Environmental Engineering Reference
In-Depth Information
96 Gt C, or roughly a net loss of 200 Gt of phytomass during the second millen-
nium of the Common Era. Other estimates of more recent losses attributable to
anthropogenic land use range from 136 Gt C for the years 1850-1998 (Bolin et al.
2001) to 200 Gt C for the years 1800-2000 (House, Prentice, and Le Quéré 2002).
Subtracting these estimates from about 900 Gt C of preindustrial terrestrial phy-
tomass would leave us with anywhere between 600 and 800 Gt C by the late
twentieth century.
In contrast to these values, the estimate of terrestrial phytomass stores for the
year 1950 offered by Whittaker and Likens (1975) seems to be too high at 1,837
Gt or about 920 Gt C, while Post, King, and Wullschleger (1997) modeled the
change in terrestrial carbon storage during the twentieth century and concluded that
it increased from 750 Gt C in 1910 to 780 Gt C in 1990. But the i rst assessment
report from the Intergovernmental Panel on Climate Change chose 550 Gt C as the
most likely value for the 1980s (IPCC 1990), and the third assessment listed two
global totals, a lower one of just 466 Gt C, from the German Advisory Council on
Global Change (WBGU 1998), and a higher one of 654 Gt C, from Saugier, Roy
and Mooney (2001), for the twentieth century's end. Of course, the compared values
for the LGM, the mid-Holocene, the preindustrial era, 1950, and the late twentieth
century come from studies that used a variety of approaches and assumptions and
whose only commonality is a signii cant margin of error.
This makes it impossible to make any clear quantitative conclusions, but the trend
appears to be indisputable and the relative magnitudes seem plausible. If we assume
that during the LGM the total continental carbon stores were about 500 Gt C lower
than at present and that at least 35%-40% of the LGM terrestrial carbon total was
in plants, the peak glacial vegetation stored 175-200 Gt C less than did the ter-
restrial biosphere around the year 2000, or close to 500 Gt C; Köhler and Fischer
(2004) put the most realistic range at 350-640 Gt C. By the mid-Holocene that
total could have more than doubled to more than 1,000 Gt C, and human activities
subsequently reduced it to no more than 900 Gt C by the onset of the industrial
era and, most likely, to less than 700 Gt C by the year 2000.
The general sequence is undoubtedly correct: a reduced phytomass during the
LGM, with a substantial gain during the Holocene (doubling does not seem exces-
sive, as the total area of tropical rain forest had roughly tripled between 18,000
and 5,000 years before present and that of cool temperature forests expanded more
than 30-fold [Adams and Faure 1998]), followed by millennia of gradual decline as
a result of the extension of cropland and wood harvests, and then by accelerated
deforestation losses since the mid-twentieth century. What lies ahead is uncertain
