Environmental Engineering Reference
In-Depth Information
Two more studies came out in 1999, the i rst one echoing the just noted consensus
and concluding that the total terrestrial carbon addition since the LGM amounted
to 550-680 Gt (Beerling 1999), the other one ending up with LGM totals ranging
from 710 Gt C less to 70 Gt C more than the present level (François et al. 1999).
And two studies published in 2002 (for some reason, these reconstructions have
not been pursued after that date) came up with a very similar result: total terrestrial
carbon storage during the LGM was 821 Gt lower (Kaplan et al. 2002) or at least
828 Gt C (and as much 1,106 Gt C) lower (Otto et al. 2002) than at the beginning
of the twenty-i rst century. After reviewing most of these studies (whose simple
mean is about 650 Gt C), Maslin and Thomas (2003) concluded that the difference
between estimates centering on 500 Gt C and those averaging around 1,000 Gt C
can be explained by taking into account isotopically light emissions of CH
4
from
gas hydrates.
For the mid-Holocene (6,000 years before the present) we have two sets of cal-
culations: François et al. (1999) offered an inconclusive range of as much 132 Gt
C less and 92 Gt C more than at present, while Beerling's (1999) two models ended
up just 103 Gt C apart, with the lower total at 750 Gt of plant carbon and with
1,363 Gt C in vegetation and soils. As already noted in chapter 1, a reconstruction
of natural ecosystems based on 106 major plant formations found that the maximum
preagricultural standing phytomass was about 2,400 Gt, or around 1,200 Gt C
(Bazilevich, Rodin, and Rozov 1971), while a reconstruction by Adams et al. (1990)
found that the Earth's potential vegetation could store 924 Gt C.
By 2010 there were are at least eight explicit values for the preindustrial phyto-
mass carbon (usually assumed to apply to the year 1700), ranging from 610 to 1,090
Gt C, with the most realistic range between approximately 620 and 910 Gt C
(Köhler and Fischer 2004). The latest reconstruction of preindustrial terrestrial
phytomass storage used an unprecedented approach as it tested a global carbon
model against nearly 1,500 surface pollen spectra from sample sites in Africa and
Eurasia. That approach yielded a total plant storage of 907 Gt C, with the largest
share (about 20%) in tropical rain forest, i tting into the previously established most
likely range (Wu et al. 2009).
Pongratz et al. (2009) combined a global climate model and a closed carbon
cycle model with previous reconstructions of anthropogenic land-use changes (Pon-
gratz et al. 2008) to estimate that between 800 and 1850 carbon emissions (direct
from destroyed vegetation and indirect due to lowered NPP) amounted to 53 Gt,
and that another 108 Gt C were added between 1850 and 2000. The net effect
of these land-use changes (after accounting for the biosphere's carbon sinks) was
