Environmental Engineering Reference
In-Depth Information
11.9 Climate forcing by greenhouse gases and other atmo-
spheric constituents (W/m
2
), 1880-2000. From Hansen,
Sato, et al. (2000).
of about 0.6
C is pending unless it is counteracted by
complex feedbacks. The eventual doubling of the prein-
dustrial CO
2
, which was about 280 ppm in 1850, would
most likely raise average tropospheric temperatures by
2
C-4.5
C and cause relatively rapid global climate
change, including unevenly distributed higher seasonal
temperatures (faster warming in polar regions), acceler-
ated water cycle, changed precipitation patterns, and ris-
ing ocean level (Alley et al. 2007). As the surface ocean
absorbs additional CO
2
, its pH falls; this may decline by
as much as 0.4 units by 2100 (Orr et al. 2006). The po-
tentially destabilizing environmental, health, and socio-
economic consequences of these changes have become a
major topic of interdisciplinary research and public policy
concern (Houghton et al. 2001; Epstein and Mills 2005).
In contrast to omnipresent carbon, nitrogen is a rela-
tively rare element in the biosphere. It is absent in cellu-
lose and lignin, the two dominant molecules of terrestrial
life, and only plant seeds and lean animal tissues have
high protein content (overall 10%-25%, soybeans to
40%). Although nitrogen is an indispensable ingredient
of all enzymes, its total reservoir in living matter is less
than 2% of carbon's huge stores, and whereas C cycling
includes large flows entirely or largely unconnected to
the biota, every major link in nitrogen's biospheric cycle
is mediated by bacteria (Smil 1985; 2000b). Human in-
terference in the nitrogen cycle is largely due to food
production: recycling of organic wastes, planting of legu-
minous crops (whose symbiotic bacteria can fix N), and
above all, applications of ammonia-based N fertilizers
that eliminated the nitrogen limit on crop production
and allowed for the expansion of human population
(Smil 2001). As already noted (section 10.2), the Haber-
Bosch synthesis of NH
3
and subsequent production of
solid and liquid N fertilizers consumes annually about 5
EJ, or roughly 1.5% of the world's TPES.
