Oil
Peak Oil has probably already occurred, if not totally, then in certain countries. If you don't know what Peak Oil is you need to read up. I'll add some good You-Tube links soon so for now - do a search.
With massive demand from India and China in 2008, the price of a barrel of crude looked like it was headed for $200 and beyond. The market crash, inevitable perhaps, intervened and demand dwindled and prices dropped back to the $60 mark. This is only an interim measure. From now on, the only way on is up. It is quite likely that, though the price will go up again, the amount of oil sold at any one time will never again reach the levels it did in 2008. Demand will be higher, but there simply will not be enough oil around in the future.
Our entire way of life is oil-dependent. If the countries of the world do not reorganize the way they grow and distribute food we are headed for a catastrophe. People seem to be waiting for some magic technology to come and save us. This is sheer stupidity. What we must do is use the remaining oil, which still relatively cheap, to build lots of power stations. Nuclear power is the only sensible option - take a look at what China is doing in that respect. Forget cars, if we don't change, we'll soon be back to horses and sails. We need electricity, and lots of it. Trams are the obvious transport of the future. And even if we do change, horses and sails will not be a bad thing.
In the meantime, war is going to appear more and more. Modern warfare has an economic purpose - think about it. Countries do not fight over land anymore. Look at what is happening in Iraq and Afghanistan.