I reckon human selfishness will be the root cause of the end as we know it. Resources are dwindling whilst desire for resources accelerates. Right on cue we have places like India and China coming on stream as potentially massive consumers.
Capitalism is the worlds way and is increasing it's penetration globally. It's key feature forms the root of our demise - capitalism demands ever-increasing growth. The undertow of this, human addiction to materialism, will vote out governments who aren't prepared to do that which is necessary to ensure the drugs keep flowing. It won't be blatant but if "it's the economy stoopid" then governments will know on which side their bread is buttered and manufacture if necessary, the potential for jumping through their masters hoops.
Oil is the current worry, but there are any number of resource issues which the world is going to have trouble circumventing - depending on how quickly they arrive. If oil is mega-strategic now, then chromium, copper, nickel and molybdenum are stacking up behind it. As pressure increases on different fronts, measures taken will gradually become more desperate. There will be less resistance to the idea of using, first malign influence, then limited force, then greater force to ensure supply. Nothing very sudden but a gradual shifting in of acceptability in that direction. We know of the corruption and abuse that ensures supplies from the African continent - we should not expect our nature to change any time soon. We cannot evolve quickly enough from our essential humanity for it to be otherwise
Good old fashioned war, for the good old fashioned reasons for war, will, I reckon, pull down the shades on this world
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