Because of Iceland's sparing use of hydrocarbon fuels, its air is cleaner than that of most industrialized nations. However, its water supply is polluted by excessive use of fertilizers, (current estimates put Iceland's yearly usage of fertilizers at 2,500 lbs per acre). Population increases in the cities also contribute to water pollution. Iceland has 170 cu km of renewable water resources with 6% used for industrial purposes. The nation's cities produce about 0.1 million tons of solid waste. Industrial carbon dioxide emissions totaled 2.1 million metric tons per year in 1996. Protected lands, which account for 9.5% of Iceland's total land area, include four national parks, with a total area of 619,300 ha (1,530,315 acres) and 27 nature reserves, covering 256,861 ha (634,714 acres). Principal environmental responsibility is vested in the Ministry of Social Affairs. As of 2001, one mammal species and one plant species were endangered. Endangered species include the leatherback turtle and four species of whales. The great auk has become extinct.