Zambia has had one of the world’s fastest growing economies for the past ten years, with real GDP growth averaging roughly 6.7% per annum. Privatization of government-owned copper mines in the 1990s relieved the government from covering mammoth losses generated by the industry and greatly increased copper mining output and profitability, spurring economic growth. Copper output increased steadily from 2004, due to higher copper prices and foreign investment, but weakened in 2014 when Zambia was overtaken as Africa’s largest copper producer by the Democratic Republic of Congo. Zambia's dependency on copper makes it vulnerable to depressed commodity prices, but record high copper prices and a bumper maize crop in 2010 helped Zambia rebound quickly from the world economic slowdown that began in 2008. Despite strong economic growth and its status as a lower middle-income country, widespread and extreme rural poverty and high unemployment levels remain significant problems, made worse by a high birth rate, a relatively high HIV/AIDS burden, and by market-distorting agricultural policies. Economic policy inconsistency and poor budget execution in recent years has hindered the economy and contributed to weakness in the kwacha, which has been Africa’s worst performing currency during the past year. Zambia has raised $1.75 billion from international investors by issuing separate sovereign bonds in September 2012 and April 2014, significantly increasing the country’s public debt as a share of GDP. On January 1, 2015, a new mineral royalty tax regime dramatically increased mining taxes, and has led to an economic impasse between the government and the mines. If the new tax is left intact, it could result in the closure of less profitable mines, the loss of thousands of jobs and could stifle Zambia’s ability to attract additional foreign investment.