Enormous amounts of chemical elements move throughout the surface of the Earth. This illustrates how much is moved by various natural processes or human activities, in units of petagrams (Pg = 1015 grams), teragrams (Tg = 1012), and gigagrams (Gg =109). Human activities include farming, mining, and construction. Natural processes include eroded material carried by the winds or transported by rivers, and chemical compounds mobilized by the growth and decay of living things, or net primary production (NPP). HANPP (human apportionment of net primary production) is a measure of the degree to which human activities, such as farming, have influenced net primary production. (Reprinted with permission from Environmental Science & Technology © 2012, American Chemical Society)
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