Geoscience Reference
In-Depth Information
Land-use/deforestation
Atmosphere 762
†
+ 3.2
per annum
*
†
2 6.3* (6.4
†
)
102
50 50
92
90
Land/atmosphere
flux
≈
-1.4*
Ocean/atmosphere
flux
≈
-1.7* (-2.2
†
)
Terrestrial
animals and plants
5
50
50
Soil and detritus
Rivers
Surface ocean 1000 + 1
per annum
1500
0.8
Plants and animals
36
3
40
4
5
37
Intermediate & deep waters
Fossil fuel
38 000 + 2
per annum
0.2
Sedimentation
Fig. 1.3
Broadestimatesoftheprincipalannualcarbonsourcesandsinksingigatonnesofcarbon(GtC),and
approximateannualmovementsofcarbonaboutthecarboncycleingigatonnesofcarbon
perannum
(GtC
year
−1
)averageforthe1990s.Itisimportanttorealisethatthereareanumberofuncertaintiesthatarethe
subjectofcurrentresearch(seetext)andalso,becausewearechangingthecycle,itwillbediferentfor
diferentdecades.Togiveasenseofhowunderstandingchangesandtheuncertaintiesinvolved,theigure
includes2001IPCCestimatesforannualluxesover1990-9,marked*,and2007IPCCestimatesforthesame
decade'sannualaverages,marked
.
†
A first impression of the biology associated with the carbon cycle might focus on
the plant activity sequestering carbon dioxide from the atmosphere. But this is just one
aspect, albeit a key one. There is also plant and animal respiration, and the respiration
of bacteria and fungi, returning carbon dioxide to the atmosphere. We might also think
of biomes (climatically determined regional groups or assemblages of ecosystems)
and how they affect the global carbon cycle in terms of marine productivity, or of
the carbon in a biomes' biomass, be it in terrestrial tropical rainforests or temperate
wetlands, for example. We might also consider the effect of climate change on such
ecosystem assemblages and individual ecosystems; and indeed we will do so later in
the topic. But there is also the biomolecular dimension.
Included in the biomolecular perspective is the role enzymes play. Rubisco
(ribulose-1,5-bisphosphate carboxylase oxygenase, which is sometimes portrayed
in print as RuBisCO) is the most common enzyme on the planet and is fundamental
to photosynthesis. It is therefore probably the most common protein on Earth; it con-
stitutes about half of leaf proteins and is synthesised in chloroplasts. All the carbon
dioxide captured by photosynthesis - in algae and multicellular plants - is handled
by this one enzyme. That is about 200 billion t (or 200 Gt) of carbon a year! Another
important enzyme is carbonic anhydrase, which catalyses the hydration of about a
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