How much revenue is hidden away in tax havens?

In light of the recent Panama Papers scandal, it is worth considering how much revenue is actually hidden away in tax havens. The most widely cited estimates seem to be from a paper by Zucman (2014; Journal of Economic Perspectives, Table 1). These estimates are cited in a recent report by Oxfam (p. 12), entitled 'Ending the Era of Tax Havens: Why the UK Government Must Lead the Way'. And very similar figures are offered in a recent report by the Tax Justice Network (p. 42), entitled 'The Price of Offshore Revisited'. 

Zucman reports revenue losses from offshore wealth of: $75 billion for Europe; $36 billion for the United States; and $190 billion for the world. Using GDP data from the World Bank, and tax revenues as a percentage of GDP from the Heritage Foundation, I calculated that these estimates represent losses of: 1% of revenues for Europe (0.4% of GDP); 0.8% of revenues for the United States (0.2% of GDP); and 1% of revenues for the world (0.2% of GDP). (Tim Worstall obtains similar figures here.)

It should be noted that these estimates are most likely conservative. As Zucman writes:

My method probably delivers a lower bound, in part because it only captures financial wealth and disregards real assets. After all, high-net-worth individuals can stash works of art, jewelry, and gold in “freeports,” warehouses that serve as repositories for valuables—Geneva, Luxembourg, and Singapore all have them. High-net-worth individuals also own real estate in foreign countries.

For comparison, tax revenues in the UK decreased by 4.5% between 2007 and 2009, over the course of the financial crisis.