Global taxes aren't realistic. This system would lack a leader, a world leader such as many places have a president and it would be hard to distribute the taxes in a manner that everyone would agree with. Also, there are too many countries who's policies are terrible and it would be unfair for people to pay into a tax system that supports them.
I do not believe global tax would be fair. There are already state taxes that help our country. I think every country should have taxes but I don't believe the taxes should be shared at a global level. People like to see where there taxes are going into and with a global tax system we would be paying taxes and not knowing where the taxes are going. Who would have control of these taxes and how would it be spent fairly? Global taxes would be very hard to manage because it covers every country and if the taxes seemed unfair to a certain country it could cause resentment and fighting.