ok, glad to see theres some sort of factual basis to that, though sounds somewhats alarmist to me.
what im confused by is the seemingly common feeling that some sort of disease or other dissaster will come along and weed out the undesirable. what on earth makes you think that? a plague. earth was hardly over populated when the plague attacked europe which wiped out between 1/3 and 1/2 of the popluation in some areas. such a disease would never be effective in our 'modern' world. the sars disease which is attacking some of china, hong kong and other asian countried has killed only 50 some odd people, and massive efforts are being put forth to contain it. hardly a major world concern. the only way people are going to die en mass as you believe is by killing each other.
the fact of the matter is that the earth isnt goin to suddenly turn brown and stop spinning. as conditions grow more dire, world policy may have to change. population control, maybe. are you aware that a large portion of farmers in the united states are paid by the government NOT to grow food, or to only gorw a small portion of what they could? this is done to prevent flodding the market with produce/bread/etc., thus lowering its value and making it impossible for anyone to live as a farmer, and in the long run acutally causing food shortanges because of a lack people willing to work for free and grow food. this food is not exported because most countries cant afford the prices americans place on their own labor. anyway, back to my point. if the world really begins to starve as you propose, it would be up to countries like the U.S. (as terrible as that may sound) to step up and help.
the population in most modern coutries (such as the U.S., western europe, south korea) is relatively stable, not growing at exponential speeds. this means that the 12 billion people you forsee living in 50 years wont be distributed equally, and the natural nature of people would likely kick in. in short, humans are likely to live on. not all of them. and it wont be up to nature to descide, it will be up to the politicians, and in 50 years that puts most of us right around the right age. so, in risk of sounding too "after school special", it may really be up to us.
sorry for the rant, but i felt like it