From The New York Times:

THE 19th century was dominated by the British Empire, the 20th century by the United States. We may now be entering the Asian century, dominated by a rising China and its currency. While the dollar’s status as the major reserve currency will not vanish overnight, we can no longer take it for granted. Sooner than we think, the dollar may be challenged by other currencies, most likely the Chinese renminbi. This would have serious costs for America, as our ability to finance our budget and trade deficits cheaply would disappear.

Traditionally, empires that hold the global reserve currency are also net foreign creditors and net lenders. The British Empire declined — and the pound lost its status as the main global reserve currency — when Britain became a net debtor and a net borrower in World War II. Today, the United States is in a similar position. It is running huge budget and trade deficits, and is relying on the kindness of restless foreign creditors who are starting to feel uneasy about accumulating even more dollar assets. The resulting downfall of the dollar may be only a matter of time.

But what could replace it? The British pound, the Japanese yen and the Swiss franc remain minor reserve currencies, as those countries are not major powers. Gold is still a barbaric relic whose value rises only when inflation is high. The euro is hobbled by concerns about the long-term viability of the European Monetary Union. That leaves the renminbi.