Well the global population is set to reach 9 million shortly after 2050; after that it will flatten out or even decrease due to the lack of vital resources such as food and water. Furthermore, a greater population contributes to more pollution and loss of habitats, thus contributing to climate change.
I'm guessing you mean 9 billion, not 9 million? Anyway,
eventually the population will flatten out out decrease due to food and water shortages. However, with increased population I reckon humanity might be too good at using/developing technology for its own good, and coupled with climate change will perhaps bring us to the point where we have completly wiped out anything we can live on and thereby eventually bring about our own extinction. That's not 2050, maybe not even 2250. However, right now fish stocks are collapsing and climate change threatens to cause a mass extinction at some point in the future. The human race could try to rise higher than they ever have before by adopting a global one child policy
now to try to save ourselves and everything else. Fail, and humanity will fall so much lower, and worse will bring the rest of life on Earth down with it.
The global population is too big already, with the population fixed at its current level we might be able to stop climate change but not I believe the collapse of fish stocks etc. There are simply too many mouths to feed.
Some have made good points, for example twins. Policy would have to be carefully planned to allow for that and other issues, but without compromising the overal aim of population reduction. Perhaps a one successful pregancy rather than a one child limit would work better.
With the population reducing, I'd suggest trying to keep the population of developed countries (like our own) relatively static (ie. a much smaller reduction than in Africa and India, but still a reduction) by moving pepole from the other areas. This has a few benifits, for example wildlife in the UK has largely been reduced to species that don't really come into conflict with humans as much as say tigers and livestock farmers in India. Another example is that sparsely populated areas do not work as well with public transport (which will always be more efficent than private cars, even if you power both by green electricity, I expect).