From the standpoint of the two parity conditions, the very long-term implication of quantitative easing is a gradual devaluation of the U.S. dollar (an increase in the dollar price $/FC of foreign currency). If this increased inflation risk was reflected in interest rates (so that real interest rates were held constant), the U.S. dollar would simply move along that gradually sloped PPP line, and likewise, foreign currencies would gradually appreciate against the dollar.
However, because of economic weakness and credit strains, coupled with the demand for Treasuries by the Fed, quantitative easing instead moves U.S. interest rates in the opposite direction, falling rather than rising. From the standpoint of interest rate parity, capital market equilibrium then requires the U.S. dollar to depreciate immediately, by a sufficient amount to set up the expectation of future appreciation in order to offset the shortfall of U.S. interest rate returns.
In short, quantitative easing is likely to induce what the late MIT economist Rudiger Dornbusch described as "exchange rate overshooting" - a large and abrupt shift in the spot exchange rate that occurs in order to align long-term equilibrium in the market for goods and services with short-term equilibrium in the capital markets.
This adjustment is depicted in the diagram below. In response to the monetary shock, a modest but long-term depreciation in the dollar (a rise in the U.S. dollar price of foreign currency) is required, depicted by the blue line. However, since nominal interest rates in the U.S. actually decline, ongoing equilibrium in the capital market requires that the U.S. dollar must be expected to appreciate over time by enough to offset the lost interest. As a result, quantitative easing is likely to result in an abrupt "jump depreciation" of the U.S. dollar (that is, a spike in the value of foreign currencies).
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