public debt was $909billion in 1980, an amount equal to 33% of America's gross domestic product (GDP); by 1990, that number had more than tripled to $3.2trillion56% of GDP. In 2001 the national debt was $5.7trillion; however, the debt-to-GDP ratio remained at 1990 levels. Debt levels rose quickly in the following decade, and on January 28, 2010, the U.S. debt ceiling was raised to $14.3trillion. Based on the 2010 United States federal budget, total national debt will grow to nearly 100% of GDP, versus a level of approximately 80% in early 2009. The White House estimates that the government's tab for servicing the debt will exceed $700billion a year in 2019, up from $202billion in 2009. The U.S. Treasury statistics indicate that, at the end of 2006, non-US citizens and institutions held 44% of federal debt held by the public. , China, holding $1.26trillion in treasury bonds, is the largest foreign financier of the U.S. public debt. The overall financial position of the United States as of 2014 includes $269.6trillion of assets owned by households, businesses, and governments within its borders, representing more than 15.7 times the annual gross domestic product of the United States. Debts owed during this same period amounted to $145.8trillion, about 8.5 times the annual gross domestic product. Since 2010, the U.S. Treasury has been obtaining negative real interest rates on government debt. Such low rates, outpaced by the inflation rate, occur when the market believes that there are no alternatives with sufficiently low risk, or when popular institutional investments such as insurance companies, pensions, or bond, money market, and balanced mutual funds are required or choose to invest sufficiently large sums in Treasury securities to hedge against risk. American economist Lawrence Summers argues that at such low rates, government debt borrowing saves taxpayer money, and improves creditworthiness. In the late 1940s through the early 1970s, the US and UK both reduced their debt burden by about 30% to 40% of GDP per decade by taking advantage of negative real interest rates, but there is no guarantee that government debt rates will continue to stay so low. In January 2012, the U.S. Treasury Borrowing Advisory Committee of the Securities Industry and Financial Markets Association unanimously recommended that government debt be allowed to auction even lower, at negative absolute interest rates. Currency and central bank The United States dollar is the unit of currency of the United