Environmental Engineering Reference
In-Depth Information
Twenty Millennia of Phytomass Change
Given the lasting uncertainties in quantifying the aboveground phytomass in today's
ecosystems (leaving aside even greater errors in estimating the belowground biomass
and its productivity), it may seem audacious to model global phytomass stores going
back 20,000 years, to the last glacial maximum (LGM), or even “just” to about
10,000 years ago, to the preagricultural era of the early Holocene, or 5,000 years
ago, to the mid-Holocene, the time of the i rst complex civilizations. The inherent
uncertainties of such attempts are obvious, but the uncertain totals should be able
to convey the magnitude of change between the LGM, when only small bands of
dispersed hunters (totaling no more than few hundred thousand people) roamed
the Earth, whose northern latitudes were covered by massive continental glaciers,
and the era of maximum phytomass storage during the preagricultural period, when
forests occupied large areas of Eurasia and North America.
Different studies have used global climate models and paleoecological, palyno-
logical, pedological, and sedimentological evidence to reconstruct past vegetation
cover and carbon storage, usually expressing the latter as a combination of vegeta-
tion and soil carbon, with some studies including and others excluding the post-
LGM peatland growth. Not surprisingly, there have been some extreme i ndings.
After reconstructing past ecosystem distributions, Adams and Faure (1998) con-
cluded that the total terrestrial carbon storage (plants, soils, and peat) 18,000 years
before the present was only 931 Gt, compared to a total of 2,130 Gt C for the late
twentieth century—the total arrived at by adding 560 Gt C in plants (Olson, Watts,
and Allison 1983), 1,115 Gt C in nonpeat soils (Post et al. 1982), and 461 Gt C
in peatlands (Gorham 1991)—a nearly 2.3-fold increase, for a net gain of about
1,200 Gt.
In contrast, Prentice and Fung (1990) claimed that (excluding peat) there was no
signii cant increase in overall terrestrial carbon storage, putting the overall LGM-
to-present carbon gain at 0
50 Gt C: this would mean that the terrestrial biosphere
was no (or not a major) carbon sink during the period of rapid deglaciation. But
Prentice et al. (1993) presented a new model that showed the late twentieth-century
terrestrial carbon storage to be at least 300 Gt C and as much as 700 Gt C higher
than during the LGM. Moreover, seven other studies published by 1998 converged
to a very similar range, indicating a gain of about 30% in the overall terrestrial
carbon storage (in absolute terms, mostly 300-700 Gt C) from the time of the LGM
to the late twentieth century (Peng, Guiot, and Van Campo 1998).
±
