Environmental Engineering Reference
In-Depth Information
biosphere's photosynthetic productivity, without rising harvests of cultivated and
wild phytomass, and without increasing contributions of animal foods produced
by domesticated species or by hunting wild mammals, birds, and i shes. The
biosphere has paid a considerable price for these great human gains as both its
total stock of standing phytomass and its overall primary productivity have declined
by signii cant amounts—as have the stocks of wild terrestrial and marine zoomass.
In closing this topic, I will review these long-term trends before making some
observations about the anthropogenic Earth and raising some possibilities, and
many more uncertainties, regarding the future of life on the third planet from
the Sun.
Biomass Changes
As I have noted repeatedly, all but a few quantii cations of global stores of biomass
rely on multiple assumptions, and such exercises can produce only more or less
acceptable approximations. Advances in remote sensing have greatly improved our
ability to monitor land-use changes, to map specii c land-cover categories, and
hence, in conjunction with i eld studies, to quantify the phytomass stocks of biomes
and ecosystems. Even so, estimates of total terrestrial phytomass at the end of
the twentieth century span an unhelpfully wide range between about 470 and
780 Gt C, and we cannot be sure if the most likely total is 550, 600, or 650 Gt C.
Not surprisingly, uncertainties regarding the evolution of phytomass stocks are even
greater.
Only a qualitative narrative is fairly clear: the most recent ice age reduced the
Earth's plant cover and hence the global phytomass stocks, which rebounded with
deglaciation; global storage peaked sometime during the mid-Holocene before the
more extensive human interferences (in the form of shifting and permanent cultiva-
tion, the grazing of domestic animals, a higher incidence of i res, and the extension
of settlements) began to change the natural land cover and reduce the phytomass
stores. These processes accelerated in the past two centuries, and even the substantial
post-1950 return of temperate forests has not been able to eliminate the overall net
loss of woody phytomass.
Quantifying all of this is another matter. The best conclusion is that during the
last glacial maximum, the land plants stored up to 200 Gt less carbon than they did
before the glaciation. A substantial gain during the Holocene—doubling does not
seem excessive, as the total area of tropical rain forest had roughly tripled between
18,000 and 5,000 years before the present and that of cool temperature forests had
