Natural systems absorb some, but not all, of the excesses of human activity. Alkaline solid can neutralize acidic gases washed out of the air; forests, grasslands and plankton can absorb carbon dioxide; some waste decomposes and some nutrients are recycled. But few human activities are sustainable in the long term.
The earth is mined for building materials, metals, chemicals and fuels. Powerful machinery, industrial processing and international trade mean that individual deposits are exploited on a scale far beyond local needs. An estimated23 billion tones of nonfuel minerals are extracted each year, about twice the amount of sediment carried each year by the world’s river systems. As the most concentrated ore deposits become exhausted, lower grade ores are used, such that to produce an estimated nine million tons of copper in 1990,990 million tons of ore had to be mined. The open cast Bingham Canyon copper mine in Utah, USA, 2500 ft deep, is the largest human excavation in the world.