Overpopulation is probably the strongest and primary factor for all the major problems: environmental pollution, shortage of potable water, food, natural resources, education for everyone. The world population according to statistics is going to double at some time. And it is not the fault of anyone, since the human being is at the top of the food chain. No species eats human beings - that is the thing.
Sadly, the HIV, diseases and wars balance the rampant population growth. Ironically saying, all the cigarette manufacturers should be awarded the Peace Nobel Prize for contributing to death of human beings. However, it does not necessarily have to be that way: it upsets a person to lose a closed/loved person who died of lung cancer or in a war.
The best solution might be the 2-child policy all over the world. It is well known that the population of a country decreases if the average number of children in a family is 1. The population remains the same with 2 children in a family, and it increases with more than 2 children. Imagine the UN strictly imposes the 2-child policy for countries with increasing population, for over-populated countries (e.g., Bangladesh, Indonesia, India etc)...The world problems might be subdued drastically. Besides, 2 children is a morally justified number: after all, any kind desires to have a brother and sister. Strict imposing would include high penalty fees for each family having more than 2 children, sanctions for a country which shows increasing population etc. Cruel? Yes. But it is a necessity.
Well, of course, the UN is an impotent organisation which is not going to go against the interests of multinational conglomerates. The latter are interested in population growth in order to always have the source of cheap labour, but imagine it considers this option. Shall we write a petition for that?