10. The global carbon cycle. Biological feedbacks
Description
This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
10. The global carbon cycle. Biological feedbacks
Here, it's tempting to list some numbers :-)
Gt = gigatonne = 10**9 metric tonnes,
the mass of one cubic kilometre of water
1 GtC corresponds to ~3.67 Gt CO2
2.12 GtC or ~7.8 Gt CO2 correspond to 1 ppmv CO2 in the
atmosphere. ppmv = parts per million by volume
Carbon reservoirs in GtC
Atmosphere (1990) 750 Surface ocean 1000
Terrestrial vegetation 600 Marine biota 3
Soils & detritus 1600 Dissolved organic carbon 700
Deep ocean 38000
Natural carbon fluxes in GtC/year, <--> denotes a two-way flux
Atmosphere --> terrestrial vegetation 120 Photosynthesis
Terrestrial vegetation --> atmosphere 60 Respiration
Terrestrial vegetation --> soils & detritus 60
Soils & detritus --> atmosphere 60 Respiration
Atmosphere <--> surface ocean 90
Surface ocean <--> deep ocean 100
Human-made CO2 in GtC/year, average fluxes 1980-1989, estimated
90 % confidence intervals in parentheses [IPCC 95, p 79]
Carbon dioxide sources:
Fossil fuel burning, cement production 5.5 (5.0-6.0)
Changes in tropical land use 1.6 (0.6-2.6)
Total emissions 7.1 (6.0-8.2)
Partitioning among reservoirs:
Storage in the atmosphere 3.3 (3.1-3.5)
Oceanic uptake 2.0 (1.2-2.8)
Northern Hemisphere forest regrowth 0.5 (0.0-1.0)
Other terrestrial sinks: CO2 fertilization,
N fertilization, climatic variations 1.3 (-0.2-2.8)
Except for atmospheric CO2, carbon reservoirs and natural fluxes are
hard to measure. Their estimates vary somewhat across the literature.
Carbon enters and leaves the atmosphere largely as CO2. Other fluxes
involve various carbon compounds. The above irreverently lumps land
animals with soils and detritus, and it omits many other details as
well. For instance, both volcanic CO2 and CO2 removal via silicate
weathering are in the order of 0.1 GtC/year and play a role on geologic
time scales only. [IPCC 95, chapters 2.1, 9, 10] [Butcher, chapter 11]
[Siegenthaler]
CO2 uptake by land plants through photosynthesis is roughly balanced
by plant and soil respiration. Depending on whether photosynthesis
exceeds or falls below respiration, the net result is CO2 drawdown
or CO2 release. Today, photosynthesis is probably slightly ahead.
In future, climatic changes or rising CO2 level may trigger feedbacks
that curb or speed up the rise of atmospheric CO2. To name a few:
CO2 fertilization should promote photosynthesis and draw down some CO2,
as long as respiration doesn't catch up. Warming may stimulate or
slow down both photosynthesis or respiration, depending, among others,
on soil moisture. The mix of species in ecosystems is likely to shift,
which in turn may affect atmospheric CO2. Dieback of vegetation can
release CO2. The overall effect of these and other feedbacks is hard
to tell. Ecosystem models tentatively suggest that carbon storage in
vegetation and soils may eventually win out. Temporarily, however,
carbon may be released, especially if large and rapid changes should
cause forests to die back. [IPCC 95, chapters 2.1 and 9]
Turning to the ocean, a sea surface warming of 1 o C may increase
atmospheric CO2 by up to 10 ppmv through degassing [IPCC 94, p 57].
More importantly, marine life, in spite of its low biomass, takes
up and releases about 50 Gt of carbon annually. Marine biological
production occurs largely in the sunlit surface and is thought to be
limited mostly by nitrogen. Surface nutrient supplies are replenished
primarily through transport from deeper ocean layers. (In the open
ocean, iron can be limiting; it enters the ocean mainly in airborne
dust and via rivers.) The export of organic carbon from the surface
to deeper ocean layers, known as the biological pump, is not or little
affected by CO2 availabilility, but it may be affected by changes in
temperature, cloud cover, ocean currents, nutrients availability,
or ultraviolet radiation.
These and other marine biological processes are complex. Researchers
cannot yet say how they will respond to disturbances. It has been
estimated that, with no biological pump, preindustrial atmospheric CO2
would have been 450 instead of 280 ppmv, whereas a marine life seizing
all available surface nutrients could have lowered this to 160 ppmv.
On the other hand, preliminary results suggest that changes in the
biological pump may affect atmospheric CO2 only by 10s rather than
100s of ppmv. [IPCC 94, p 57-8] [IPCC 95, p 79-80, chapter 10]
Biological feedbacks on climate are not limited to the carbon cycle.
For instance, dimethyl sulfide (DMS) from the ocean is a major natural
source of tropospheric sulfate aerosols. Shifts in DMS production may
affect marine cloud cover and surface temperature. DMS production is
hard to predict, because it depends, among many others, on the local
biomass and mix of species. [IPCC 95, p 488, 504-6]
Back to the land, spreading of boreal forest into tundra may lead to
warmer winters. Trees protrude above the snow-covered ground, they
reflect less sunlight back to space than snow-covered tundra. During
and after deglaciation, the expansion of boreal forests amplified the
warming of northern land areas. The reverse process, displacement
of boreal forest by tundra, probably played a role in the onset of the
last glaciation. For another example, rising CO2 tends to improve
the water-use efficiency of vegetation. Plants may then release less
water vapor to the ambient air. Regionally, this may warm the surface
and affect precipitation and soil moisture. [Gallimore] [IPCC 95,
p 217-21, 450, 469-71]
These few illustrations should do to show that, for better or for
worse, human land-use changes like de- or reforestation can make
a difference.
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