Starting to reduce the world population is only the first step. The population needs to decrease substantially, to 300 million people on the planet. Why decrease it so much? Because a smaller population can more easily protect themselves from natural disasters. People then could live in consolidated groups of 15 to 20 million people. Each urban group would stockpile provisions to permit them to survive an ice age and even to take in another urban group in the event the other groups looses it's habitat. Man is the first species who can prevent extinction. The world needs to unite before depopulation can begin in earnest. Good luck homo sapiens.