I originally asked that question in a post dated May 17 2005. It was a speculative question at the time, but along some dimensions the question has been answered. From the Financial Times:
The euro has displaced the US dollar as the world’s pre-eminent currency in international bond markets, having outstripped the dollar-denominated market for the second year in a row.
The data consolidate news last month that the value of euro notes in circulation had overtaken the dollar for the first time...
Outstanding euro-denominated debt accounts for 45 per cent of the global market, compared with 37 per cent for the dollar. New issuance last year accounted for 49 per cent of the global total.
That represents a startling turnabout from the pattern seen in recent decades, when the US bond market dwarfed its European rival: as recently as 2002, outstanding euro-denominated issuance represented just 27 per cent of the global pie, compared with 51 per cent for the dollar.
What's up?
The rising role of the euro comes amid growing issuance by debt-laden European governments. However, the main factor is a rise in euro-denominated issuance by companies and financial institutions.
One factor driving this is that European companies are moving away from their traditional reliance on bank loans – and embracing the capital markets to a greater degree.
Another is that the creation of the single currency in 1999 has permitted development of a deeper and more liquid market, consolidated by a growing eurozone.
This has made it more attractive for issuers around the world to raise funds in the euro market. And, more recently, the trend among some Asian and Middle Eastern countries to diversify their assets away from the dollar has further boosted this trend.
Are the days of the US dollar as the predominant world currency over? Maybe not. As was noted in the research of Menzie Chinn and Jeffrey Frankel cited in my 2005 post, there are many factors that determine which country's monetary assets become the leading international reserve currency. But to the question of what currency will emerge, we are increasingly ticking off items on the list that would make the answer "the euro."