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Many people will say that humans are the exception, but I think it is nonetheless interesting to look at this from an ecological point of view. A common pattern of population growth for animals includes a period of exponential growth (as we are currently seeing), which ends with an "overshoot" where the population continues to grow past the "carrying capacity" of the environment. The population then decreases (possibly due to starvation or disease), and begins to cycle around the carrying capacity, creating an equilibrium. If one believes that this is a reasonable model to apply to the human population on Earth, the real question is: "What is the carrying capacity of Earth?"

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"If one believes that this is a reasonable model to apply to the human population on Earth,"

It almost certainly isn't. The extremely strong propensity for rich countries to have fewer-than-replacement children kills this model. Animal models have no equivalent to this.

We're headed into uncharted territory with human population. Usually this is supposed to be a sort of implicitly scary thing to say, but since the charted territory here is "guaranteed major ecological catastrophe", it's net good news. Disaster is not assured! (Of course, it's still on the table. It's always on the table.)

On the flip side, we're in thoroughly uncharted territory in terms of carrying capacity as well. On the relevant timeframes (decades, minimum), nobody can correctly predict what our technological carrying capacity will be. There's no guarantee it will continue going up, but contrary to the doommongers there's no guarantee it will stay static or go down significantly either; if in 40 years we've licked nuclear energy and cheap robots we could well be growing huge amounts of food in robot-tended greenhouses stacked into skyscrapers, powered by the nuclear energy, or growing meat in vats with chemical energy more efficiently than going through the full biological cycle. And while that may sound like sci-fi, those actually feel like conservative extrapolations of existing trends. (Just about the only prediction I won't buy is the one where we make no further progress and everything stays the same but we keep consuming resources at the same rate, but that's the one you hear most often.)

I'm not a biologist, but surely the fewer-than-replacement birth rate could be considered as just a new way for the population to drop to carrying capacity? It may be a response unique to humans, but wouldn't the effect be the same?

It's not a response to the carrying capacity of the planet/environment, though. It's a response to the pressures and culture of human societies in developed countries.

As another answer says, global carrying capacity depends on people's living standards. Wars normally start when there is a regression in standards. I am sympathetic to the idea that in the West this might not apply as much, i.e. regressing to 1970s living from 2011 wouldn't be a huge tragedy even if it was a loss. However, as current protests and riots in the West show, there is also a psychological element. Additionally, large portions of the world do not have that padding, and regression actually means physical pain (food shortages) rather than psychological pain. Large groups in that category are also nuclear-equipped (Russia, Pakistan, India and China) and so I guess your sine-wave of population gently dipping down from carrying capacity after briefly going over might actually be a very sharp movement downward, measured in the units of the nuclear age: megadeaths.[1]

[1]http://en.wikipedia.org/wiki/Megadeath

Unless this model doesn't apply to the human population. While I'm sure it does in the general aspect, it seems likely that once the overshoot has begun, we'll be able to stave off the decline (starvation, disease, and therefore war) more effectively than other species, leading to a larger overshoot and eventually, a more drastic decline.

> What is the carrying capacity of Earth?

Probably depends on the quality of life people are willing to put up with.

"Willing to put up with", but also ready to create and maintain.

Agreed.

A better question should be: "What are the limits of energy production on Earth?". We are far from reaching this limit.

I'm not so sure. You'd be surprised how much energy we already consume compared to the solar energy received by the earth. At some point the waste heat will become a problem: http://news.ycombinator.com/item?id=2985303

And if we exploit available nuclear fusion and fission, we could generate more waste heat than the total energy available from the Sun. At this point we'd have no other place to dump heat but outer space itself.

Of course, by this time it would be energy-cheap to expand beyond Earth.

Human societies with different development levels could be considered as different species competing for more abstract resources, such as money. Assuming the world order will remain constant for a few decades, maybe you could apply such a model.

The difference is that with our technology and through our politics, humans can drastically effect the carrying capacity of Earth. We choose.

Humans are technological. We do not have an ecological niche as other animals do, we make our own. On the one hand we use agriculture and industry to make our own food and homes. On the other hand we choose our own behaviors based on our culture, economy, etc. That makes the idea of a simple carrying capacity meaningless. We can change our diet and the effective carrying capacity will change. Or we can change our technology, our land use, etc, etc, etc.

As far as the absolute limits, it's unlikely we're anywhere near them. If humans switched to becoming significantly more vegetarian in their diet we could support quite a lot more people with current levels of food production. Yet food production is not even remotely running up against fundamental limits at current levels.

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