Environmental Engineering Reference
In-Depth Information
totals of specii c harvests. As expected, different authors have their own preferred
numbers and simplii cations.
The i rst study of this kind produced a range of appropriation shares from just
3% to about 39% (Vitousek et al. 1986). The second one (Wright 1990) produced
a share of about 24%; the third one (Rojstaczer, Sterling, and Moore 2001), 32%
(but its coni dence interval indicated possible appropriations ranging from 10% to
55%); the fourth one (Imhoff et al. 2004a), a range of 14%-26%; and the last one
(Haberl et al. 2007) added up to nearly 24% of potential NPP that is “appropri-
ated” or “co-opted” by humans. There is no need to extend this small list of ques-
tionable rates: even the most careful and eminently defensible selections of analytical
approaches and key data inputs could still result in shares differing by impractically
wide margins (easily 50%), too uncertain to offer any practical policy guidance.
And, as I have already noted in my detailed deconstruction of the concept in chapter
11, this poorly dei ned measure offers only rough quantitative impressions, as it
completely ignores any qualitative consequences of phytomass harvests.
Accounting just for the biomass that is actually harvested by humans should be
a simpler task, with more reliable results: after all, i eld crops, i sh landings, and
timber cuts are now overwhelming part of national and global markets, and their
output and consumption are closely monitored. But as we have seen, some major
uncertainties remain, and hence any claims of high accuracy must also be suspect.
Historical records of crop harvests (often given as multiples of planted seed mass)
are only good enough to trace centuries of very low and stagnating yields. Plant
improvement proceeded very slowly until the i rst determined experiments of the
seventeenth and eighteenth century, and it really took off only after Mendelian
genetics opened up new opportunities (Kingsbury 2010).
The most important result of these efforts has been a steady rise in harvest indices
(HI), and the most obvious outcome of that trend has been the shortening of cereal
straws. Traditional cultivars were taller than an average person (see the i gures at
the beginning of chapter 4 and part II). Even in 1900 many wheat cultivars were
still more than 1 m tall, while today the shortest varieties, carrying a dwari ng
Norin-20 gene, are only about 50 cm tall. Higher HI values, denser planting, an
optimum nutrient supply, and applications of herbicides and pesticides boosted the
cereal yields (with national averages often more than doubling) during the twentieth
century.
Better data allow fairly reliable global reconstructions of crop harvests for the
entire twentieth century. In 1900 the worldwide harvests of food and feed crops
amounted to about 400 Mt of dry matter. By 1950 that total had doubled, by 1975
