The world’s major central banks have expanded their balance sheets to unprecedented levels during the past five years. The Bank for International Settlements (BIS) estimates the aggregate assets of all the world’s major central banks now stands at $18 trillion, roughly 30% of global GDP and double the ratio of a decade ago, according to its annual report published on June 24. Most of that sits on the balance sheets of the Federal Reserve, the European Central Bank, the Bank of Japan, the Bank of