[Porsche-O-Phile]

Religious differences will likely lead to a nuclear exchange between two or more nations/powers leading to the eradication of millions, with billions more dying off slowly in a polluted, radioactive world with little capacity to produce food and agriculture in the years following.

This is the most likely scenario as I see it. Frankly I'd be shocked if the human race survives another 500 years. Honestly, I think this scenario will happen in the next 100. Quite possibly in our generation.

Our restraint has historically never kept pace with our technology and we still (as a species) are inclined to cling to silly ancient mysticism, religious superstition and tribal-based xenophobia rather than trying to see ourselves as part of a collective race of inhabitants on one very small planet capable of supporting life in a vast ocean of barren emptiness.