How to start a novel set in the future?

I'm writing a story that takes place in the future, around the time when population growth, dwindling resources, and pollution are taking their tole on our species. Tensions are growing, war is imminent, and only minor space exploration has succeeded (such as mining/colonization of the moon, asteroids, mars, and not much else). Can anyone tell me in what time frame these events are likely to occur? 50 years? 100 years? 200 years? 500? A short explanation would be greatly appreciated as well :) Gracias!