The Zambian economy has historically been based on the copper mining industry. The output of copper had fallen to a low of 228,000 metric tons in 1998 after a 30-year decline in output due to lack of investment, low copper prices, and uncertainty over privatisation. In 2002, following the privatisation of the industry, copper production rebounded to 337,000 metric tons. Improvements in the world copper market have magnified the effect of this volume increase on revenues and foreign exchange earnings.