Environmental Engineering Reference
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yielded an unrealistically low assessment, while the high estimate required specula-
tion as to the NPP lost as a result of human actions. Their evaluation differed
from the 1986 account of Vitousek et al. in three important ways: it was limited
to the terrestrial NPP; they used the most recent data available at the time of
their analysis (many derived from continental- or global-scale measurements);
and they estimated the uncertainty ranges for all parameters with inadequate lit-
erature references and derived an estimate of NPP variability by using a stochastic
simulation.
Their mean result of HANPP—39 Gt of dry matter, or 20 Gt C—was equal to
32% of terrestrial NPP, matching almost precisely the intermediate value estimated
by Vitousek et al. (1986). This was a mere coincidence because most of the param-
eters used in this analysis had substantially different values. More important,
Rojstaczer, Sterling, and Moore (2001) concluded that the variance in their estimates
of parameters resulted in poorly constrained coni dence intervals of
27 Gt (14
Gt C) and hence in a more than i vefold range for HANPP of 12-66 Gt of dry
matter. Not surprisingly, they had to admit that “the error bounds are so wide that
mean estimates of HANPP like that obtained here and earlier have limited utility”
(Rojstaczer, Sterling, and Moore 2001, 2550). Translated into shares of NPP, their
results mean that at the end of the twentieth century, humans “appropriated” as
little as 10% and as much as 55% of all terrestrial photosynthesis.
The fourth attempt to quantify the global rate of human appropriation of pho-
tosynthetic products dei ned the measure as the amount of terrestrial NPP that is
required to produce foodstuffs and i bers consumed by humans, including the phy-
tomass that is lost during harvests as well as during the processing of harvested
tissue into i nal products (Imhoff et al. 2004a). Unlike the previous two attempts,
which were based on biome and global averages, this account also presented a rough
spatial distribution of the appropriation by using the FAO's data for 230 countries
in seven key categories: plant food, meat, milk, eggs, and wood for construction and
fuel, paper, and i ber.
To acknowledge the many inherent uncertainties, Imhoff et al. (2004a) presented
the HANPP estimates in low, intermediate, and high versions (amounting, respec-
tively, to 8, 11.54, and 14.81 Gt C), and assumed the annual NPP was 56.8 Gt C;
as a result, human appropriations in the different versions represented about 14%,
20%, and 26% of the biosphere's annual primary production. The global HANPP
mean of about 20% was derived from continental averages ranging (for intermedi-
ate values) from about 6% for South America to 80% for South Asia, with Western
Europe just above 70% and North America just below 25%. As would be expected,
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