As Japan needs a labor market to support an aging population it will be interesting to see the impact it would have on their country and culture once 40% or 50% of the country is from sub-Saharan Africa or predominantly Muslim nations. Mass population migrations will becoming increasingly common and essentially everywhere will be a melting pot.

Hans Rosling had this prediction down a long time ago.

Human population growth is the most serious problem the world has. Forget global warming, habitat loss, decreasing biodiversity, they are only symptoms of the burgeoning human population.
Solve the population growth crisis and things like controlling green house gases etc., become viable, without tackling population its a losing battle.
The working/retired population might not be so much of an issue with AI, robotics and better health services.

Daishi - I disagree. I think the world is moving to a more isolationist position, and Japan's labor market is already struggling, hence the huge robot push over there.

An Earth of nine to ten billion humans will be a living hell devoid of wildlife and decency.

Soylent Green!...... .its people.

One of the problems with decreasing support ratios are the pyramid scheme such as Social Security and state and federal pensions in that what the person paid into is either woefully inadequate or the people paying for the benefits being paid out are not the ones who enjoyed the work produced, it was a past generation and the retirement payout was kicked down the road. A 401K like system is far more workable.
Medical is another issue and this is where I part from conservatives in that I think end of life needs to be handled better. It is a complete waste of public resources giving a 80 yo person an organ transplant yet it happens on state dime all the time. In fact we keep demented people alive when their own bodies are failing due to natural causes mostly because the family either does not want to let the go or wants the benefit checks.

@Roomie, are you mixing up Hans Rosling with Thomas Mathus?
Us humans will eventually exhaust Mother Nature to the limit, but it doesn't have to be like that. For sure, there has to be ways to get the developing world to have less children. Without any human rights protests from developed nations, China's one-child policy was a bold step that lasted an incredible 36 years (with a couple of adjustments, that estimates say prevented 400 million births!), but it eventually got more complicated because of too many boys and an aging population.
A combination of consuming less resources per capita and smaller birth rates would be ideal. Personally, I'm doing my part, but capitalism is an insatiably hungry system.
@CAVUMark, occasionally when I go shopping for food and find myself in the meat section (I'm mostly vegetarian), I think of Soylent Green. That sci-fi film made a big impression on me.

What good is peaking at 10.9 billion if the environment collapses at 9 billion?

All great comments (especially @Nelson Hyde Chick) and proof that there is at least some intelligent life on this planet. Noticing a complete absence of the problem in our political discourse, I wrote a letter to all the POTUS candidates concerning our species' numbers. I'm anxious to see if I get any replies but the main point I want to make is that if those that grasp the gravity of this problem don't make it a political issue then nothing is going to change and as noted - the planet can't support the current population let alone 11 billion.