The world public will never support a global tax, no matter how much it will improve people's lives. It is just not feasible or possible in any way because in many parts of the world people suffer from extreme poverty. For instance, in Eastern Europe people are very poor and there are hardly any jobs, yet allow people to pay a tax on whatever it may be.
No, the world public would not support such a tax, because there is no way that all nations would agree on a way to make it fair. There would also be no penalty that could actually be enforced for nations that did not pay. The purpose of the tax, which would likely be to help developing nations, already happens through the United Nations, and countless charities around the world.