Environmental Engineering Reference
In-Depth Information
expanded more than 30-fold (Adams and Faure 1998)—could have raised the stocks
to more than 1,000 Gt C, with subsequent land-use changes reducing it to no more
than 900 Gt C by the eighteenth century, with a total of 750-800 Gt C the most
likely value. Plant carbon losses during the last two centuries amounted most likely
to 150-200 Gt C, lowering the late twentieth-century terrestrial stocks to no more
than 650 Gt C, and perhaps even below 600 Gt C.
Human actions have thus reduced the biosphere's phytomass by as much as
35%-40% from its preagricultural level. During the twentieth century the net reduc-
tion in global phytomass was about 110 Gt C, or about 15% of the 1900 total—but,
concurrently, the phytomass of i eld crops increased i vefold.
Accurate knowledge of long-term changes of the terrestrial phytomass would
make it possible to quantify with greater coni dence the partitioning of carbon
emitted from fossil fuel combustion, as well as the magnitude of potential terrestrial
carbon sinks and the interactions of carbon and nitrogen cycle. And, obviously,
our assessment of cumulative anthropogenic losses of biodiversity would be very
different if we could assert that we have lost only 25% rather than 40% of the peak
postglacial phytomass.
We are on i rmer ground when appraising the conversion of natural ecosystems
to i elds and the global expansion of cropping driven by growing populations and
by the universal dietary transition from an overwhelmingly vegetarian diet to one
containing a higher share of animal protein. By the middle of the eighteenth century
the global area of farmland was still only about 350 Mha; by 1900 it was 850 Mha,
and by 2010 the land used for annual and permanent crops had surpassed 1.5 Gha,
claiming about 12% of all ice-free land, but the peak seasonal preharvest phytomass
of global cropland is less than 0.5% of all plant mass. The obverse of these gains
was a major loss of temperate grasslands and tropical forests. After 1850, most of
North America's and Russia's new cropland came from plowing up grasslands; in
the tropics, most new i elds came from deforestation. In total, ecosystem conversions
led to the loss of at least 150 Gt of plant carbon between 1850 and 2000 (Houghton
2003).
But perhaps the most impressive way to illustrate the extent of human impacts
on the stocks of global organic matter is to trace the gains and losses of mammalian
biomass, that is, the increasing mass of humanity (anthropomass) and its domesti-
cated animals and the declining zoomass of wild terrestrial animals, particularly
of the largest herbivores and anthropoid primates. Again, the need to resort to
chained assumptions precludes a high degree of accuracy, but conservatively made
calculations not only reveal the magnitude of indisputable secular trends but can
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