A decrease in global human population is absolutely necessary. There is no way to sugarcoat this. In October 1999 world human population careened past six billion individuals. Perhaps we can put this in perspective. I am a relatively young man, born in the early 1960's. The human population of the planet has nearly tripled since my birth. We are approaching numbers that Thomas Malthus (one of the earliest and most cited population researchers) could only theorize about 300 years ago. And his hypotheses are proving to be correct. War, hunger, disease, and ecological devastation are the order of the day.
What is clear is that, like all unsustainable situations, we cannot proceed on course. Nor can we ignore this. A conscious effort to stabilize the current level of human saturation is necessary. Once this is achieved, a steady decrease can be implemented. I am not suggesting this will be easy, nor met with consensus from the world's peoples and religions. In fact, there is no way of broaching this subject without igniting a firestorm.
Imagine a time traveler from the distant past making their way to America 2002. Let us suppose they come to us from the Middle Ages, a time that is only a blink of an eye by geologic standards. One can imagine the scene would be surreal for this traveler.
They would notice that the air is polluted, the soil leached of nutrients, the water dirty. Crime and violence are so prevalent that people are desensitized to it. Open spaces are few and far between, and often degraded by overuse. Despite huge advances in technology and living standards for some regions, the majority of the world's people live in abject poverty. Children in these countries often die of malnutrition before reaching school age, while others live in sewers and are hunted for sport. Smog alerts shut down entire cities, while a monstrous hole in the ozone layer looms over Antarctica. Melting ice from this region threatens to wipe out entire island nations, all because the affluent countries refuse to alter their 'quality of life'. Energy demand is so high that 'oil wars' have become a reality. 'Water wars' are on the horizon. What we would call science fiction. Yet, this is not science fiction. This is our reality.
Let us look at this from another perspective. If one puts a culture of microorganisms in a petri dish and gives them a source of food, they will multiply until the food source runs out. Then they die. They breed themselves out of existence. I trust that we are more intelligent, and have more foresight, than a dish of microorganisms.