It has been estimated that global net primary productivity (NPP), terrestrial, aquatic and marine, equals about 225 petagrams. Of this, humans divert or use 42.6 petagrams, of which 7.2 petagrams are utilized and 35.4 petagrams are diverted. Thus, 19% of the total NPP is consumed or diverted by humans: 31% of the terrestrial and 2% of the marine NPP. But this is not all. Humans, by altering surface and atmospheric conditions, diminish the potential NPP of the planet. We cover potential productive land with asphalt and concrete; we make agricultural land from forests and savannahs; we desertify many formerly productive areas of the earth’s surface; we lower or destroy the productivity of many waterways and lakes. If we add the amount of NPP lost by these activities to the amount of NPP usurped by humans, we approximate 60.1 Pg, or 39% of total potential NPP. Whether or not these figures are accurate, we know that humans appropriate a huge amount of global NPP for their purposes. Ehrlich and Wilson (1991) and Vitousek, et al., (1986), among others, estimate that 40% of global NPP is co-opted by humans (the same amount as is consumed by animals and decomposers combined). A new paper (Rojstaczer, Sterling and Moore, 2001) estimates that human use of global terrestrial NPP amounts to between 10% and 55% (the great variability occurs because of the uncertainty in the important parameters).
1) Agriculture: Nearly 15% of the land surface of the earth is being used for crop agriculture, and another 6-8 % for pastureland. By converting forests to agricultural land (and by other means – pollution, sedimentation, etc.), we are reducing the primary productivity of the land. Agricultural land, pastures and other converted areas are much less productive than are forests, because of lower plant biomass and lower biodiversity. Additionally, only about 10% of the organic material produced on agricultural land is usable as food. If we include these losses, we can estimate that humans utilize almost 40% of the potential net primary productivity (NPP) of all of the terrestrial plant life on earth for their purposes (Vitousek, et al., 1986; Diamond, 1987). In addition to our direct usage of NPP, we affect the remaining 60% as well. Thus, we are shifting from ecosystems with very high NPP to those with low NPP.
According to one calculation, by converting seven million km2 of forest to pasture (a figure from about 15 years ago), we have lost 1.4 petagrams of net primary productivity, about equal to the amount of organic material that humans use directly per year (or 3% of the biosphere’s total net primary productivity). This is out of an estimated total of 149.6 petagrams of NPP per annum for the terrestrial areas of the planet (Vitousek, et al., 1986).
2. Timber: The conversion of land from forest to other purposes also reduces the NPP of the land, as less than 50% of the NPP of forest land is being utilized as timber.
3. Habitations, recreation: When land is converted for purposes of lawns, golf courses, etc., the organic matter produced on the converted land is not used, for the most part.