Environmental Engineering Reference
In-Depth Information
biomass in just two generations, well before any eventual leveling off of the global
population count.
I will abstain from any time-specii c forecasts, as they are bound to be wrong.
But I will identify those major factors whose interaction will determine future devel-
opments, and hint at the probabilities of the most important long-term trends. Three
major concerns stand out when probing the future of biomass productivity and
storage from the perspective of natural responses and feedbacks. The i rst is the
basic photosynthetic response, the future productivity of plants in a warmer world
with higher CO
2
levels: will they do, on the whole, better than now, will there be
little discernible difference on the global scale, or will the primary productivity
suffer? The second one is a closely connected concern about carbon uptake by the
biosphere, particularly by the forests: will they remain substantial sinks of carbon
or will their storage capacities weaken or will they shift into being net sources of
carbon to the atmosphere? The third one concerns the primary production in the
ocean: its waters, too, will be warmer and also more acid, conditions that raise
worries about the future phytoplanktonic productivity.
A great deal of recent research has explored the impacts of climate change on
primary productivity, particularly as far as agriculture and forestry are concerned:
convenient summaries are in the impact volume of the Intergovernmental Panel on
Climate Change's Fourth Assessment Report (Parry et al. 2007). The earliest studies
were coni ned to small-scale, short-term laboratory phytotron experiments; later
came all-season i eld experiments with crops subjected to elevated CO
2
levels, and
these arrangements were then replicated (with more difi culty and at a much greater
expense) in enrichment trials with trees; at the same time, advances in computer
modeling have provided somewhat more realistic global simulation of the bio-
sphere's response to a warmer and CO
2
-richer world. The existing atmospheric CO
2
concentration (now approaching 400 ppm, or 0.04% by volume) is much below
the level needed to saturate the photosynthesis of C
3
plants(whose species dominate
all forests) as well as to inhibit photorespiration, and we also know that higher
concentrations of CO
2
will improve the water-use efi ciency of C
3
species.
Warming will lengthen the growing seasons everywhere, and as it will intensify
the global water cycle, many regions will receive higher precipitation. As a result,
in a warmer world with more CO
2
, the NPP could increase even in areas where
precipitation would decrease, and entire biomes could continue to act as substantial
net carbon sinks. And while autotrophic respiration (R
A
) tends to increase with
increasing temperature, this response may be only transient, and plants growing
in warmer climates may acclimate and eventually respire at the same rate as those
