The UN has published their population projection for the next 45 years
Details. un.org/news/press/docs/2005/pop918.doc.htm

Briefly, world pop is expected to increase by 2.6 billion to 9.1 billion. in
2050
Almost all growth will take place in least developed countries where 5.3
billion of the 6.5 total pop. lives now. Their pop. will grow to 7.8 billion

Demographers plot several ,(usually three) scenarios.
Here I am quoting the medium variant

By July 2005, the world will have 6.5 billion inhabitants. That's 380 million
more than in 2000. Increases are therefore 76 million per year
by 2050, when pop reaches 9.1 billion, pop will still be increasing at rate
of 34 million annually

Fertility rates are the driving force. By 2050 fertility rates are expected
to decline from 2.6 children per per woman today to slightly over 2 per woman.
The fractions are important. If fertility were to increase a half child per
woman above the median projection, the total population increase would jump
to 10.6 billion. But if it were a half child below, the total pop increase by
2050 would be a mere 7.7 billion
So even when fertility declnes accelerates, pop grows for a few more years

Pop. of developed countries like N America, Europe, Japan will stay stable
at 1.2 billion

The most rapid growth of Pop. will occur in the least developed countries.
Pop's will at least triple in Afghanistan, Burkina Faso, Burundi.Chad, Congo,
DemocraticRepublic of Congo. Timor-Leste, Guinea Bissau, Liberia, Mali, Niger
and Uganda

Eight countries will account for half the projected increases of 2.6 billion:
India, Pakistan. Nigeria, Democraiic republic of Congo, Bangladesh, Uganda,
USA, Ethiopia and China

A huge unknown factor is AIDS, In Africa and in Russia nad its former SSR
colonies, AIDS is wide spread. AIDs wil affect pop. figures but no one knows
how much. AIDs is is terra incognita to UN demographers...any everyone else

The other factor in addition to fertility is mortality rate. All countries
are looking at increasing longevities. THe UN report provides many statistics
for those interested

Several developed countries woud be in pop. decline were it not for
immigration. Typically, immigrants bring higher fertility rates with them to
their new
homes

The major countries to which immigrants will go to are: USA, Germany ,
Canada, UK, Italy, Australia.
The major countries from which immigrants will flee are: China, Mexico.
India, Phillipines, Pakistan, Indonesia

The UN report does not comment on the consequences of pop increases. The
section of the UN that publishes this report just does demographics

But as these projections come true, there will be consequences. we can see
some of them right now if we care to look (we don't)
Dave S