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because our understanding of NPP in a warmer world with higher atmospheric CO 2
levels is a mixture of coni dent conclusions and unsatisfactory speculations.
Thanks to our knowledge of the biophysical and biochemical processes that
govern photosynthesis, we can make some useful predictions regarding the short-
term plant response. Indisputably, the current partial pressure of the atmospheric
CO 2 (at about 390 ppm, or nearly 0.04% by volume) is considerably below the
concentration needed to saturate the photosynthesis of C 3 plants, the dominant
species in both natural and managed ecosystems: species-specii c responses show
saturation at levels that are twice to three times the current tropospheric concen-
tration. Moreover, C 4 species, whose optimum productivity is above 30°C, would
benei t from warmer temperatures. This means not only enhanced but also more
efi cient photosynthesis operating (thanks to reduced stomatal conductance) with
higher water-use efi ciency.
Models of global NPP based on satellite observations indicated that climate
change has already eased the CO 2 and temperature constraints on plant productivity
and that between 1982 and 1999, the annual rate rose by 6%, or about 3.4 Gt C
(Nemani et al. 2003). But during that time global photosynthesis was also boosted
owing to volcanic aerosols that were emitted all the way to the stratosphere by the
Mount Pinatubo eruption in 1991: that effect resulted from the fact that plant cano-
pies use diffuse radiation more efi ciently than they use direct-beam radiation (Gu
et al. 2003). On the other hand, a pronounced European heat wave during the
summer of 2003 depressed GPP by nearly a third and turned the continent tempo-
rarily into a anomalously signii cant (about 500 Mt C) net source of carbon (Ciais
et al. 2005). And the best estimates for the decade between 2000 and 2009 indicate
that the record-breaking average temperatures accompanied by extensive droughts
had a global effect on terrestrial NPP, reducing it by as much as 2 Gt C/year (in
2005) and depressing it, on the average, by 0.55 Gt C/year (Zhao and Running
2010).
Should extreme heat events become more common as the average global tem-
peratures rise, then both trends would have a signii cant cumulative impact on
biospheric carbon storage. Long-term responses that would require acclimation
and shifts of vegetation boundaries are extremely difi cult to quantify because
of dynamic links among NPP, radiation, temperature, and precipitation (both in
terms of averages and in terms of seasonal and monthly l uctuations), atmospheric
CO 2 levels, and nutrient availability. A single modeling exercise sufi ces to illustrate
these uncertainties. Schaphof et al. (2006) applied different climate change scenarios
based on i ve global circulation models—with the global temperature averages in
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