Why not just kill off the undesirables

I don't think there is any real limit on the population of the Earth. Sure it will be harder to provide for everyone. But we just need to make sure our technologies advance just as fast as the population is growing.

Isn't it like 8-9 billion could live sustainably or something in that neighbourhood?

I think technology can keep things going for a long time yet. England's population density is about 380 people per km2. If you got the whole world to that density, the total population would be about 57 billion. To do this, you would need to have essentially unlimited energy resources, to get water to all areas of the globe, and presumably some sort of geo-engineering projects to deal with the various repercussions. But I don't see any actual limits.

They should just sterilize a % of children when they are born in the hospital. Although most people wouldn't approve it would help control the population.

I have a modest proposal for overpopulation that also rids of poverty and hunger

Sustainably is a very bad word.

How do we define sustainable? I mean, of the 7 billion we have, we don't actually sustain all of them - there is famine and water issues in many parts of the world, so we're not sustainable as is.

On the other end, maybe we are sustaining them since the overall population is going up? If we hold to current levels of famine, water issues and poverty, we could probably grow a bit more than 8-9 billion.

In fact, we could grow a lot more if we found better ways of doing things:

We could build Arcologies to ensure that density per capita is handled very efficiently. 20 acres of Acros could replace a medium sized city.

We can desalinize water from the ocean to ensure drinkable water for the population.

We can genetically engineer more crops to yield better harvests, and there is also far more farmland to utilize than what is currently used.

So it is really a sliding scale based on corruption, infrastructure, and technology. We probably can't handle more than 4-5 billion 'well' as of today, but we could theoretically hold tens of billions under the right circumstances.

World war 3 will take care of this.

China and India are destined for all out war in the future, one great drought casing famine and millions of deaths and the civilians will look to the military for help. And that means invasion of neighboring country with massive water deposits.

I don't know. Ask the Chi-Coms and see what they say then get back 2 me.

Yes, cause giving the government the power to tell you how many kids you can have or if any at all is just great.

No one has the right to force me not to have kids.