I've already seen other studies showing world population growth increasing to approx 9 - 9.5 billion by 2050, then decreasing during the latter half of the century.

Generally social, religious, and above all economic development have the greatest impact of demographic trends. Russia and Eastern Europe, still reeling from the shock and trauma of crash course conversion to capitalism from stagnant socialism, have actually experienced drops in population. America and Western Europe's declining fertility and birth rates are principally due to economic factors pressuring family size decisions, legalized abortion, and societies freeing themselves from the more backwards and foolish dictates of organized religion.

The USA is a special case, where population increase is almost entirely due to immigration. While moderate immigration has been very healthy for America, and Western Europe, efforts to help 3rd world nations in Africa, Asia, and Latin America to develop are vital to lower the current high birth rates, and prevent or lessen illegal immigration in the future.
One reason why NAFTA was such a good idea.