Environmental Engineering Reference
In-Depth Information
complexity of stores and l uxes of soil organic carbon precludes any unequivocal
conclusions (Davidson and Janssens 2006).
All of this makes any large-scale, long-term appraisals of future net trends highly
uncertain, and there is no consensus even as to the recent net global values. Between
mid-1991 and mid-1997, the biosphere sequestered annually 1.4 (
±
0.8) and 2
0.6) Gt C, in contrast to the 1980s, when the land biota were basically neutral
(Battle et al. 2000). Wigley and Schimel (2000) put the additional sequestration
due to the CO
2
fertilizing effect at 1.2-2.6 Gt C/year, but Potter et al. (2003)
concluded that between 1982 and 1998, the carbon l ux for the terrestrial biosphere
ranged widely between being a source of 0.9 Gt C/year to being a large sink of
2.1 Gt C, and Houghton (2003) thought that the terrestrial biomes were a source
of 0.7 Gt C during the 1990s. But Ballantyne et al. (2012) concluded that between
1960 and 2010 the net global carbon uptake (by land and ocean) has increased
signii cantly, by about 50 Mt C/year, and that the total uptake doubled to about
5 Gt C/year. Moreover, global terrestrial carbon l uxes appear to be about twice
as variable (with precipitation and surface solar irradiance being the key drivers)
as ocean l uxes, and at different time periods they can be dominated by either
tropical or mid- and high-latitude ecosystems (Bousquet et al. 2000).
These are some of the basic qualitative conclusions that might withstand the test
of time: the carbon sequestration potential of croplands has most likely been over-
estimated (Smith et al. 2005); the role of old forests (generally thought to be insig-
nii cant sinks of carbon) has been almost certainly (and signii cantly) underestimated
(Carey et al. 2001); the availability of nitrogen will be a key factor in carbon seques-
tration in many forests (De Vries et al. 2006); and, given the pressures to reduce the
nitrogen loading of European ecosystems, it is reasonable to expect that the conti-
nent's future net carbon intake will be limited (Janssens et al. 2005). Obviously, the
actual outcomes will be critically dependent on the eventual level of CO
2
enrich-
ment, on the degree of other atmospheric and biospheric changes associated with
climate change, and on the supply of nitrogen.
Not surprisingly, when Schaphoff et al. (2006) tested a global vegetation model
in conjunction with i ve general atmospheric circulation models, they found that the
additional carbon storage by the year 2100 ranged from -106 to +201 Gt, making
even the direction of the likely change uncertain. And when it is unclear whether
the biosphere will respond as a source or a sink of carbon, then the only truly
defensible conclusion regarding the long-term effects of global climate change on
plant productivity, carbon sequestration, and associated changes in water supply,
diseases, and pests is not a string of catastrophic predictions but an honest acknowl-
edgment that our ignorance still trumps our knowledge. That is why I was glad to
(
±
