Environmental Engineering Reference
In-Depth Information
can be used to constrain the global estimate of GPP, and found a shorter cycling
time of CO
2
with respect to the terrestrial biosphere and oceans than did previous
estimates. The authors concluded that the fast response is most plausibly accounted
for by revising the global GPP upward to the range of 150-175 Gt C/year.
That is an increase of 25%-45% above what Cuntz (2011) calls carbon's “gold
standard” of global GPP, around 120 Gt C/year; he also notes that gold does not
tarnish easily—and a closer look at the new value shows how some small shifts in
assumptions can bring the total closer to the prevailing standard.
Most notably, Welp at al. (2011) assume that 43% of all CO
2
molecules entering
through the stomata are eventually i xed, a rate that is highly dependent on other
assumptions and on the global partitioning of photosynthesis between the more
efi cient C
4
species (whose i xation rate is about 60%) and the less efi cient but
dominant C
3
plants. Reducing the overall i xation rate lowers the global GPP esti-
mate, and at 34% it would be 120 Gt C/year. Consequently, the only solid conclu-
sion we can make is that despite all the recent advances, our understanding of global
primary productivity will have a substantial margin of uncertainty (at least
±
15%)
for years to come.
As for the spatial and temporal variability of the NPP, satellite observations have
coni rmed many well-known phenomena (for example, during the i rst four years
of monitoring, the global NPP exhibited annual l uctuations up to nearly 4%), but
they have also brought some unexpected results. The NPP reaches its annual ter-
restrial maxima in excess of 1 kg C/m
2
in the equatorial zone: in the Amazon basin
and in the regions located immediately north and south of it, in the Congo basin,
in parts of eastern and western Africa, and across the Indonesian archipelago. High
productivities also prevail in the highlands of Central America and in the wettest
parts of Southeast Asia. A secondary NPP maximum is in the temperate mid-
latitudes of the Northern Hemisphere: most of Europe, eastern North America, and
China average 400-600 g C/m
2
, while most of Asia's interior and Australia's interior,
as well Siberia and western North America, have means below 300 g C/m
2
.
