Environmental Engineering Reference
In-Depth Information
the fertilization process plays different roles in rich and
poor countries. In affluent nations it helps to produce
rich diets with plenty of animal foods and surplus food
for export; in low-income countries it prevents wide-
spread malnutrition.
Relative levels of agricultural energy subsidies depend
on TPES. In affluent countries, with high energy use
in households and transportation, they claim 3%-5% of
TPES, whereas in some large populous nations the shares
may be closer to 10% (Smil 2006; FAO 2000). Several
detailed accounts of energy use in U.S. agriculture
(Steinhart and Steinhart 1974; USDA 1980; Stout, But-
ler, and Garett 1984) found very similar shares in 1970
(3%), 1974 (2.9%), and 1981 (3.4%). By the late 1990s
more efficient production and use of fertilizers, a slower
growth rate in the use of pesticides, and more widespread
reliance on reduced tillage lowered the share of energy
subsidies in U.S. crop farming to about 2% of the coun-
try's TPES (Collins 2000). As China's TPES expanded,
the share of agricultural subsidies, as high as 15% during
the 1980s, fell to about 10% by the year 2000 (Smil
1992; NBS 2000).
A conservative approximation of energy subsidies used
by global crop farming at the beginning of the twenty-
first century is 12.8 EJ, equivalent to 300 Mtoe or 8.4
GJ/ha under annual or permanent crops. About 2 EJ
goes to produce and maintain agricultural machinery
(tractors, combines, implements, irrigation systems),
about 5 EJ to power it, the same amount to extract, syn-
thesize, and distribute fertilizers, 500 PJ to make pesti-
cides and herbicides, and 300 PJ to build irrigation
systems and deliver water to fields. This prorates to aver-
age global power density of about 25 mW/m
2
. The
highest regional means are 1 OM higher. Such low
rates reflect the intermittent tasks of crop farming spread
over large areas. The energy cost of animal husbandry
(excluding the energy content of feed), aquaculture, and
fishing would increase this total by no more than 4 EJ. A
17-EJ share used in global food production would repre-
sent less than 5% of the world's TPES in the year 2000.
For comparison, Giampietro (2002), whose global calcu-
lation also included the cost of preparing animal feeds
and energy invested in the cultivation of forages, arrived
at a total of 18.2 EJ (433 Mtoe) for 1997.
Unfortunately, there is no suitable measure to evaluate
the efficiency of these energy subsidies, a fact that has not
stopped such efforts. Black (1971) introduced the quo-
tient of harvested food energy and energy invested in
the growing process (animate or inanimate) as an effi-
ciency ratio of farming systems. This approach has been
used repeatedly to illustrate relatively high returns of tra-
ditional cropping powered by animate energies (ratios of
10-30; see chapter 6), low and declining energy gains of
modern intensive crop cultivation (grain cropping 2-8,
fruit growing@1, vegetable cultivation 0.1-1), and sub-
stantial energy losses incurred by all modern animal pro-
duction systems (ratios as low as 0.05 for lean red meat
and no higher than 0.5 for milk).
Critics of modern agriculture see such ratios as perfect
proof of the dubious nature of subsidized farming, but
the ratios are inappropriate and should be avoided (Fluck
1979; Smil, Nachman, and Long 1983). To the un-
informed they misleadingly suggest a direct link, a direct
conversion of input energies into food outputs. Any
inference that fuel energy is converted to food is wrong.
The relevant energy conversion is photosynthesis, and
the subsidies merely remove or moderate some factors
limiting NPP and help to channel the photosynthates
into target harvest tissues (that is, maximizing NEP and
the harvest index; see section 3.3). If higher energy
