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OCR for page 88
7
BIOGEOCHEMICAL FEEDBACKS
AND THE CARBON CYCLE
SUMMARY
Both the marine and terrestrial carbon cycles contain potentially
important feedback processes. There are, however, major gaps in
understanding. No definitive explanation has been given for the vast uptake
of CO2 by the terrestrial biosphere, and no confident prediction can be given
of future biological uptake or release of CO2, particularly over the long term.
Few observations are available to guide the necessary scaling of vegetation-
climate feedbacks from the scale of an individual leaf to a landscape mosaic
of vegetation and soils. In the marine realm the strengths of a wide variety of
potential feedback mechanisms involving CO2 and DMS are yet to be
determined.
Research into carbon uptake by the land and ocean as outlined in the
U.S. Carbon Cycle Plan (Sarmiento and Wofsy, 1999) and North American
Carbon Program (Wofsy and Harriss, 2002) should be undertaken to
characterize and reduce the uncertainty associated with carbon uptake
feedbacks. The Panel also recommends that research outlined in the Surface
Ocean Lower Atmosphere Study (SOLAS) Science Plan be adopted in order
to improve our understanding of DMS-climate feedbacks as well as carbon
cycle feedbacks that involve air-sea transfer (such as iron-CO2 feedbacks).
The U.S. Carbon Cycle Science Plan outlines a strategy to "deliver
credible prediction of future atmospheric carbon dioxide levels . . . by means
of approaches that can incorporate relevant interactions and feedbacks of the
carbon-cycle climate system." The plan advocates strong multiagency
collaboration to carry out specific program elements, which include (1)
expanded, long-term observational networks in the atmosphere, ocean and
terrestrial systems; (2) historical reconstructions of CO2 emissions and
terrestrial carbon inventories; (3) intensive ocean and land process studies;
and (4) modeling and synthesis, including the development of models that
88
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
89
couple the carbon cycle to the rest of the climate system. SOLAS is an
international research initiative designed to "achieve quantitative
understanding of the key biogeochemical-physical interactions and
feedbacks between the ocean and atmosphere, and of how this coupled
system affects and is affected by climate and environmental change." To
achieve this goal the SOLAS Science Plan recommends increased
cooperation between atmospheric and marine scientists in order to develop
process studies, monitoring programs, process-level models, and Earth-
system models.
AS greenhouse gases increase in the atmosphere and warming is
produced, the net exchange of carbon between the atmosphere and reservoirs
of carbon in the land and ocean may be altered. Temperature and
precipitation changes may alter the uptake of carbon by plants. Increased
temperature in high latitudes may change the storage of carbon by frozen
soils and associated biomass. Changes in ocean temperature and circulation
may alter the storage of carbon in the ocean. All these potential feedback
processes will alter the amount of atmospheric carbon dioxide increase that
results from fossil fuel combustion by humans. The production and uptake
of other radiatively active gases in the land and ocean may also be modified
as a result of climate change.
The land and ocean currently exchange approximately 120 and 90
petagrams of carbon per year with the atmosphere, respectively (Prentice et
al., 2001~. Although the ocean constitutes a much larger reservoir of carbon
than the land biosphere, both land and ocean carbon exchanges are important
for understanding the anthropogenic effect on atmospheric carbon dioxide.
Both land and ocean also have the potential to produce feedbacks between
climate change and uptake of anthropogenic carbon.
The quantities of carbon stored as plant biomass and soil organic matter
on land, or carbonate species and organic carbon in the sea, vastly exceed
CO2 in the atmosphere. Analysis of long-term changes in atmospheric CO2,
~3CO2/~2Co2, and O2 show that the atmospheric increase in CO2 was less
than half of the fossil fuel input between 1991 and 1997, with the remainder
approximately equally partitioned among the land and ocean (Battle et al.,
2000).
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9o
UNDERSTANDING CLIME TE CHANGE FEEDBACKS
TERRESTRIAL CARBON FEEDBACKS
Climat~Plant~O2 Feedbacks
Atmospheric CO2 is regulated by complex processes involving
terrestrial and marine plants, which fix inorganic carbon as organic matter,
heterotrophic organisms that mineralize organic matter back to CO2, and a
variety of geochemical and biogeochemical processes that convert CO2 to
and from mineral carbonates (e.g., Cached. All of these processes are
sensitive to climate. Nevertheless, assessments of climate change have long
regarded feedbacks in the carbon-climate system as basically simple two-
step processes, as depicted in Figure 7.1 for the terrestrial biosphere:
Faster
I growth I ~ +
~1
W ' ~
Conventional views of CO2-climate
feedbacks.
(a) Temperature(T)-respiration feedback.
Terrestrial systems respond to climate
. . . . . .
warming Dy increasing respiration,
adding CO2 to Me atmosphere from
stocks of soil organic matter,
increasing CO2, arid enhancing
warming.
| CO2 1 (a) CO=growthfeedback. Plants increase
_ ~ ~ Am+ rates of photosynthesis when grown at
~ ~ _ ~ , elevated concentrations of CO2,
~, ~ Enhanced
photosynthesis,
reduced ET
especially in dry climates or in nutrient-
rich soils where other factors do not
inhibit the response to CO2.
Evapotrasporation (ET) is reduced,
lowering the water requirement for
vegetation.
FIGURE 7.1 Climate-land biosphere feedback processes: Conventional view.
The positive feedback loop (a) is based on the increased rate of
respiration observed for almost all organisms as temperatures increase. This
factor underlies the paradoxical distribution of soil organic matter with
latitude. Rates of production of organic matter are slower in cold versus
warm climates, but rates of decomposition decline faster than production.
Huge stocks of organic carbon, several times larger than the quantity of CO2
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
91
in the atmosphere, are locked up in the soils of boreal and sub-boreal
regions, and feedback (a) could thus have a major impact on future levels of
CO2.
The negative feedback (b), plant growth accelerated by CO2, is also a
well-known biophysical process. Green plants all use the enzyme rubisco to
bind CO2 during photosynthesis. Rubisco takes carbon dioxide and attaches
it to ribulose bisphosphate, a small sugar with five carbon atoms; then it cuts
the molecule into two identical pieces with three carbon atoms. In spite of its
central role rubisco is remarkably inefficient. Typical enzymes process 1,000
molecules so, but rubisco fixes only about three carbon dioxide molecules
per second. High concentrations of CO2 (roughly 260 ppm) are needed to
bind with the enzyme in a cell. Plants compensate for the inefficiency by
allocating substantial resources to rubisco, and most plants must allow rapid
gas exchange with the interior tissues of the leaf to provide the needed high
concentrations of CO2. This circulation, through opening of the stomates of
the leaf, allows water loss by evaporation. Elevated CO2 thus allows plants
to increase growth with fixed (or reduced) allocation to rubisco and with
lower requirements for water.
The CO2-growth feedback modifies the quantity of atmospheric CO2 by
altering the amount of organic matter in living biomass and the inputs of
fresh organic matter to soils, in contrast to the respiration feedback that
alters the quantity of dead organic matter in soils, much of which is old and
recalcitrant. The stocks of biomass and short-lived organic matter that may
be maintained on the land impose the limit for the CO2-growth feedback.
These stocks are subject to manipulation by harvesting, preservation of
wood and paper, and other management. The limit on the temperature-
respiration feedback is imposed by the available stores of soil organic
matter, generally assumed to be larger than potential biomass stocks.
Real ecosystems do not however behave just like simple organisms
exposed to a single, instantaneous change in the environment. For example,
some ecosystems show quite small stimulation by elevated CO2, and
responses typically decline during extended studies. Several factors are at
work. Stomates may remain open despite higher CO2, to restrain the rise in
leaf temperature; moreover, reduced water use provides little help to plants
in well-watered environments. Other resources, such as nutrients (N. P. Ca,
K), often limit plant growth, inhibiting any stimulation by CO2 (Bauer et al.,
2001).
Some critically important feedbacks occur only on long time scales. For
example, the length of the growing season has been shown to provide the
dominant effect of climate on carbon sequestration by mid-latitude forests.
Years with warm temperatures in spring have greater net uptake of CO2 than
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92
UNDERSTANDINrG CLIMATE CHANGE FEEDBACKS
cold years (Barford et al., 2001~. Greater rates of growth and carbon uptake
are observed for mid-succession forests in warmer parts of the temperate
zone. These effects far exceed any increase in respiration, contradicting
expectations of the temperature-CO2 feedback (a).
The peatlands of Alaska, Canada, and Siberia represent a very
important, potentially positive, feedback between CO2 and climate (Chapin
et al., 2000~. Enormous quantities of carbon have accumulated as peat since
the end of the last ice age, equivalent to 200 ppm or more of atmospheric
CO2 (Gorham, 1991~. Peat is preserved by being saturated with water,
maintained in the low-precipitation boreal environment by very slow
evaporation, or by being frozen. Peatlands that become drier are subject to
fairly rapid oxidation, either by microbial activity or by natural fires
(Goulden et al., 1998; Harden et al., 2000~. Evidence suggests that this
process is occurring at present, and it could accelerate markedly according to
some climate scenarios. The key lies in future changes in regional
precipitation at least as much as with temperature.
Figure 7.2 illustrates two of the feedbacks between the climate system
and the terrestrial system. The same interactions viewed at the landscape
scale and long times may have strong feedbacks opposite to those inferred
for single organisms subjected to instantaneous perturbations of a single
environmental variable (cf Fig. 7.1~.
A Scientific Strategy for Terrestrial Carbon Feedbacks
There is currently no definitive explanation for the vast uptake of CO2
by the terrestrial biosphere, nor is there a confident prediction of future
uptake or release of CO2 from the terrestrial biosphere. The major issue is to
determine the responses of whole ecosystems and landscapes to the full
diversity of environmental changes attending climate change. Warming per
se is likely less important than other factors, such as precipitation,
evaporation, humidity, cloudiness, CO2 concentrations, land use, and land
management. Physiological processes responsible for vegetation-climate
feedbacks that operate at the scale of an individual leaf need to be scaled to a
canopy of leaves and then to a landscape of thousands of plants. There are
few observations to guide this scaling, as most studies of stomata!
conductance and its response to CO2 are obtained from leaf measurements.
In addition, most studies examine the short-te~ response of plants to CO2.
Longer-term acclimation to high CO2 will change the short-term reduction in
stomata! conductance.
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
IT sneer 1' Q $~
~ season - ~ +
L CO2 1
_/+ 1
_ 1
T
rReduced ETI
93
System interactive CO2-climate feedbacks.
(a) Temperature (T)-respiration
feedback Terrestrial systems respond
to climate warming by increasing
growth due to longer growing seasons,
removing CO2 from the atmosphere
and storing in biomass and fresh
organic matter.
(b) CO=growth feedback/
Evapotranspiration (ET) is
reduced, lowering latent heat
fluxes and increasing climate
warming through reduced cloud
cover and increased sensible heat
FIGURE 7.2 Climate-land biosphere feedbacks: System interactive views.
The key to understanding the terrestrial biosphere's uptake of CO2 is to
undertake observations and analysis at large spatial scales for extended
times. These observations should integrate measurements of the carbon
cycle with measurements of the energy and water cycles. The Panel supports
the strategy of the U.S. Carbon Cycle Science Plan (Sarmiento and Wofsy,
1999) and the North American Carbon Program (Wofsy and Harriss, 2002)
in this regard. For the purposes of this report the Panel supports the U.S.
Carbon Cycle Science Plan's focus on the following two questions:
1. What has happened to the carbon dioxide that has already been emitted
by human activities?
2. What will be the future atmospheric CO2 concentration trajectory
resulting from both past and future emissions?
These fundamental questions were articulated into six specific goals,
two of which focus on the terrestrial carbon cycle.
1. Quantify and understand the Northern Hemisphere terrestrial carbon
sink.
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94
UNDERSTANDING CLIMA TE CHANGE FEEDBACKS
2. Determine the impacts of past and current disturbance, both natural
(e.g., boreal forest and anthropogenic (e.g., land use) on the carbon budget.
These goals are considered to be feasible steps over the next five or so
years to address uncertainties in the carbon cycle and interactions with
climate change. The focus on North America is intended as a first step to
define global feedbacks involving CO2 and climate. Implementation of these
goals has been laid out in plans for the North American Carbon Program
(NACP) (Wofsy and Harriss, 2002~. The NACP includes radically new
networks of long-term atmospheric observations and ecosystem studies.
Data assimilation systems are described that for the first time would allow us
to combine these data with high-resolution assimilated winds to define CO2
net exchange at landscape and continental scales. The plan also prescribes
extensive manipulations and field measurements to elucidate the factors
regulating CO2 uptake or release by major ecosystems. Thus, the NACP
represents a systematic effort to address the carbon-climate feedbacks at the
time and space scales relevant for understanding the mutual interactions of
the carbon cycle and the climate system. This program, if implemented,
would provide the basic information and analytical framework needed to
quantify and understand climate-carbon feedbacks for North America, and it
would provide the template for extension to other major land masses. In
addition to the goals outlined above, the NACP will also be concerned with
emissions of CO2, CH4, and CO. (Improving accounting of carbon emissions
and uptake is also important for reasons other than the objectives of this
report; they are vital for developing and maintaining effective greenhouse
gas mitigation policies.)
Previous carbon cycle research largely focused on studies of single
components, such as the atmosphere or ocean, or through small-scale
process studies. But carbon is exchanged continuously through the
atmosphere, land biosphere, soils, and oceans. The temporal and spatial
scales of the program must be appropriately large for addressing climatic
issues, and data and models from all components must be brought together
to develop information on global carbon balances. Results must be scaled up
from process studies and inventories and rigorously compared to information
gained at a regional or continental scale. These integration objectives are
shared by and are embodied in the program's major elements for integration,
including innovative new assimilation and data fusion systems that bring
together diverse data and models, linking information at various scales to
provide a consistent continental-scale carbon balance, resolved temporally
by season. This coordination of science activities requires similar
coordination among agencies involved in implementation.
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
95
Major Program Elements of the Carbon Measurement and Analysis
Strategy
Long-term atmospheric measurements of the carbon budget are required
from the ground, aircraft, and satellites, which should provide spatially and
temporally resolved, three-dimensional atmospheric data for the major
carbon gases, CO2 CH4, and CO, to enable reliable estimates for North
American sources and sinks of these gases. These observations are required
to obtain regional and continental sources and sinks for atmospheric CO2,
CH4, and CO. The network planned by NACP extends present remote
monitoring networks (Tans et al., 1996) of atmospheric observations to
provide dense coverage and vertical soundings in the interior of the
continent. Present networks of flux stations (Baldocchi et al., 2001) will be
enhanced to provide traceable absolutely calibrated concentrations, and
coverage will be extended to include many more representative regions.
Intensive field programs that are planned by NACP, including large-
scale airborne and field campaigns, should be launched to provide datasets
to evaluate and to improve the design of atmospheric and surface
measurement networks, to develop and test models, to interpret
observations, and to provide atmospheric snapshots to constrain fluxes.
These efforts should provide continuous feedback on uncertainties in
modeling and assessment tools for carbon accounting.
Inventories of carbon in major ecotones (e.g., the Forest Inventory
Analysis ~Goodale et al., 20023) will need to be enhanced to encompass full
carbon accounting and complemented by remote sensing and models to
provide a complete carbon budget for the land. Lands (peatlands, scrub land,
suburban landscapes) and carbon pools (roots, coarse woody debris, shrubs)
not currently inventoried must be included. A hierarchical conceptual
approach is planned in the NACP to support a multiscale interpretation, with
intensive studies providing access to details and mechanisms that are
extended using remote sensing, extensive inventories, and mechanistic
models and join the atmospheric and ocean studies as components in a
unified analysis framework.
As outlined in several other disciplinary chapters of this report, the
integration of models and model-data assimilation will be important. Such
efforts could provide knowledge of the atmospheric concentrations of CO2
over the entire continent and adjacent waters at frequent intervals. We
support the flow of information and the integration outlined by the NACP to
obtain regional carbon accounting (see Figure 7.3~.
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96
UNDERSTANDING CLIMATE CHANGE FEEDBACKS
| Forecast winds
4-D atmospheric data: Remote sensing of
CO2, CH4, CO (surface, land, oceans
airbome, satellite)
/ ' Forecast
/ tower data
Meteorological
input data
(sondes,
radiances)
/ ~ And cover, land
use, historical,
inventory data, in-
situ ocean data
~ .
Data fusion
Diagnostic models (inverse Assimilation) |
· Retrospective, real-time |
North American sources and sinks for CO2, CH4, |
FIGURE 7.3 Data flow and integration in the NACP. Complexity and level of synthesis
increase down the figure. Valuable data products are delivered at each level. Note the
central role played by the model-data fusion systems that combine observations Tom
diverse sources, using data-driven models and advanced data assimilation arid
optimization methods.
A critical step will be to develop new classes of diagnostic models to
determine sources and sinks of CO2 and other gases. Data-driven models of
carbon dynamics in vegetation and soils will be combined in a data fusion
framework with high-resolution meteorological information, surface flux
data, and atmospheric concentrations to derive fluxes and a quantitative
representation of the state of the atmosphere and of the carbon cycle.
The Panel recommends that the NACP be implemented with major
initiatives in the aforementioned key areas. We also support its plans for
regular state-of-the-art assessments of carbon cycle science and carbon
inventories for North America, with eventual extension of the observations
and analysis framework to the entire globe. Linkage to the Global Carbon
Project () of the International
Geosphere-Biosphere Program me (IGBP), World Climate Research
Programme fWCRP9, and International Human Dimensions Programme on
Global Environmental Change (IHDP) would be useful in this regard.
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
MARINE BIOGEOCHEMICAL FEEDBACKS
97
Marine carbon feedbacks have been evaluated almost exclusively with
models. This is unfortunate because the marine carbon cycle models being
used for climate change studies are not capturing processes that may be key
elements of feedback mechanisms. This is particularly true for the biological
component of the models. The most advanced marine carbon cycle model
that has been used in climate change simulations (Cox et al., 2000) does not
include, for example, multiple phytoplankton species, iron limitation,
nitrogen fixation, variable carbon-to-nitrogen ratios, and dissolved organic
matter, all of which appear to be important features of the marine carbon
cycle. Most other models used for such purposes are even simpler. The
reasons for these omissions are various, but they include the lack of data for
developing defendable parameterizations as well as the additional
computational expense of increasing the complexity of the models.
Models nevertheless can help to put rough boundaries on the strength of
various feedback mechanisms. In terms of the overall feedback of the marine
carbon cycle on climate on the time scale of a century, the models vary from
showing almost no impact on ocean carbon uptake (Jogs et al., 1999; Maier-
Reimer et al., 1996) to a reduction of about 10-15 percent (Friedlingstein et
al., 2001; Matear and Hirst, 1999; Sarmiento et al., 1999;~. This overall
effect represents the sum of individual feedbacks that may be considerably
larger. Some of the potentially important marine biogeochemical feedbacks
are described briefly below.
Physical and Chemical Feedbacks on Atmospheric CO2
Solubility-Temperature Feedback
The solubility of CO2 and the degree to which it reacts to form other
inorganic and nonvolatile forms of carbon decreases with increasing
temperature with an accurately known functionality that is described by
temperature-dependent equilibrium constants. Thus, there is a positive
feedback on atmospheric CO2 associated with temperature changes and the
inorganic chemistry of CO2 in seawater. The few modeling studies of this
feedback regard it to be of modest strength, amounting to a 10-15 percent
reduction of the cumulative anthropogenic CO2 uptake by the ocean on the
century time scale (Jogs et al., 1999; Matear and Hirst, 1999; Sarmiento et
al., 1999).
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98
CO2 Uptake-Ventilation Feedback
UNDERSTANDING CLIMA TE CHANGE FEEDBACKS
In order for substantial carbon to be taken up by the oceans, it must be
first moved across the air-sea interface and then from the surface ocean to
deeper in the ocean. The resistance of the air-sea interface is relatively small,
and so it is largely the vertical circulation in the ocean, including the
ventilation of the thermocline and the formation of intermediate and
deepwaters, that regulates the uptake of anthropogenic CO2 by the ocean. As
noted in Chapter 5 some models predict that the rate of overturning by the
thermohaline circulation will decrease in a warmed world, which would
result in a positive feedback on atmospheric CO2. The few studies on this
feedback are in disagreement with regard to its strength, varying between
essentially no impact on ocean carbon uptake (Maier-Reimer et al., 1996) to
as much as 17 percent (Sarmiento et al., 1999~. Differences are primarily due
to the sensitivity of the ocean circulation to CO2 changes. This underscores
the point that marine carbon cycle models are only as good as the circulation
models in which they are embedded.
Stratification Mixing Feedback
As discussed above, many models predict changes in ocean circulation,
which can alter the CO2 balance of surface waters and therefore atmospheric
CO2. For example, stratification of high-latitude waters would inhibit the
upward flux of deepwaters, which are enriched in CO2 due to the
decomposition of organic matter sinking from the upper ocean, resulting in a
negative feedback on atmospheric CO2. This would be counteracted to some
degree by a reduction in carbon export from surface waters due to the
reduced upward flux of nutrients. For example, Bopp et al. (2001) found this
effect to dominate the 6 percent decrease in carbon export from surface
waters for a CO2 doubling in their models. The few studies on the overall
feedback disagree with regard to its strength, though they generally agree
that this feedback tends to have a similar magnitude (but opposite in sign) to
the CO2 uptake-ventilation feedback described above (Jogs et al., 1999;
Matear and Hirst, 1999; Sarmiento et al., 1999~.
ENSO~O2-Upwelling Feedback
A similar feedback may operate in low latitudes, as indicated by some
models that predict increased frequency of El Nino events with increased
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
99
CO2 (Timmerman et al., 1999~. Such an increase would reduce the natural
marine source of CO2 to the atmosphere (due to upwelling), creating a
negative feedback on atmospheric CO2. Other models show a reduction in
equatorial upwelling, which would have a similar effect (Bopp et al., 2001~.
This feedback has not been quantified, though at least one modeling study
suggests that the equatorial Pacific does not exert a strong control on
atmospheric CO2 on the century time scale (Sarmiento and Orr, 1991~.
However, the relative roles of high and low latitudes in regulating
atmospheric CO2 are active areas of research (Broecker et al., 1999~.
Overview of Biological Feedbacks on Atmospheric CO2
Carbon Export-Temperature Feedback
Phytoplankton growth rates generally increase with temperature
(Eppley, 1972) and so the potential for a negative feedback exists. The
fraction of photosynthetically derived material that is exported to deeper
waters, however, is suggested by a recent synthesis of many field studies to
decrease with increasing temperature (Laws et al., 2000), which would
constitute a positive feedback. Bopp et al. (2001) found very little sensitivity
of carbon export to climate warming using a simple ecosystem model. That
model, however, did not include the findings of Laws et al. (2000), and so
this feedback remains poorly quantified; even its sign is not known.
Carbon Export-Light Feedback
The exposure of phytoplankton to light depends on the surface
irradiance, the opacity of the water column and the depth of the mixed layer
(deeper mixed layers result in more time that phytoplankton spend in the
dark). Changes in cloudiness could therefore change photosynthesis. The
opacity of the water column is largely due to changes in phytoplankton
abundance but also to colored dissolved organic matter, the dynamics of
which are poorly understood. Finally, many climate models (e.g., Bopp et
al., 2001) predict shallower mixed layers due to decreases in surface density,
which could enhance light levels and therefore photosynthesis. Bopp et al.
(2001), the only study to quantify this feedback, found increases in carbon
export of as much as 20 percent over large regions of the high latitudes due
to decreases in mixed layer depth induced by a CO2 doubling.
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100
Carbon Export-Iron Input Feedback
UNDERSTANDING CLIMATE CHANGE FEEDBACKS
Phytoplankton growth in many parts of the ocean is limited by the
availability of iron, a substantial fraction of which is derived from wind-
blown continental dust (Martin et al., 1991~. Thus, climate-induced changes
in continental aridity, wind speed, and wind direction may influence
phytoplankton production. Ice core data, which show higher levels of
atmospheric dust during glacial times, suggests that iron may be part of a
positive feedback loop (Martin, 1990~. This feedback has not been
quantified using models because the incorporation of iron into marine
ecosystem models is just beginning (Moore et al., 2002~. Many questions
remain about how and in what form iron is delivered to the ocean, how it is
made available to phytoplankton and how it is cycled in the marine
ecosystem.
CO2kalcification Feedback
The calcification rates of coccolithophores and coral reefs have recently
been shown to decrease with increasing atmospheric CO2 (Kleypas et al.,
1999; Riebesell et al., 2000~. Because calcification is a source of CO2, such
organisms are potentially part of a negative feedback on anthropogenic CO2.
Using a simple model, Zondervan et al. (2001) suggest that this feedback is
rather small for the coccolithophores, which dominate global calcification.
Feedbacks Involving DimethylsulB~de
Dimethylsulfide (DMS) is thought to be a major precursor of cloud
condensation nuclei in unpolluted air (see Chapter 7~; therefore the release
of DMS from the ocean may influence cloud albedo and climate.
Phytoplankton, bacteria, and zooplankton all play important roles in marine
DMS cycling, so any change to the marine ecosystem as a result of climate
change is likely to affect the DMS concentration in seawater and hence its
flux to the atmosphere. The turnover of DMS in the ocean mixed layer is so
rapid that the flux to the atmosphere is only a small residual of much larger
fluxes. Thus, modest changes in internal cycling have the potential of
producing large changes in the air-sea flux. However, because the response
of marine ecosystems to climate change is uncertain, the response of marine
DMS emissions is also uncertain. Additional uncertainty is caused by
production of the DMS precursor, dimethylsulfoniopropionate (DMSP),
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
101
which varies greatly among phytoplankton species (Keller et al., 1989~.
Zooplankton play a role in DMS cycling through grazing, which is an
important mechanism for releasing DMSP from phytoplankton cells (Dacey
and Wakeham, 1986~. The bacterial impact on DMS cycling is through the
effect on DMS yield during DMSP consumption, as well as through the
direct consumption of DMS (Kiene and Bates, 1990~. A significant sink of
DMS also occurs through abiotic photochemical consumption (Kieber et al.,
1996~. These points underscore the complexity of DMS cycling in seawater
and the difficulty in predicting its response to climate change.
It is not surprising that there is no simple relationship between DMS
concentration and temperature, salinity or chlorophyll, as revealed by a
recent synthesis of over 15,000 measurements by Kettle et al. (1999~.
However, DMS flux tends to increase with increasing solar radiation (Bates
et al., 1987), with seasonal maxima in flux and concentration occurring in
the summer (Kettle et al., 1999; Kettle and Andreae, 2000~. Simo and
Pedros-Alio (1999) suggest that this relationship is due to photo-inhibitory
effects on bacteria (which consume DMS and reduce the DMS yield from
DMSP) during conditions of high light and shallow mixed layer depth. Thus
there is some support for the hypothesis of a negative feedback on the
climate system involving DMS and sunlight (Charlson et al., 1987; Shaw,
1983~. However the magnitude of the feedback is not known nor is the
underlying mechanism well elucidated.
Ice core data provide additional insights regarding DMS-climate
feedbacks. Ice core records of methanesulfonate (MSA), an atmospheric
oxidation product of DMS, show that its atmospheric concentration during
glacial times was substantially different compared to the present. Glacial
concentrations were higher in the Southern Hemisphere (Legend et al.,
1991) and lower in the Northern Hemisphere (Saltzman et al., 1997),
suggesting that the sign of the feedback may vary with location.
There have been a few modeling studies that have attempted to quantify
DMS-climate feedbacks. The empirical model of Lawrence (1993)
suggested that a CO2-induced warming could be reduced by 10 percent to 50
percent due a DMS-climate (negative) feedback. Gabric et al. (1998) applied
temperature and wind speed changes from a doubled-CO2 climate model to
an ecosystem model with DMS dynamics in the Southern Ocean. They
found a modest (2-8 percent) increase in the flux of DMS to the atmosphere
due to an increase in the gas transfer velocity and phytoplankton growth rate,
both of which increase with temperature (wind speeds actually decreased
slightly in the simulation). This study also supports the potential of a
negative feedback, albeit a weak one. In light of the complexity of DMS
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102
UNDERSTANDING CLIMATE CHANGE FEEDBACKS
cycling, these models are extreme simplifications, but they nevertheless
provide a framework for attempting to quantify DMS feedbacks.
Feedbacks Involving Methane and Nitrous Oxide
The emission of methane and nitrous oxide from the ocean currently
constitutes a very small fraction of the total greenhouse gas forcing of the
atmosphere, however, there is the potential of large releases of these gases.
Abundant reservoirs of methane are stored in ocean sediments in the form of
clathrates, which are nonvolatile. Warming could release the methane into
the water column and atmosphere, providing a positive feedback. This
feedback has not been quantified, though the paleoclimate record suggests
that the feedback may have been activated many times in the past (Bains et
al., 1999~. The volume of methane available for release is poorly known
(Gornitz and Fung, 1994~. One modeling study suggests an upper limit of 10
percent to 25 percent increase in warming over the next century due to this
feedback (Harvey and Huang, 1995~.
Nitrous oxide is formed in the ocean during respiration, and the rate of
release appears to be a function of the dissolved oxygen concentration (Law
and Owens, 1990), particularly at low oxygen levels. Because both
respiration and oxygen abundance are sensitive to climate change, there is
the potential for climate feedbacks involving marine N2O. This is
particularly true given the fact that the amount of N2O release is only a small
fraction of the total cycling of nitrogen.
A Scientific Strategy for Marine Biogeochemical Feedbacks
Marine biogeochemical feedbacks are to a large extent unquantified.
First order questions related to even the sign of certain feedbacks exist in
some cases.
The rate at which the ocean takes up carbon will very likely continue to
increase because of the increasing atmospheric CO2 level. Changes in ocean
carbon dynamics driven by changes in circulation and biology will modulate
this increase. The degree of this modulation is very poorly known due to
large uncertainties in the projections of future changes in ocean circulation
and of the response of ocean biota to these ocean circulation changes. While
primary production is important for evaluating the overall intensity of
carbon cycling in surface waters, it is the exported fraction (from surface
waters) of primary production that is important to surface ocean and
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BIOGEOCHEMICAL FEEDBACKS AND TlIE CARBON CYCLE
103
atmospheric CO2 levels. Our ability to quantify export and its variability on
large scales is improving, but it is still poor. The rate of decomposition of
organic matter exported from surface waters is also very important but even
more poorly known.
The feedback between marine DMS emissions and cloud albedo Is
potentially very large. Over the past 15 years substantial progress has been
made in evaluating the mechanisms of ocean DMS cycling, including its
production, consumption, release to the atmosphere, oxidation in the
atmosphere, and contribution to the cloud condensation nuclei (CCN) pool.
However, the nature of the overall feedback has remained elusive. The ocean
is currently a minor source of methane and nitrous oxide to the atmosphere.
However, there is a poorly understood potential for a large release of these
gases to the atmosphere. Marine sedimentary clathrates are a very large
reservoir of methane that could be abruptly released. Large amounts of
nitrogen are cycled in the marine environment and the fraction released as
N2O is currently small, but the controls on this fraction are poorly
understood.
.
Observations for Improving Understanding and Models
The main areas that deserve attention in the context of marine
biogeochemistry and climate are the rate of CO2 uptake by the ocean and the
release of DMS from the ocean. If ocean circulation and biology do not
change in the future, these rates can be projected with relatively high
accuracy. Model uncertainties exist because we do not know to what extent
changes in ocean physics and biology will modulate the cycling of carbon
and sulfur in the sea. Thus concerted studies need to be undertaken to assess
the response of the marine carbon and sulfur cycles to changes in ocean
circulation and other climate variables, such as solar radiation and
temperature. This will be best achieved by monitoring the ocean carbon and
sulfur cycles over time scales ranging from months to decades. The annual
cycle in ocean physical properties and other climate variables represents the
major temporal forcing on marine biogeochemical systems and should be
monitored intensively. Interannual and decadal climate variations represent
another major forcing that needs to be understood in terms of feedbacks on
the marine carbon and sulfur cycles.
Four observing system components are selected for special attention;
1. Continued satellite-based monitoring of ocean color is needed to derive
information about changes in plankton biomass and CO2fxation.
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UNDERSTANDING CLIME TE CHANGE FEEDBACKS
2. Expanded monitoring of the atmospheric oxygen-nitrogen ratio and
atmospheric DMS concentration is needed to derive information about
seasonal and interannual variations in the CO2 fixation and export to the
ocean interior and sea-to-air DMSflux, respectively, on basin-wide scales.
3. High-resolution (monthly) time-series measurements are needled of the
carbonate system (e.g. CO2 concentration and dissolved inorganic carbons,
nutrients, oxygen, chlorophyll, dissolved" organic carbon, primary
production, verticalflwres of carbon, and the main sulfur pools (particulate
and dissolved DMS and DMSP) at a wide variety of ocean locations.
Currently, open ocean time-series measurements are limited to the carbon
cycle at a few sites, mainly in the subtropical oceans.
4. Periodic surveys of ocean chemical and physical properties are needed
to evaluate the uptake and processing of carbon in the marine environment.
There have been a few such surveys in the past, including the Geochemical
Ocean Sections Study fGEOSECSJ of the 1970s and the WOCE Joint Global
Ocean Flux Study (JGOFS) CO2 survey of the 1990s, and it is critical that
they occur everyfive to ten years.
These measurements should be made through a combination of
autonomous buoys to derive temporally continuous time series; ship-based
measurements to produce spatially extensive repeat surveys; and remote
sensing.
In addition to these observational strategies, increased efforts are needed
to develop new technologies for measuring carbon and sulfur fluxes in the
sea, particularly the air-sea flux of CO2, the sinking flux of organic carbon,
the rate at which organic matter decomposes (respiration), and production
and consumption of DMS and DMSP.
The primary obstacle to making projections about the marine carbon and
sulfur cycles is the lack of observations to inform the models. The
aforementioned observations will be critical in helping to provide adequate
descriptions of the relevant processes, which can lead to refined and
observationally tested model representations. The transition between
observation and the development and testing of corresponding model
representations of the key processes should be a seamless one; we advocate
facilitating this by incorporating numerical modeling into field studies
during the development and execution as well as in the data synthesis phase.
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BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE
Evaluating Progress
105
Factors critical to the uptake of carbon by the ocean that might be
derivable from observations and that can be used to test models include the
following.
1. The change in surface pCO2 for a given change in temperature or the
change in inorganic carbon contentfor a given change in heat content. This
is necessary to evaluate the solubility-temperature feedback.
2. Change in inorganic carbon inventory for a given change in ocean
ventilation rate. This will allow the circulation-uptake feedback to be
assessed. The ventilation rate can be estimated from various tracers of ocean
circulation, such as chlorofluorocarbons.
3. Change in export production and surface nutrient concentration for a
given change in stratification. This will allow the stratification-CO2-mixing
and stratification-production feedbacks to be assessed. Export production
can be crudely estimated on large scales from satellites and variations in
atmospheric oxygen.
4. Change in export production for given changes in temperature, light,
and iron dust inputs. This will allow the feedbacks between carbon export
and various controls on it to be assessed. Iron dust inputs on large scales can
be crudely estimated from precipitation and aerosol fields derived from
satellites.
5. Change in cloudfi-action and albedo for given changes in surface ocean
DMS. This will allow feedbacks involving DMS and climate to be assessed
in a crude sense. Monitoring at a more detailed level (e.g., MSA, CCN
densities, wind speed, SST, DMS community production) would be valuable
as well.
6. Changes in concentrations of the isotopes of methane in the atmosphere
and select areas of the ocean for given changes in ocean temperature. This
would allow for feedbacks between warming and release of methane from
clathrates to be assessed.
7. Changes in the concentrations of the isotopes of nitrous oxide in the
atmosphere and select areas of the ocean for given changes in a variety of
ocean physical and biological properties, including stratification,
temperature, and primary production. This would allow feedbacks related to
N2O release from the ocean to be evaluated.
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UNDERSTANDING CLIME TE CHANGE FEEDBA CKS
Programmatic Efforts
The U.S. Global Change Research Program has developed an
interagency Carbon Cycle Science Programi with a Science Plan whose
goal is specifically to reduce uncertainties in understanding the carbon cycle.
In addition, as part of a new international initiative the fledgling U.S.
Surface Ocean Lower Atmosphere Study (SoLAS)3 has a mission to
"achieve a quantitative understanding of the key biogeochemical-physical
interactions between ocean and atmosphere, and of how this coupled system
affects and is affected by climate and environmental change." To a large
extent a successful approach toward improving understanding and modeling
of biogeochemical feedbacks is directly linked to the success of these
programs. We recommend that agencies work to ensure that the goals of the
U.S. Carbon Cycle Science Program and SOLAS are met through adequate
and sustained funding These agencies should continue to ensure that U.S.
Carbon Cycle Science Program and SOLAS activities fit within the
framework of international activities.
' http://www.carboncyclescience.gov
2 http://www.carboncyclescience.gov/PDF/sciplan/ccsp.pdf
3 http://w~7vw.aoml.noaa.gov/ocd/solas/
Representative terms from entire chapter:
organic matter
