of major political pressure there is no reason at all to become optimistic that Americans will change their piggish, planet greedy ways on their own. (E.g. the U.S. with 5% of the world's population consumes more than 25% of its oil)
Technology is great BUT: 1) Technology means nada if the political will to put the money where gums are flapping doesn't exist. One could as well be talking about zero point energy as savior. 2) Technology is not an energy source but energy consumer and often an energy use accelerator as Matt Savinar has noted in his excellent online text, ‘Life After the Oil Crash’. .
Thus again, technology and even maximal brain power to develop it means nada if the energy sources aren't there for its expeditious use and distribution. To put it another way, Prof. Wizardo may design a super-duper automatic crop creator to use in deserts. However, if the energy E isn't there to: a) run the damn thing (say 10^7 Joules per month), and the water W (hectares) isn't available to irrigate the result, his device amounts to a waste of time, effort and more resources. And Prof. Z could have an IQ high enough to qualify got the 'Triple Nine' society and it still wouldn't matter. Without the resources to enable his invention, it's dead on arrival.
Isaac Asimov in one of his early essays (in his anthology ‘The Stars in Their Courses’) on population and carrying capacity, put it bluntly:
“Humans have a simple choice: they can either decrease their numbers on their own to live within the Earth’s carrying capacity or they can let nature do it for them”.
With the emergence of antibiotic resistant bacteria such as c. diff. and MRSA in recent years, it more and more appears the latter is what is transpiring.