The U.S. remains the leading retirement market, accounting for more than half of the world’s total retirement assets of nearly $28 trillion, according to a new report.

The report by Allianz Life Insurance Co. of North America, Minneapolis, Minn., forecasts the U.S. retirement market will dominate the world pension market until the end of the decade even though it is only expected to grow by a 3.6% compound annual growth rate (CAGR).

Despite its low growth, the net increase in the U.S. during this period will equal the total volume of continental Western Europe today, the report says.

In size, the U.S. retirement market is followed at 11.5% by the United Kingdom. Western Europe’s combined retirement assets came to slightly more than 20% while Australia and Japan each claim 3% of the global market.

The emerging economies of Asia and central and eastern European, which are still in the early stages of building up their individual funded pension systems, represent only minor shares (1.8% and 0.4%, respectively), Allianz finds.

The report expects global retirement assets to accumulate to $46 trillion by 2020. The global retirement market is forecast to grow by 66%, representing an annual growth rate of 4.7%.

Total pension assets would increase to $46 trillion from $28 trillion, the study says.

At year-end 2008, global pension assets stood at $26 trillion, down by roughly 15% from year-end 2007 ($30 trillion), the study finds.