Tensions are rising over Chinese economic policy, and rightly so: China’s policy of keeping its currency, the renminbi, undervalued has become a significant drag on global economic recovery. Something must be done.

Paul Krugman

To give you a sense of the problem: Widespread complaints that China was manipulating its currency — selling renminbi and buying foreign currencies, so as to keep the renminbi weak and China’s exports artificially competitive — began around 2003. At that point China was adding about $10 billion a month to its reserves, and in 2003 it ran an overall surplus on its current account — a broad measure of the trade balance — of $46 billion.

Today, China is adding more than $30 billion a month to its $2.4 trillion hoard of...