How many billion people do you think the world can hold before the planet just says "fuck it".

I think the planet has been mumbling "fuck it" under its breath for a while already...

Also, I think China had half of a right idea in general (you need to stop population growth, the other being that you need to stop taking out debts, whether in fossil fuels or biomass - think ocean depletion or soil erosion - to underwrite our living expenses), but got the implementation all wrong. The only thing that reliably seems to help with the first goal is to empower and educate women and give them access to reliable birth control. I don't think anyone has yet demonstrated a way to achieve the second goal.

Too many by far, but what can anyone DO about it? That's the terrible thing--even if you could get everyone the education and easy access to birth control required to stop it, shrinking a population is ALWAYS going to be painful. Look at individual countries where this is happening... aging populations without enough young people to care for them, strained pension systems, and various other problems.

I try to be optimistic about most things, but this is something I look at and can't help but think that no matter how it plays out, we're kind of screwed. :/

shrinking a population is ALWAYS going to be painful.

Bunner is right, if we were somewhat smarter in how we use and distribute the resource we have, feeding even the projected 9 billion of 2050 should be well achievable. Which means that we don't really need to shrink any population, anywhere.

All we need to do (hah!) is stop population growth or at least slow it down considerably. But we need to do that quickly: Even if we were able to successfully institute a two-child-policy (i.e. birth rates at or slightly below replacement level) right NOW, we'd still grow to at least 8 billion people, simply because, while hundreds of millions of people will die over the next 25 years, there are about 2 billion children in the world right now who would grow up within that time span and likely produce another, say, 1.5 billion children themselves, even under that policy.

We'd be fine if they'd get off their asses and start terraforming Saturn's moons.

Edit to clarify: This remark is at least 98% flippant.

Terraforming really isn't a solution either. If we wanted to use terraforming as a way to reduce Earth's population, we would have to have a way to remove colonists from the planet faster than the population growth rate. I just don't see space travel technology improving fast enough to allow migrations of the necessary scale to occur at any time in our lifetimes.

It's not space travel technology that's the true bottleneck, although it's true that it would need to grow many orders of magnitude. It's space habitat technology. There is no other place in this solar system that can be slapdash rapidly turned into a habitation for humans, or for any Terran organism with an aerobic metabolism. We are just now starting the survey to determine how far it is to the next world we could live on unaided without habitats or centuries of work. We don't know how far it is. It could be as close as Alpha Centauri... or hundreds of light-years away. But the plain and simple fact is that we don't know where they are.

Kim Stanley Robinson's Mars Trilogy explored this problem. The story arc tracks the growth of a Mars colony from first settlement through a few million people. Earth at the time was experiencing problems with overcrowding. They were able to send a few million people to Mars, mostly against the wishes of the existing colonists. This action had no significant effect on Earth's population, but it completely overwhelmed the Martian society with people who did not share the same vision for Martian culture as those who had come before.

Correct. One of the characters does the math on this; it's essentially the same no matter whether there is a lack of colony space or infinite colony space. Imagine there are a hundred space elevators serving Earth. That's, on average, one every two hundred and fifty miles, all around the Equator. (Except that there aren't too many places to put space elevators, on land or sea; you need good weather conditions, and on land, a stable location with good access. So they're likely to be more crowded, like one every hundred miles. You can't put them much closer without risking them getting tangled, since they have to be able to maneuver to avoid collisions with satellites and space debris.)

Imagine every one of those elevators has a car leaving every hour, day or night, nonstop, carrying a hundred people off Earth. This is an astounding rate, considering that the cars will probably move at something like 1000 miles/hour and the cable is, minimum, 25,000 miles long. That's fifty cars running up or down the elevator, all the time. But let's posit it. Every hour, on the hour, ten thousand people migrate out. The math's pretty simple. Ten thousand an hour is 240,000 a day... 1.68 million a week. Sounds good, right? You could clear out a small country every week. Surely this will work?

Nope. Because it's still only 86 million a year. That means it would take twelve years to move a billion people off the Earth. I hate to tell you, but a billion people were born in the last twelve years. And that's with the one-child policy AND the onward march of women's rights and blah blah blah. If Earth collectively decides that migration means they can let up on the birth-rate brakes... they can easily, EASILY make up the deficit that ANY amount of out-migration causes.

Bottom line: Earth's population problem is Earth's population problem. If we want to reduce our population, there is no substitute for making changes to our birth rate.

But! There are problems to doing THAT too fast, as Japan has learned and the rest of the world -- especially China -- will soon find out. Fewer babies means MORE OLD PEOPLE. And you may have noticed, but old people require more attention... or they don't live as long. For ten thousand years and more, humans have based their societies on the assumption that the young will outnumber the old. Within our lifetimes, that will cease to be true... and everything will change.