Instantman points out a link to the Voluntary Human Extinction movement.

The funny thing is, that these folks seem to be getting their wish. Current demographic trends indicate that the global human population will peak a little later this century, and then start to shrink. This is because industrialized societies tend to have fewer children–reduced infant and child mortality, and the diminished need of them for subsistence labor, act to reduce optimal family size. This trend will accelerate if we bring the third world into the second and first.

Much of Europe and Japan are already below replacement rate, which is a major problem for their government pension systems.

Of course, it only takes one group, like the Mormons, who decide that it’s their religious duty to procreate, to cause an expansion of that group, whether on world, or off. But the next century could also bring transhumans of various varieties, which may have much longer lifespans, or be effectively immortal.

Neither the UN, or the Voluntary Extincters take this possibility (probability?) into consideration.