Environmental Engineering Reference
In-Depth Information
any case, the net l uxes in the Amazon and Congo basins will be driven primarily
by deforestation, and continued warming should eventually see a gradual expansion
of forests into the latitudes and altitudes formerly occupied by tundra and mountain
grasses, but the eventual extent of this process is unclear.
The outlook for crop response is also uncertain. There will be a direct effect on
C
3
crops, as higher CO
2
levels increase the stomatal uptake of the gas, but the
eventual boost in average yields could range from marginal to substantial. Early
chamber studies with plants usually grown in pots indicated that a doubling of
preindustrial CO
2
levels (concentrations of about 550 ppm) boosted rice and soybean
yields by about 20% and wheat yields by as much as 30%, but numerous subsequent
FACE (free-air concentration enrichment) studies (typically using 20-m-diameter
plots within actual crop i elds) indicated gains only about half those seen in enclo-
sure experiments (Long et al. 2006). The rice yield could be thus less than 10%
higher and wheat yields no more than 13% better. But higher gains are possible
with concurrent nitrogen enrichment, and lower gains may be caused by excessive
tropospheric ozone levels, now a major concern in many agricultural regions sur-
rounding the megacities (Fuhrer 2009).
C
4
crops—corn, sorghum, and millets, amounting to about 40% of the world's
grain harvest, and sugarcane—do not respond directly to higher CO
2
levels, and
their yields will increase only in times and places of drought (Leakey 2009). And in
food production, quality also matters. All hard spring wheat cultivars released
between 1903 and 1996 responded to higher CO
2
levels with increased vegetative
growth and increased seed yield, but the gains were lower with the most recent
cultivars, and the quality of grain was lower in all cases (Ziska, Morris, and Goins
2004). Hogy et al. (2009) found not only signii cant decreases in total grain protein
but, more important, a decline in gluten proteins, the all-important gliadins that
make the best bread and pasta dough.
And plant carbon, whether in herbaceous or woody species, cannot be considered
in isolation. All densely inhabited regions now receive unprecedented amounts of
reactive nitrogen from atmospheric deposition, and this enrichment has already
enhanced the growth and carbon storage of not only annual crops but also perennial
herbaceous species and trees (Smil 2000; Elser et al. 2010; Thomas et al. 2010). At
the same time, a warmer world could also produce a very worrisome feedback by
releasing signii cant amounts of soil carbon. Soils store more than twice as much
carbon as do biota or the atmosphere, and accelerated decomposition producing
additional releases of CO
2
could reinforce the warming. This response was con-
i rmed not only by models and small-scale experiments but also (as already noted)
by some large-scale analyses (Bellamy et al. 2005). At the same time, an exceptional
