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2
Integrated Modeling of the
Earth System
OVERVIEW
The possibility of major changes in the global environment presents the
scientific research community with a difficult task: to devise ways of ana-
lyzing the causes of and projecting the course of these shifts as they are
occurring. Purely observational approaches are inadequate for providing
the needed predictive or anticipatory information because response times of
many terrestrial ecosystems are slow and there is a great deal of variability
from place to place. Furthermore, many important processes cannot be
measured directly over large areas, such as those processes that occur in
soils. We need models to express our understanding of the complex sub-
systems of the earth and how they interact with and respond to and control
changes in the physical-climate and biogeochemical systems.
By the year 2000, a fully coupled, dynamical model of the earth system
(Figure 2.1) could be a reality. Such models would significantly improve
capabilities for projecting changes in the earth system on a decadal time
scale. The focus of this chapter is on the efforts required to achieve this
goal. For instance, it is necessary to begin now to develop models that are
more completely coupled albeit still partial-than those that are currently
available. Even though these prototypes may themselves not be successful,
This chapter was prepared by the working groups on Integrated Earth System
Models established under the Committee on Global Change. Members of the group
on Terrestrial-Atmosphere Modeling were Berrien Moore III, University of New
Hampshire, Chair; John Aber, University of New Hampshire; Guy Brasseur, Na-
tional Center for Atmospheric Research; Robert Dickinson, National Center for At-
mospheric Research; William Emanuel, Oak Ridge National Laboratory; Jerry Melillo,
16
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INTEGRATED MODELING OF THE EARTH SYSTEM
cn I
~ i_
\
Biogeochemical Cycles
17
~ E
~ cat
a, c
On
0
~0
~ ._
cn ~
a,
c'
~ ~It -it 1 ' 1~' ~
| Tropospheric Chemistry
4 Pollutants |
Atmosphenc Physics/Dynamics :=
;! a!;
IT Ti 1 ~ _
Terrestrial
Or ean Dynamics Energy/Moisture
~ Marine l l Terrestrial
Biogeochemistry I I Ecosystems
| Global Moisture 1 ( Soil
. ~ ~ ' ~'
~-
Land _
Use
Human
Activities
FIGURE 2.1 Status of earth system science in the year 2000 (ESSC, 1988~.
they will teach us what is needed to realize our goal of a fully coupled,
dynamical earth system model with a multidecadal scale of analysis. How-
ever, it should not be overlooked that much of the real science is in the
simple models and empirical observations that guide our understanding and
give us a framework for interpreting (and creating) the more complex mod-
els that evolve later. The early linking of complex models and the subse-
quent addition of existing approaches should be balanced by efforts to cre-
ate new, insightful simple models. Such insights provide the basis for
qualitative improvements in model structures.
It should be recognized at the outset that the muliidecadal temporal scale
places important constraints and demands upon the character of earth sys-
tem models (Bolin et al., 1986; ESSC, 1988; NRC, 1988~. For instance, the
Marine Biological Laboratory; David Schimel, Colorado State University; Piers Sellers,
University of Maryland; and Herman Shugart, University of Virginia. Members of
the group on Ocean-Atmosphere Modeling were Berrien Moore III, University of
New Hampshire, Chair; Mark Abbott, Oregon State University; Curt Covey, Lawrence
Livermore National Laboratory; Nick Graham, Scripps Institution of Oceanography;
Dale Haidvogel, Johns Hopkins University; Eileen Hoffman, Old Dominion University;
Christopher Mooers, University of New Hampshire; James O'Brien, Florida State
University; Albert Semtner, Naval Postgraduate School; and Leonard Walstad, Oregon
State University. Members of the group on Atmospheric Physics-Atmospheric Chemistry
were Berrien Moore III, University of New Hampshire, Chair; Guy Brasseur and
Robert Dickinson, National Center for Atmospheric Research; Bill Gross, NASA
Langley Research Center; and Chris Morris, University of New Hampshire.
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18
RESEARCH STRATEGIES FOR THE USGCRP
temporal scale demands inclusion of the biosphere and coupling across critical
interfaces: terrestrial ecosystems and the atmosphere, the chemistry of the
atmosphere and the physics of the atmosphere, and the oceans and the
atmosphere. Advances at these interfaces are essential for progress.
Differences in characteristic rates of change and fundamental processes
of different components of the system will impose subsystem-specific de-
mands and requirements on component models (Rosswall et al., 1988~. Ecological
systems will most likely rest upon functional groups rather than species;
understanding biogeochemical fluxes will require process-level models, but
initial implementation at global scales will certainly require extensive pa-
rameterization. Similarly, the nonlinear chaotic dynamics of the fluid sub-
systems the oceans and atmosphere will continue to require a careful,
step-by-step buildup in complexity; the simplistic thinking that must go into
all initial modeling advances will tend to be eventually superseded by
computationally intensive three-dimensional approaches. This is, in fact,
occurring in many of the geophysical and biological-biogeochemical sci
ences.
The most complex models to date are the atmospheric and oceanic gen-
eral circulation models (GCMs). These have structures largely determined
by the need to solve the Navier-Stokes fluid equations, but they are rich in
other physical processes as well. The atmospheric models and their climate
role are especially strongly governed by water processes; however, it is
precisely these aspects, including questions of scale and parameterization,
that are among the least satisfactory of the models.
Resolution is a problem in that the spatial scales of many of the impor-
tant atmospheric water structures are poorly resolved by existing models.
For example, many of the cloud systems that are most important for atmo-
spheric radiation have vertical scales of less than the thickness of the layers
in most existing GCMs. The horizontal structure of precipitating systems
suffers not only from inadequate resolution but also from severe difficulties
with the currently available numerical schemes that were designed prima-
rily for effectiveness (minimal computational demands) in treating the model
hydrodynamics. One obvious defect of these schemes is the tendency of
truncated spectral series to give negative mixing ratios for water in high
latitudes, a consequence of the failure of the series to represent properly the
fields in going from relatively large mixing ratios to relatively small ones.
The same difficulty can be encountered for any model tracer. For example,
it was difficult to get models to treat global smoke fields properly in nuclear
winter computations. The hope is that the new semi-Lagrangian schemes
will cure these numerical difficulties.
Another question in the treatment of water vapor in various atmospheric
GCMs is whether vertical transport in the models resembles the process in
nature, again because much of the real vertical transport occurs on scales
that are small in comparison with that of the model. The subgrid-scale moist
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INTEGRATED MODELING OF THE EARTH SYSTEM
19
convection parameterizations in the models are still fairly crude and have
not improved much in the last decade, although considerable effort is now
going into them (Anthes, 1983~.
Adding the important chemical constituents and the reactions to an atmo-
spheric GCM causes the issues of scale and computational challenges to
become daunting. Many of the important chemical reactions are concentra-
tion dependent and hence grid-scale dependent, and important processes
often occur in the boundary layer, which generally is not well enough resolved.
Further, the addition of atmospheric chemistry to a GCM places greater
demands upon the terrestrial and oceanic boundary conditions and dynamic
simulations (Lenschow and Hicks, 1989; NBC, 1984; Schimel et al., 1989~.
In considering coupling atmospheric GCMs to terrestrial models, where
the coupling transfers not only energy and water but also important gases,
such as carbon monoxide, methane, and carbon dioxide for the carbon cycle,
temporal- and spatial-scale issues again emerge. The macrobalance of ter-
restrial carbon stocks, which determine the net flux of carbon dioxide, are
difficult to derive by integrating across the short time scales at which en-
ergy, water, and carbon dioxide and oxygen are actually exchanged because
of the high degree of variability that these processes exhibit. Longer time
step integrations have generally been more successful. On the other hand,
the flux of methane and other short-lived species cannot be treated by simple
mass balance and crudely time-averaged responses. Ecological changes,
such as successional sequences of tree species, are not well treated on time
steps that are appropriate for considering photon input and: water exchange
or even trace gas fluxes and require some intermediate parameterization or
model.
The relatively simple coupling issue of land hydrology and atmosphere
remains elusive, and yet it is quite important. The exchange of many re-
duced gases (e.g., methane) depends on soil moisture conditions, and en-
ergy fluxes are influenced by water balances. Modeling sensitivity studies
have shown that if evapotranspiration were turned off over continental-scale
areas, summer precipitation would be severely reduced and temperatures
would be as much as 10 K higher than with normal fluxes. They also show
that over tall vegetation the integrated resistance to transpiration implied by
the stomata will have a major effect on Bowen ratios over the diurnal cycle.
Since the rates of sensible heat exchange over the diurnal cycle determine
the height reached by the planetary boundary layer as well as diurnal variations
of precipitation in tropical and summer conditions, it is evident that it is
important to include the role of vegetation in simulations of the hydrologi-
cal cycle. Better field data are helping to establish the parameters needed
for linking plant physiology to surface evapotranspiration, but considerable
further effort is needed before the appropriate submodels can be applied
with confidence over a wide range of vegetation cover (e.g., Dickinson,
1984; Eagleson, 1986; Sellers et al., 1986~.
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20
RESEARCH STRATEGIES FOR THE USGCRP
The coupling between the ocean and the atmosphere is central to the
question of climate change. Atmospheric GCMs with prescribed oceans,
long the mainstay of three-dimensional climate modeling, are inherently
incapable of simulating the actual time-evolving response of the climate
system to increasing greenhouse gases because this response involves heat
uptake by the oceans. This is particularly clear when one realizes that the
heat capacity of the atmosphere is roughly equivalent to that of the upper 3
m of the ocean. While it is true that the ocean may, partially, act in a
passive manner, studies of the E1 Nino/Southern Oscillation (ENSO) show
that the ocean-atmosphere system responds in a coupled fashion on interannual
time scales, and paleo-oceanographic investigations suggest that aspects of
longer-term climate change are associated with changes in the ocean's ther-
mohaline circulation. The capability to predict these changes in circulation
and heat exchange is necessary to describe the future evolution of global
climate (e.g., Bryan et al., 1982; Cess and Goldenberg, 1981; IPCC, 1990;
Sarmiento et al., 1988~.
Fortunately, exciting and encouraging progress is being made in cou-
pling key aspects of the major subsystems. Results from linking atmo-
spheric and oceanic GCMs have already been reported in the literature and
have shown significantly different behavior from that of simulations in un-
coupled modes. Similarly, interactive simulations between atmosphere and
land vegetation have been reported, and these have also exhibited new dynamical
characteristics. The inclusion of biology in oceanic GCMs has begun, al-
though the models are still simplistic and do not yet include climatic feedback
in a coupled system. Representations of terrestrial biology are also preliminary
and again without critical biogeochemical feedbacks. Finally, progress is
being made toward model structures and data sets that will allow implemen-
tation of atmospheric-oceanic-terrestrial models that include key biological-
biogeochemical feedbacks.
For the near term, developments in modeling the earth system should
continue to focus on linking previously unlinked components, adding spe-
cific subsystems to existing models (e.g., coupling oceanic and atmospheric
GCMs or adding a marine biospheric model to an oceanic GCM), or im-
proving existing linked treatments. In this spirit, the committee has arranged
the following discussion around three interface models:
1. Atmosphere-terrestrial subsystem.
2. Physical-chemical interactions in the atmosphere.
3. Atmosphere-ocean subsystem including interactions with the biosphere.
These three subsystems obviously overlap and do not include all interfaces.
Further consideration is required on the issue of the role of the cryosphere
and its coupling on multidecadal time scales (see OIES, 1989~.
In the following sections the committee presents a brief general discus
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INTEGRATED MODELING OF THE EARTH SYSTEM
21
sion of the current status of models at these three interfaces, including for
each a focused report on recommended initiatives and themes. The two
final sections deal with the cross-cutting issues of model tests and infra-
structure. In order to provide perspective on the remainder of the chapter,
the following considerations for each of the three interface models are pro-
vided:
For models that couple the terrestrial ecosystems and the atmosphere:
· The coupling must address questions such as how will a changing
climate affect terrestrial carbon dioxide uptake and storage; how will
· · .
evapotransp~rat~on change; how will the distribution of vegetation and its
seasonal pattern change; what are the effects on climate of changing pat-
terns of vegetation, including large-scale deforestation; and what is the ef-
fect of changing chemical conditions on terrestrial vegetation and trace gas
exchange?
· The primary research issue in understanding the role of terrestrial
ecosystems in global change is that of analyzing how processes with vastly
differing rates of change, from photosynthesis to community change, are
coupled to each other and to the atmosphere.
· Modeling these interactions requires coupling successional models to
biogeochemical models to physiological models. Of these, only the physi-
ological models can currently describe the exchange of water and energy
between the vegetation and the atmosphere at fine time scales.
.
Terrestrial models should focus on linked models addressing plant
community change, biogeochemistry, and physiology and~biophysics. Mod-
els of the physics of the atmosphere couple directly to terrestrial physiology
models; biogeochemical models serve as a bridge between physiology and
community change as well as coupling to the chemistry of the atmosphere.
· The coupling must address how changes in the global environment,
including the effects of land use and chemical stress, affect terrestrial eco-
systems and how ecosystem changes affect the global system.
Formidable problems of scale and parameterization are raised in three-
and four-dimensional simulations of biology and atmospheric chemistry be-
cause of nonlinear concentration-dependent phenomena.
For models that couple physics and chemistry in the atmosphere:
· The coupling must address questions such as what is the spatial-tem-
poral distribution of carbon monoxide, methane, and tropospheric ozone
and how might it change; what is the effect of changing climatic or chemi-
cal conditions on the aerosol-initiated stratospheric ozone depletion in the
Arctic and the Antarctic; how might the exchange of water vapor between
the troposphere and the stratosphere change in a changing climate; and what
is the vertical transport of trace species by cloud convection and how might
it change?
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22
RESEARCH STRATEGIES FOR THE USGCRP
Progress in the modeling of the coupled chemical-physical atmospheric
system requires a better knowledge of surface sources of trace gases and
their dependence on climatic conditions; chemical processes and reaction
channels, both in the gas and in the aqueous phase, and their dependence on
atmospheric conditions; and transport processes by advection and convec-
tion, including the development of high-resolution transport models coupled
to atmospheric GCMs with detailed representation of physical processes
including cloud formation and associated transport, boundary layer trans-
port, and troposphere-s~atosphere exchange. This progress is dependent on
the acquisition of global data sets for validation of these treatments.
Future progress will be dependent both on available computational
resources and on progress in developing our understanding of fundamental
physical and chemical processes and the nature of their coupling.
For models that couple the ocean and the atmosphere:
· The coupling must address questions such as how will changing cli-
mate affect oceanic carbon dioxide uptake and storage; how will oceanic
heat storage and transport change; how will the amount and distribution of
primary production change; how will the marine hydrological cycle change;
and how will a changing ocean affect a changing climate?
Critical issues include widely differing temporal and spatial scales,
inclusion of biological and biogeochemical dynamics, and sparse data. Par-
ticularly important and difficult tasks are the scaling of the biological-bio-
geochemical components from local-regional domains to basin-global do-
mains, formation of the upper mixed-layer physics, and inclusion of possible
biological feedbacks on mixed-layer dynamics.
· Progress in the development of coupled oceanic-atmospheric models
including biological-biogeochemical dynamics is limited, in part, by an in-
adequate theoretical or observational understanding of certain key processes
and a corresponding and continuing uncertainty as to how best to incorpo-
rate or parameterize them in oceanic GCMs.
.
The set of field programs (JGOFS, WOCE9 the Coupled Oceans At-
mosphere Research Experiment (COARE) organized under TOGA, and the
Global Ocean Ecosystem Dynamics (GLOBEC)) required to acquire the
data needed to advance our knowledge of fundamental oceanic processes is
already well defined. These programs also offer valuable opportunities for
simultaneous observational efforts, and these should be encouraged.
· The development of fully coupled models should be encouraged along
two parallel paths: the first devoted to developing basin- and global-scale
models with increasing levels of coupling, and the second leading to a
series of regional fine-scale models that could provide boundary conditions
and parameterization tests for the larger-scale models.
Several overarching issues exist regarding approaches to and testing of
models and the infrastructure necessary for their development:
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INTEGRATED MODELING OF THE EARTH SYSTEM
.
23
Validation is extremely difficult; models should be subjected to natu-
rally occurring perturbation tests that exercise the coupling. In addition,
large-scale phenomena offer a valuable opportunity for focusing model de-
velopments and testing model dynamics. Studies of these large-scale pro-
cesses will serve not only as diagnostic tests but also as prognostic tools.
It is urgent that testing of models and model combinations begin as
soon as possible. Experiments with global models will initially use simple
representations, but the lessons learned and data bases developed will be
critical to future improvements. Prototype global experiments will be espe-
cially important to exploring feedbacks between the production of long-
lived trace gas species and climate.
Two important themes are important in early testing of partial earth
system models: the global carbon cycle (carbon dioxide, methane, and
carbon monoxide) and the transient response to a changing greenhouse forc-
ing. The former exercises the chemistry and biology, whereas the latter
stresses the physics and biology. The obvious next step is coupling these
.
.
two themes.
.
The importance of experience gained through prototype modeling ex-
periments, including failure, should not be underestimated. Careful analy-
sis of failures can provide valuable information.
· Earth system modeling should serve as a focus and catalyst for inter-
disciplinary science. No one institution or group of investigators has more
than a fraction of the interdisciplinary talent necessary for the development
of an earth system model focused on multidecadal time scales. Thus sev-
eral teams and talented individuals should be supported, who with some
coordination could help perform the incremental steps toward the integrated
earth system model. Some of these groups may act primarily as synthesiz-
ers, their principal interest being in linking component pieces, while in
other groups the interest would be in component development.
Also needed in an overall modeling strategy are centralized facilities
and associated staff to serve the common needs of the various teams and
individuals and focus on issues of synthesis, continuity, documentation, and
extensive numerical experiments.
ATMOSPHERE-TERRESTRIAL SUBSYSTEM
The primary research issue for coupling atmosphere-terrestrial models is
understanding how processes with vastly differing rates of change, from
photosynthesis to community change, are coupled. Representing this cou-
pling in models is the central challenge to modeling the terrestrial biosphere
as part of the earth system (e.g., Allen and Wyleto, 1984; Huston et al.,
1988; King et al., 1990; Moore etal., 1989b, Smith et al., 1989~.
Terrestrial ecosystems participate in climate and in the biogeochemical
cycles on several temporal scales. The metabolic processes that are respon
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24
RESEARCH STRATEGIES FOR THE USGCRP
sible for plant growth and maintenance, and the microbial turnover associ-
ated with dead organic matter decomposition, move carbon and water through
rapid as well as intermediate time scale circuits in plants and soil. More-
over, this cycle includes key controls over biogenic trace gas production.
Some of the carbon fixed by photosynthesis is incorporated into plant tissue
and is delayed from returning to the atmosphere until it is oxidized by
decomposition or fire. This slower carbon loop through the terrestrial com-
ponent of the carbon cycle, which is matched by cycles of nutrients required
by plants and decomposers, affects the increasing Rend in atmospheric car-
bon dioxide concentration and imposes a seasonal cycle on that trend (Fig-
ure 2.2~. The structure of terrestrial ecosystems, which responds on even
longer time scales, is the integrated response to the intermediate time scale
carbon machinery. The loop is closed back to the climate system since it is
the structure of ecosystems, including species composition, that sets the
terrestrial boundary condition in the climate system from the standpoint of
surface roughness, albedo, and, to a great extent, latent heat exchange.
These separate temporal scales contain explicit feedback loops that may
modify the system dynamics. Consider again the coupling of long-term
climatic change with vegetation change. Climatic change will drive vegeta-
tion dynamics, but as the vegetation changes in amount or structure, this
355
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8
345
340
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325
320
315
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_ ~,.1., I ,l,,,,,l,,,,,l,,,,,l,,,,,l, l , lL,.,,l,, ,l l l l l l l , ,,,l,.,,,,,,,,,l,,,,,,, ~
58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90
YEAR
FIGURE 2.2 Concentration of atmospheric carbon dioxide in parts per million of
dry air (ppm) versus time for the years 1958 to 1989 at Mauna Loa Observatory,
Hawaii. The dots indicate monthly average concentration. (From C.D. Keeling et
al. (1989). Copyright (3 1989 by the American Geophysical Union.)
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INTEGRATED MODELING OF THE EARTH SYSTEM
25
will feed back to the atmosphere through changing water, energy, and gas
exchange. Biogeochemical cycling will also change, altering the exchange
of trace gas species. The long-term change in climate, driven by chemical
forcing functions (carbon dioxide and methane) will drive long-term eco-
system change. Modeling these interactions requires coupling successional
models to biogeochemical models to physiological models that describe the
exchange of water and energy between the vegetation and the atmosphere at
fine time scales. There does not appear to be any obvious way to allow
direct reciprocal coupling of GCM-type models of the atmosphere, which
inherently run with fine time constants, to ecosystem or successional mod-
els, which have coarse temporal resolution, without the interposition of a
physiological model. This is equally true for biogeochemical models of the
exchange of carbon dioxide and trace species. This cross-time-scale coupling
is important and sets the focus for the modeling strategy.
A Modeling Strategy: Prognosis for Progress
Intuitively, we might develop a global model of terrestrial ecosystem
dynamics by combining descriptions of each of the physical, chemical, and
biological processes involved in the system. In such a scheme, longer-term
vegetation changes would be derived by integrating the responses of rapidly
responding parts of the model. But we cannot simply integrate models that
describe the rapid processes of carbon dioxide diffusion, photosynthesis,
fluid transport, respiration, and transpiration in cells and leaves in order to
estimate productivity of whole plants, let alone entire ecosystems. The
nature of the spatial averaging implied in the selection of parameters and
processes to consider is difficult because of nonlinearities, which means
that the choice of scale influences the calculation of averages (see Rosswall
et al., 1988~.
To progress in the development of terrestrial ecosystem models, we choose
processes to treat in different models based on the phenomenological scales
involved. As is common in physical models, terms in fundamental equa-
tions can be included or ignored depending on the temporal and spatial
scales of interest (e.g., ignoring gravitational effects in quantum physics
and including Coriolis effects in large-scale fluid motion). Careful organi-
zation of a suite of models, each describing processes that operate at differ-
ent rates, is crucial to the practical development of terrestrial ecosystem
models for use in earth system models of global change.
Based on current model structures, atmosphere-biosphere interactions can
be captured with simulations operating with three characteristic time con-
stants (Figure 2.3~. The first level represents rapid (seconds to days) bio-
physical interactions between the climate and the biosphere (Figures 2.4a
and b). The dynamics at this level result from changes in water, radiation,
OCR for page 26
26
C
L
1
M
A
T
1
C
D
R
1
V
E
R
S
RESEARCH STRATEGIES FOR THE USGCRP
| TIME STEP
1 ,
SECONDS - DAYS
TIME STEP
DAYS - WEEKS
TIME STEP
1
ANNUAL
H2O
EVAPOTRANS PI RATI ON
ENERGY / WATER / CO2
LAI (SEASONAL)
FOLIAR C / N (SEASONAL)
HYDROLOGY / SOIL CHEMISTRY / TRACE GASES
DECOMPOSITION / MINERALIZATION / UPTAKE
LAl COTS)
NPP TOTAL)
DECOMPOSITION / MINERALIZATION / UPTAKE
NET CARBON EXCHANGE / NET ECOSYSTEM PRODUCTION
FIGURE 2.3 Three different time steps at which existing models of terrestrial
ecosystems use climatic information to modify rates of ecosystem function.
LEVEL 1
A 4) C H2O B HZO
~ ~ ~1! CO2
3- TOTAL
PRIMARY
r , PRODUCTION
_ I TEMPERATURE - _
l | l _
WATER, LIGHT
FAST TIME
1
_: 1
DECAY
POOL
FIGURE 2.4 Two diagrammatic representations of models converting short-time-
step environmental data (minutes to hours) into balances of energy, water, and
carbon. For these models, ecosystem structure, including leaf display and canopy
structure, are fixed. Nutrient fluxes other than emission and consumption of trace
gases are not dealt with.
OCR for page 56
56
RESEARCH STRATEGIES FOR THE USGCRP
measurements will be required to define the subsurface variability (satel-
lites do not detect many key processes, for example, new production and
vertical fluxes) and to provide a baseline for satellite measurements. Cur-
rently, an important concern is whether or not there will be an ocean color
satellite in orbit during the major field campaigns (i.e., WOCE and JGOFS).
This represents a potential critical gap in being able to link biological and
physical phenomena.
Data assimilation is an emerging reality in physical oceanographic mod-
eling and observational studies and may be used advantageously by biologi-
cal and biogeochemical oceanographers. This methodology aids in the in-
terpolation of physical observations by adding dynamical constraints. While
the quantity of physical data required to describe oceanic phenomena may
be reduced by the use of data assimilative models, it is more likely that
field estimates will be improved as a consequence of data assimilation.
The first attempts are now being made to develop the techniques neces-
sary to assimilate ocean color measurements into regional physical-biologi-
cal models. This is a promising direction for the development of models
that ultimately will have predictive capability for biological distributions in
the ocean.
One aspect that makes data assimilation into physical-biological models
challenging is that updating one ecosystem component (e.g., phytoplankton
from ocean color) requires that all other ecosystem components be adjusted
so that they are in equilibrium with the updated field. More specifically,
assimilative models will require estimates of the error fields of both the
assimilated data sets and the processes that are being parameterized. This
will allow quantitative estimates of the confidence in the forecast (or hindcast)
fields being produced. For example, in assimilating ocean color data into a
multicomponent ecosystem model, one needs to have an estimate of the
errors in the satellite data in time and space (i.e., particularly those associ-
ated with gap filling) as well as an estimate on the effect of zooplankton
grazing within the model. Such error estimation will require synergy be-
tween the modeling effort and the field programs.
In spite of these difficulties, the initial attempts at assimilating ocean
color data into physical-biological models have shown that the accuracy of
the model is improved, but that the improvement of the model diminishes
after a short time. The implication that data are needed at frequent intervals
for assimilation into physical-biological models is a potential area of re-
search that could be an important aspect of developing models to address
problems of carbon dioxide uptake by the ocean. Specifically, given the
inherent nonlinearity of biological processes as well as their occasional
"switching circuit" behavior, present data assimilation techniques are inad-
equate. Research into techniques involving nonlinear and nondifferentiable
forms is needed.
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INTEGRATED MODELING OF THE EARTH SYSTEM
57
From a more classical approach, biological models in which the flow
field is set as a boundary condition have been in use for about 15 years for
specific regional studies. Consequently, the dynamics and limitations in-
herent in these models are beginning to be understood. If physical-biologi-
cal models are to be developed to address the larger question of carbon
dioxide uptake by the ocean, then the question arises as to how to extend
the knowledge gained from regional, physically forced biological modeling
studies as well as from geographically restricted fully coupled models to
models developed for basin or global domains. While, in principle, it seems
straightforward to simply increase the model domain, in practice, this is not
so. At least three issues must be addressed: (1) how to link the biological
dynamics to biogeochemical changes important for global carbon studies,
(2) how much of the complexity that characterizes coastal biological sys-
tems needs to be transferred to larger-scale domains, and (3) how to match
the space and time scale requirements of coastal processes with those of
larger-scale systems. These three issues represent fundamental problems
that must be addressed if coupled oceanic-atmospheric-biogeochemical models
are to be developed to investigate carbon uptake by the world oceans.
Biogeochemical models link biology and chemistry at the level of nutri-
ents and carbon dioxide and are generally based on the forcing of nitrate or
phosphate fields. The full effect of the biology on the chemistry is gener-
ally not included, nor is the full effect of the chemistry on the biology. Yet
it is likely in a changing climate system that these processes may be impor-
tant. We need to better understand the sensitivity of the climate system to
changing biogeochemical systems, and, if found to be relevant, biogeochemical
systems must be included in our climate models. Including these relation-
ships is expected to be computationally expensive, and yet ignoring the
interaction of the physical-chemical-biological systems may lead to poor
predictions of climatic change.
Local to regional three-dimensional, coupled oceanic-atmospheric-bio-
logical models that use circulation fields obtained from sophisticated re-
gional primitive equation circulation models to produce "predictions" of
biological distributions are currently under development. The development
of this type of model should be given particular encouragement since this
development is essential for the realization of fully coupled global carbon
cycle models. In particular, these regional models can be used to test and
validate parameterizations and generalizations that are used in larger-scale
models. The development of realistic fine-scale regional models is also
desirable in that the output from these models can be used to specify the
boundary conditions for the basin- and global-scale models. Perhaps the
appropriate direction for modeling is along two parallel paths: one to develop
basin- and global-scale models with increasing levels of coupling and the
second to develop a series of regional fine-scale models that could provide
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RESEARCH STRATEGIES FOR THE USGCRP
boundary conditions and parameterizations tests for the larger-scale models.
Each regional model could, thereby, include the complexity and dynamics
appropriate for simulating the processes in a specific region, and at the
same time the necessity for maintaining reasonable and consistent interfaces
with the basin- and global-scale models would give the entire modeling
effort an overall framework.
Finally, much can be learned from extant models despite their limita-
tions. Careful analysis of the sensitivity of the systems, which are approxi-
mations to the climate and biogeochemical systems, will indicate the emphasis
needed in observational and modeling studies. The importance of experience
gained through modeling experiments, including failure, should not be un-
derestimated. Failure, when carefully analyzed in the refereed literature,
can be valuable to the scientific community as a whole.
Summary
The strategy is to acquire data through field experiments (e.g., TOGA/
COARE, WOCE, JGOFS; see chapter 7) designed to develop an under-
standing of processes and a description of phenomena. Models may aid in
the design and execution of these experiments as well as in the analysis and
interpretation of the measurements. New understanding of key processes
will be used to improve models and reduce or improve parameterizations.
These model enhancements are expected to lead to the ability to describe
biogeochemical and physical phenomena. This enhanced modeling ability
should lead to improved climate estimates, including error bounds from
which rational decisions may be made. Hence the development of the fully
coupled models should be encouraged along two parallel paths: one to
develop basin- and global-scale models with increasing levels of coupling
and the second to develop a series of regional fine-scale models that could
provide boundary conditions and parameterization tests for the larger-scale
models. Each regional model could include the complexity and dynamics
appropriate for simulating the processes in a specific region, and the neces-
sity for maintaining interfaces with the larger-scale models would provide
an overall consistent structure for model development.
In seeking to develop models of the coupled atmosphere-ocean-marine
biosphere and biogeochemical system, it is important, as mentioned earlier,
to recognize the value of "great failure." Linking atmospheric-oceanic-
biospheric models, even though costly in terms of human and computer
resources, should begin sooner rather than later. These early, relatively primitive
attempts will shed light on the difficult issues of scale, both spatial and
temporal, and the associated questions concerning the degree to which vari-
ous process complexities or details are required. This clarification may be
of particular importance as we scale the biological-biogeochemical compo
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INTEGRATED MODELING OF THE EARTH SYSTEM
59
nents from local-regional domains to basin-global domains, or as we seek to
better define and formulate the upper mixed-layer physics and possible bio-
logical feedback, such as shading due to phytoplankton blooms.
At the least, such attempts will encourage the creation of needed infra-
structure and will provide a basis for assessing better the required resources.
Part of the infrastructure enhancement would be the establishment of mod-
eling teams. Obviously, several parallel efforts will be needed.
Validation of these models is both difficult and critical. The first step is
to ensure that they reproduce major climate phenomena (e.g., the spring
bloom and E1 Nino). Testing (not validating) the "interfacing" models as
well as the earth system models that link them can be addressed in the U.S.
Global Change Research Program.
CRITICAL MODEL TESTS
The earth system modeling program should include three interface mod-
els, as well as models of the fully coupled system. This approach allows for
the rapid development of science and its inclusion into the less computationally
demanding (although still challenging) interface models. Also, certain av-
enues of validation are open to the interface models that will be difficult to
use for a full earth system model. The required abilities of the models and
the critical tests needed before they can be used with confidence are dis-
cussed below. As concepts are developed and tested in the interface models,
they should be included into an evolving earth system model that will form
the basis for long-term prediction.
The Challenge and Critical Tests
All of the models described below must be able to simulate system re-
sponse to the forcing induced, for example, by a carbon-dioxide-equivalent
doubling in the atmosphere. That is, all of the models must be able to
simulate the transient response of oceans, ecosystems, chemistry, or physi-
cal atmosphere to a change in physical climate induced by a greenhouse gas
~ .
forcing.
Other drivers and critical feedbacks (e.g., land surface albedo, clouds,
and oceanic heat transport) should be included when developing physical
climate scenarios for use as forcing functions. The forcing functions given
to the interface models will evolve as tested concepts from the interface
models are incorporated into the earth system model, presumably modifying
its predictions of whole-system response to changing greenhouse forcing.
Thus a continual interplay of interface and earth system models is required,
allowing for cyclic validation, failure, and modification.
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RESEARCH STRATEGIES FOR THE USGCRP
Finally, validation is impossible in the classical con~ol-experiment mode.
We have no other earth system to serve as a control, not to mention the
difficulties imposed by the multidecadal time frame and policy-relevant
aspects of this science. As a step in appraising these models, a series of
critical tests is described below that exploit various data sets, including
satellite data (e.g., Earth System Science Committee, 1988) and paleo-records
(see chapter 3~. These "tests" of these interface models provide an evalua-
tion of the models' capabilities prior to either their use in a predictive mode
or their inclusion in an earth system model.
The Interface Models
Atmosphere-Terrestrial Subsystem
The challenge for an interface model of the atmosphere-land biosphere is
to predict responses to changes in such phenomena as
water and energy exchange (and more generally the hydrological cycle
per se),
trace gas biogeochemistry,
· primary productivity and ecosystem carbon storage, and
· vegetation composition and structure due to the changing macroclimatic
forcing andlor atmospheric chemical composition.
Critical tests of this model prior to its use in the predictive mode will be
to
· reproduce current patterns of biogenic trace gas and carbon exchange,
using past and current climate as drivers;
· reproduce key aspects of coupling between the paleoclimate and pa-
leoecological records within regions of interest;
· simulate contemporary spatial and seasonal patterns of vegetation
properties, including primary productivity worldwide, using satellite indi-
ces as validation data;
· capture patterns of ecological change along anthropogenically induced
chemical gradients in the land component of this interface model; and
· simulate surface fluxes of radiation, especially solar, and including
spectral surface albedos in the atmospheric component of this interface
model. Adequate simulation of amounts and spatial and temporal distribu-
tion of precipitation must also be addressed.
Physical-Chemical Interactions in the Atmosphere
For this interface model of the physical atmosphere and the chemical
atmosphere, the challenge is to predict the change in chemical climate through
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INTEGRATED MODELING OF THE EARTH SYSTEM
61
a macroclimatic transient forcing. This will, of course, require either simu-
lation or forcing functions for the biospheric sources, which could be de-
rived from the atmospheric-terrestrial-biospheric model.
Critical tests will include simulation of
contemporary variations in global trace gas fields, especially of"inte-
grator species" such as methyl chloroform and carbon monoxide;
· methane and carbon dioxide concentrations and isotope ratios;
large-scale tropospheric ozone features such as are observed in the
tropics;
· high-latitude stratospheric ozone; and
exchange of water vapor between troposphere and stratosphere.
Atmosphere-Ocean Subsystem
For the interface model of the atmosphere-ocean subsystem, the chal-
lenge is to predict the responses of
· water and energy exchange,
· carbon dioxide exchange and carbon storage,
· pattern of the spring bloom, and
shifts in ecosystem composition and resultant shifts in oceanic mixed-
layer chemistry (e.g., alkalinity).
The critical tests of such models are whether they can capture the key
aspects of such large-scale phenomena as
· E1 Nino/Southern Oscillation (ENSO),
· North Atlantic spring bloom,
cross-shelf exchange,
· poleward heat flux, and
biogeochemistry of aeolian deposits.
INFRASTRUCTURE
. . . . .. . . . ..
The global change modeling effort, particularly on these longer time
scales, encompasses a class of scientific problems far broader than those
cnaractenzed by the physical climate system alone. The biogeochemical
system (see Figure 2.1) merits an equal emphasis. An overall strategy that
favors diversity is required, in that no one institution or group of investiga-
tors has more than a fraction of the interdisciplinary talent necessary for the
complete task. Research teams in a range of sizes should be supported.
Larger groups are needed for an overall integration role; smaller groups (5
to 10 people) would achieve the incremental steps (i.e., the linkages be-
tween the interface models) toward integrated earth system models. Indi
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62
i
RESEARCH STRATEGIES FOR THE USGCRP
vidual investigators will obviously also make important contributions along
these same lines. Some of these groups may act primarily as synthesizers
whose principal interest would be in linking component pieces; others would
develop the components.
The larger groups would most likely be associated with various central-
zed facilities, which would also serve the common needs of the various
teams for computational resources and linkages with large-scale models.
Examples of such needs include the preparation and analysis of large-scale
observational data sets (such as those that will be developed in preparation
for the NASA Earth Observing System (EOS), not to mention the essential
data set that EOS will provide following launch); operation of the large,
computationally intensive GCMs; documentation and maintenance of baseline
codes and protocols for information exchange used by the community; and
diagnostics of model output. These larger facilities would provide the physical
locations for the most capable supercomputers.
One of the more intriguing advances in computer technology is in the
area of massively parallel architectures. It appears that many of our current
models may be recast to operate in a parallel mode. Centralized facilities
could devote resources to this longer-term investment that would be diffi-
cult for smaller modeling teams to provide.
Given the rapid improvements in CPU power, especially in the worksta-
tion class and with parallel architectures, the gap between hardware and
software is increasing. Many of the model codes that we use are many
years old, and it is difficult to find the funds or the researchers required to
convert such codes to take advantage of new hardware. In addition, many
of the codes are unwieldy and poorly documented. Again, this is an area to
which larger, central teams could commit resources. Specifically, more
effort needs to be placed on software development than simply applying
existing codes in faster machines.
To aid in this process, all modeling teams, large and small, should be
encouraged to take advantage of various debugging and software develop-
ment tools. For example, code profilers that aid in parallelizing or vector-
ing codes should be used. Object-oriented methods that allow codes to be
reused or reconfigured more easily should be incorporated into new models.
One of the limitations of such efforts is that such tools are often cumber-
some and difficult to learn. Thus it is essential that new partnerships be
formed between the various hardware and software vendors and the scien-
tific users so that appropriate tools can be developed.
Lastly, the realm of visualization is becoming increasingly important in
handling the volume and increasing dimensionality of the data sets. Visual-
izat~on tools also need to be made more accessible. In addition to facilitat-
ing the analysis of the model output, such tools can play an important role
in testing and debugging by allowing the modeler to see every time step of
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INTEGRATED MODELING OF THE EARTH SYSTEM
63
the model, rather than relying on summary data sets. Visualization also
requires close coupling between He model and a data base system to Rack
the model output.
Clearly, these centers and the smaller modeling groups and individual
investigators must be mutually supporting. Smaller groups must be pro-
vided with the capability to run process experiments on full GCMs and full
earth system models at larger centers; moreover, they must have on-site
computer support, including workstations, advanced graphics, geographical
information systems, and ma~nfrarnes and, most importantly, the technical
staff to allow a full application of the on-site computer facilities as well as
the off-site supercomputers. Correspondingly, the centers must be able to
incorporate advances in subsystem and interface representations formulated
by the smaller modeling groups. Such a strategy must necessarily involve
multiagency support over many years.
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Representative terms from entire chapter:
research strategies
