Investors across the globe are scooping up Treasurys, an asset whose main selling point until recently was safety. Overseas buyers, led by China, are buying U.S. debt to gather dollars and help weaken their currencies as a way to boost exports. The purchases are taking place while the supply of U.S. government debt is shrinking, due to an improving economy and narrower budget deficit. Strong global demand, shrinking supply and uneven economic growth have helped to limit the impact of reduced Treasury purchases by the Federal Reserve, fueling a price rally that few analysts predicted.