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10 Modeling
Pages 445-516

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From page 445...
... We therefore need models -- numerical representations of the Earth system -- to express our understanding of the many components of the system, how they interact, how they respond to perturbations, and how they feed back to provide dynamical controls on overall system behavior. It is thus evident that the study of global environmental changes -- their causes, their impacts, and strategies for mitigation -- inescapably requires models that encompass the mutual interactions of the principal components of the Earth system.
From page 446...
... models of the full Earth system (see Figure 10.11) that can be used on multidecadal timescales and at spatial scales relevant to important policy formulation and impact assessment.
From page 447...
... MODELING 447 Physical Climate System : WCRP Climate Atmospheric Physics / Dynamics Change Sun Chemistry / Dynamics Ocean Dynamics Terrestrial Energy / Moisture Water External Forcing Stratospheric Greenhouse Human Human Global Moisture Soil : IHDP Gases Activities Activities Volcanoes Marine Terrestrial Land Biogeochemistry Ecosystems Use Tropospheric Chemistry Pollutants / Greenhouse Gases Biogeochemical Systems : IGBP FIGURE 10.1 Conceptual model of the Earth system. SOURCE: Adapted from NASA (1986)
From page 448...
... In sum, interactions among components over these longer timescales are likely to be as important as processes within each. Models must therefore deal with interactions between terrestrial ecosystems and the atmosphere, physical and dynamic interactions between the ocean and the atmosphere, the chemistry and physics of the atmosphere and ocean themselves, the land-ocean interface, and even the challenge of incorporating the human component.
From page 449...
... ; • ocean circulation and overturning; • aerosol forcing, requiring information on aerosol character and extent; • decadal to centennial variability; • land-surface processes, including the climate-induced changes in the struc ture and functioning of ecological systems with resultant changes in global chemical cycles; • short-term variability affecting the frequency and intensity of extreme and high impact events (e.g., monsoons, hurricanes, mesoscale storm systems, etc.) ; • non-linear and threshold effects that create the potential for surprises; and • interactions between chemistry and climate change and improved represen tation of atmospheric chemical interactions within climate models, thereby leading to improved understanding of the causes of trends in CH4, N2O, O3, CFCs, and aerosols."2
From page 450...
... Despite the difficulties that face modelers of terrestrial ecosystems, a coordinated strategy has been developed over the past five years to improve estimates of terrestrial primary productivity and respiration by means of measurement and modeling (see Box 10.1 and Chapter 2) .6 For terrestrial ecosystems at the global scale, there has been a focus on the carbon cycle.
From page 451...
... With the one-half degree gridscale, it is now possible to investigate the magnitude and geographic distribution of primary productivity on a global scale by a combination of monitoring by remote sensing and modeling of the biogeochemical aspects of terrestrial ecosystems. These models range in complexity from fairly simple regressions between key climatic variables and biological production to quasi-mechanistic models that attempt to simulate the biophysical and ecophysiological processes occurring at the plant level (including their scaling to large areas)
From page 452...
... Connected to this question is the development of dynamic vegetation models, which treat competitive processes within terrestrial ecosystems and their response to multiple stresses. For the atmosphere, a central question has been, is, and likely will continue to be the role of clouds.
From page 453...
... In addition, important processes often occur in the boundary layer, which generally is not adequately resolved. Adding atmospheric chemistry to a GCM thus places greater demands on the terrestrial and oceanic boundary conditions and dynamic simulations.
From page 454...
... clear that humans can cause environmental change, even on a global scale. It is equally clear that environmental changes, whether human caused or not, can have impacts on humans.
From page 455...
... Circulation FIGURE 10.2 Impact of increasing CO2 on the Earth's climate as simulated in a Geographical Fluid Dynamics Laboratory coupled ocean-atmosphere climate model. Shown are time series of (a)
From page 456...
... f Atmosphere-terrestrial systems focused on carbon and/or trace gases tend to have only one-way coupling (atmospheric forcing) and do not yet include critical biogeochemical feedbacks.
From page 457...
... Water entering terrestrial ecosystems directly affects plant growth, soil water properties, recharge of groundwater pools, and discharge into river systems. Water is redirected back into the atmosphere through the processes of canopy interception, followed by evaporation, transpiration, and direct soil evaporation, all of which moderate surface temperatures and provide a mechanism to recycle water for further precipitation.
From page 458...
... The loop is closed back to the climate system, since it is the structure of ecosystems, including species composition, that largely sets the terrestrial boundary condition in the climate system in terms of surface roughness, albedo, and latent heat exchange. In sum, terrestrial ecosystems influence climate and biogeochemical cycles on several temporal scales that involve feedback loops that may modify the climate and biogeochemical system dynamics.
From page 459...
... At fine spatial scales, such as the scale of a forest watershed, there is often coherence between water, energy, and carbon. Terrestrial models of water and energy exchange between the atmosphere and land surface operate at the subhourly to daily timescale, as do models of net photosynthesis.17 However, biogeochemical models capable of extrapolation over large spatial scales generally operate at weekly to monthly timescales with finer-scale dynamics being used as constraints.18 To properly model the exchange of water, energy, and important biogeochemical elements like carbon and nitrogen between the atmosphere and the land surface, it will be necessary to resolve differences in both temporal and spatial scale between linked atmospheric and biogeochemical models.
From page 460...
... Second, and more subtle, are the possible changes in net ecosystem production (and hence carbon storage) resulting from changes in atmospheric CO2, other global biogeochemical cycles (particularly nitrogen)
From page 461...
... From a broader perspective, the prognostic models of terrestrial carbon cycle and terrestrial ecosystem processes are central for any consideration of the effects of environmental change and analysis of mitigation strategies; moreover, these demands will become even more significant if countries begin to adopt carbon emission targets.25 Finally, while progress will be made (and is needed) on modeling terrestrial processes, more integrative studies also are needed wherein terrestrial systems are coupled to models of the physical atmosphere and eventually to the chemical atmosphere as well.26 Tying in the human component is clearly important.27 Soil Moisture Modeling studies of extreme (theoretical)
From page 462...
... Accurate prediction of soil moisture is crucial for simulation of the hydrological cycle and of soil and vegetation biochemistry, including the cycling of carbon and nutrients at local, regional, continental, and global scales. It thus plays a significant role in atmospheric models, hydrological models, and ecological models.
From page 463...
... This topic resurfaces in the subsection that addresses modeling perspectives at the mesoscale in the land-ocean subsystem discussion later in this chapter. Trace Gases The broad question is the role of terrestrial ecosystems and human activities in the regulation of atmospheric concentrations of CO2 and other radiatively active atmospheric constituents.
From page 464...
... In turn and as previously noted, terrestrial ecosystems recycle water vapor at the land surface/atmosphere boundary, exchange numerous important trace gases with the troposphere, and transfer water and biogeochemical compounds to river systems. This section33 focuses on this latter exchange and addresses the development of models to explore the possible changes in fluxes in rivers of water, carbon, nitrogen, phosphorus, and silicon from terrestrial biomes to the world's oceans.
From page 465...
... This water forms the basis of rivers and the recharge of aquifers; moreover, by definition it is tied to the coupled dynamics of the terrestrial ecosystem and the land-atmosphere water cycle.i The drainage basin "transforms" complex pat h In effect, consideration is then passed to the ocean-atmosphere section; however, the linkage through the coastal ocean of the inputs from the land at the land-(coastal) ocean boundary and the "open" ocean needs further consideration.
From page 466...
... The extent to which each biogeochemical process is specifically modeled would depend on the state of understanding, the availability of data, and the purpose for which the model was constructed. Multiple component models would be required dealing with terrestrial ecosystems (Chapter 2)
From page 467...
... MODELING 467 a b FIGURE 10.3 a and b The Simulated Topological Network (STN) for potential river systems at 30-minute (longitude × latitude)
From page 468...
... This latter will be important in regions such as the Amazon, where realistic hydrographs cannot be generated without an explicit consideration of these intermediate wetlands effects, lasting typically about six months per year. In addition to meteorological forcings such as precipitation, temperature, and radiation, addi j We note the discussion in the previous subsection on soil moisture, which raises important modeling issues that impact these considerations on runoff.
From page 469...
... Investigators41 proposed a general mesoscale model formulation that aggregates a simplified soil-vegetation-atmosphere transfer scheme with respect to a statistical distribution of topographic and soil properties. The resulting mesoscale hydrological model may significantly advance the issue of the appropriate land surface parameterization in climate models, which was highlighted in the soil moisture discussion in the land-atmosphere subsystem.
From page 470...
... 470 GLOBAL ENVIRONMENTAL CHANGE 0 1/2 KEY FEATURES 1/2 0 SOIL OF A GLOBAL CHARACTERISTICS PPT, PET, T HYDROLOGY LAND COVER MODEL TOPOGRAPHY Input 0 1/2 0 1/2 TRANSFER / FLOOD PARAMETERS WATER BALANCE MODEL TOPOLOGY ENGINEERING WORKS Single Grid Cells Predictions RUNOFF EVAPO TRANSPIRATION Inputs SOIL MOISTURE WATER TRANSPORT MODEL Inputs Multi-Grid Prediction TERRESTRIAL TRACE ECOSYSTEM GAS CUMULATIVE MODELS MODELS FLOW FIGURE 10.5 Key features of a global hydrology model. SOURCE: Vörösmarty et al.
From page 471...
... Global Applications Understanding secured through such case study work should be carried for k Model state variables include surface temperature, canopy water storage, soil moisture in two layers, and local water table depth as a bottom boundary. Potential evapotranspiration is computed from a nonlinear energy balance equation, and actual evapotranspiration is determined as the minimum of the potential and soil-vegetation-controlled moisture limitation.
From page 472...
... An alternative to increasing the resolution of global models is to use climatic boundary conditions to drive regional models with sufficient resolution. On the other hand, transient climatic time series and monthly discharge data for past climate over several decades at selected locations provide the opportunity for important tests of models, including appraisal of the impact of episodic events, such as El Niño, on surface water balance and river discharge in South America.
From page 473...
... Moreover, at both the river basin specific and the continental to global scales, the model outputs could be linked to complementary studies of coastal ocean productivity. There are, as discussed, significant issues regarding the adequacy of the needed data and the availability of the data that currently exist.
From page 474...
... . The following discussion focuses attention on three specific critical areas: clouds in atmospheric models, carbon in the ocean, and the problem of linking ocean circulation models with models of atmospheric circulation.
From page 475...
... Some methods involve focusing on quantifying well the statistics of the initial conditions and then evaluating the time response under a distribution of initial conditions. For models of the oceanatmosphere system this careful analysis of the statistical variability in the initial fields is problematic because of a lack of three-dimensional oceanic data fields with statistical information.
From page 476...
... The importance of clouds is best summarized by the recent Working Group 1 report of the IPCC: "The single largest uncertainty in determining the climate sensitivity to either natural or anthropogenic changes are clouds and their effects on radiation and their role in the hydrological cycle."57 Handling the physics and/or parameterization of clouds in climate models remains a central difficulty. Recently, it was reported (The Feedback Analysis of GCMs and In Observations [FANGIO]
From page 477...
... Will that be as true in the future? Given that the current generation of global climate models represent the Earth in terms of gridpoints spaced several hundred miles apart, many observed features on smaller scales, such as individual cloud systems, are not explicitly resolved by the global models.
From page 478...
... The primary controls are the circulation of the ocean and two important biogeochemical processes: the solubility pump and the biological pump, both of which act to create a global mean increase of dissolved inorganic carbon (DIC) with depth and therefore to maintain atmospheric CO2 at a level considerably lower -- about a factor of three -- than it would otherwise be.65 The interplay between the circulation of the oceans and the biogeochemical "pumps" determines the sea surface PCO2 and hence the primary determinants (with atmospheric PCO2 and sea surface winds)
From page 479...
... . This exploratory study found large regional discrepancies between ocean carbon models in estimating CO2 fluxes and carbon storage (see Plate 8)
From page 480...
... 75; the latter has been particularly focused on the various biogeochemical pumps at work in the ocean carbon system. Biogeochemical Pumps The "biogeochemical pump," which transfers CO2 in the surface ocean to other physical, chemical, and biological components, is actually three pumps: the solubility pump and the biological pump, which is itself two pumps -- the organic matter pump and the calcium carbonate pump.
From page 481...
... It has been pointed out by a number of modeling studies that if there were no biological pump the preindustrial atmospheric CO2 concentration would have been 450 ppm instead of 280 ppm. Any complete model of the natural ocean carbon cycle should therefore include the biological pump; however, most recent assessments of the oceanic uptake of anthropogenic CO2 have assumed that the biological pump would not be affected by climate change and have therefore only modeled the physical solubility pump.76 A recent exception is a coupled ocean-atmosphere model used to show that including a simple parameterization of the biological pump significantly altered the calculations of total uptake of CO2 over timescales of 70 to 140 years.77 This result demonstrates the importance of including simple models of the biological pump in ocean carbon cycle models.
From page 482...
... must be simulated. Fortunately, the calcium carbonate pump contributes relatively little to the vertical DIC gradient compared to the organic matter and solubility pumps.
From page 483...
... Ocean carbon cycle models, however, provide an attractive means for estimating the relative strengths as well as potential future patterns and rates of exchange. Ocean Tracers: A Diagnostic Tool for Ocean Carbon Cycle Models Since ocean circulation plays a key role in the natural and anthropogenic marine carbon cycle, we need to quantify its current effect to understand better its potential future role.
From page 484...
... By carefully separating out the nuclear component through correlations with other ocean tracers, oceanographers have been able to use the bomb 14C signal in the ocean as a primary dataset with which to evaluate ocean circulation models.86 Many new high-precision measurements of oceanic 14C are now becoming available from samples collected during WOCE and JGOFS. Because of its much finer resolution, validations using both natural and bomb 14C will benefit from this dataset.
From page 485...
... Finally, chlorofluorocarbons (CFCs) are excellent tracers of ocean circulation for several reasons.
From page 486...
... Research Priorities First and foremost, long-term consistent data are needed to support modeling investigations.p The various reanalysis projects are immensely valuable and should be extended to include coherent boundary forcing fields such as surface temperatures, wind stress, and sea-ice extent as well as independent estimates of precipitation and evaporation. As mentioned earlier, a central activity for the coming decade is to compare models with data; for the oceans this means careful comparison of models with WOCE and JGOFS ocean data fields.
From page 487...
... The oceans are very energetic on spatial scales of 10 to 100 km, yet these motions are not resolved in the current generation of global climate models. A major unsolved problem in oceanography is to determine the effects of these unresolved mesoscale ocean eddies on large-scale circulation and climate.
From page 488...
... Advanced three-dimensional atmospheric models were developed to study the interaction of chemistry, dynamics, and radiation in the stratosphere. These extensive calculations were necessary for evaluating the simpler models used in the policy assessment studies as well as for understanding the climatic impact of the Antarctic ozone hole.
From page 489...
... . Most effort in three-dimensional atmospheric chemistry models over the past decade has been in the use of transport models in the analysis of certain chemically active species (e.g., long-lived gases such as N2O or the CFCs)
From page 490...
... The additional burden imposed by incorporating detailed chemistry into a comprehensive GCM has made long-term simulations and transient experiments with existing computing resources impractical. Current three-dimensional atmospheric chemistry models that focus on the stratosphere seek a compromise solution by combinations of expedients: using coarse resolution (both vertical and horizontal dimensions)
From page 491...
... Chemical Transport Models Coupled to the NCAR CCM2 The chemical transport model coupled to the NCAR Community Climate Model (CCM2) is very similar to IMAGES for the surface sources of trace constituents, chemical reactions, and surface deposition.
From page 492...
... Perhaps an even more important deficiency in models used to study atmospheric chemistry is the failure to include, or to treat adequately, cloud processes and the hydrological cycle. This fault results from both inadequacy of computational resources and incomplete understanding of the hydrological cycle.
From page 493...
... Understanding will be advanced partly by systematic observations of different terrestrial ecosystems and surface marine ecosystems under variable meteorological conditions and by the development of ecosystem and surface models that will provide para meterizations of these exchanges. • Global empirical models of surface emissions are necessary to ex trapolate and interpolate individual measurements provided in differ ent environments under different conditions.
From page 494...
... 3. Transport models coupled to GCMs, with detailed representation of physical processes, including cloud formation and boundary layer trans port, are required to simulate how advection, turbulence, and convection affect the chemical composition of the atmosphere.
From page 495...
... These changes affect the atmosphere through altered physical properties such as albedo and roughness and consequently an altered energy balance over the more intensively managed parts of the land surface, as well as through changed fluxes of H2O, CO2, CH4, and other trace gases between soils, vegetation, and the atmosphere. Changed land use also greatly alters the fluxes of carbon, nutrients, and inorganic sediments into river systems and consequently into many oceanic coastal zones.
From page 496...
... Predicting the future response of the Earth system to changes in land use and land cover will require projections of trends in the human contributions to these global changes, and this sort of modeling presents difficult challenges because of the multiple factors operating at local, regional, national, and global levels to influence local land use decisions.106 The complex linkages between human activity and global change are equally important for activities other than land use. In particular, anthropogenic changes in material and energy fluxes, resulting from such activities as fossil fuel combustion and chemical fertilizer use, are expected to increase in the coming decades.
From page 497...
... Studying human linkages with other components of the Earth system may be the most difficult challenge in modeling global change and the most important. Understanding human impacts and potential responses is a central purpose of the endeavor of global change research.
From page 498...
... It is a key component in the land surface schemes in GCMs, since it is closely related to evaporation and thus to the apportioning of sensible and latent heat fluxes, and accurate prediction of soil moisture is crucial for simulation of primary production and of soil and vegetation biochemistry, including trace gas exchanges.
From page 499...
... These developing models can simulate ecosystem responses with particular emphasis on vegetation dynamics on timescales from decades to centuries and provide a means of investigating responses to disturbances such as deforestation. However, a fundamental problem in assessing the results of terrestrial ecosystem models is a lack of good validation data.
From page 500...
... There are also greatly differing degrees of parameterization with insufficient understanding as to their effects. For this subsystem attention is focused on three specific areas: clouds in atmospheric models, carbon in the ocean, and the problem of linking ocean circulation models with models of atmospheric circulation.
From page 501...
... Ocean models are far from perfect, however, and much work is required if reliable predictions of future oceanic CO2 uptake are ever to become feasible. There is a fundamental need to compare ocean carbon cycle models and thereby to clarify key physical and biogeochemical processes.
From page 502...
... • Chemical models with a detailed set of reactions, in which transport is ignored, need to be developed. • Transport models coupled to GCMs, with detailed representation of physi cal processes, including cloud formation and boundary layer transport, are required to simulate how advection, turbulence, and convection affect the chemical composition of the atmosphere.
From page 503...
... Hence, representing the linkages between humans and other components of the Earth system poses a challenge in modeling the Earth system, and hence understanding them is essential to understanding the behavior of the whole system and to providing useful advice to inform policy and response. Causal models of social processes have large uncertainties and pose deep problems, which may be of qualitatively different character than those associated with modeling nonhuman components of the Earth system.
From page 504...
... , the Atmospheric Model Intercomparison Project (AMIP) was established in which the 10-year period 1979 to 1988 has been simulated by 30 different atmospheric models under specified conditions (see http://www.wmo.ch/)
From page 505...
... B Mitchell's working paper for the JSC/CLIVAR Working Group on Coupled Models, September 1997.
From page 506...
... 83. The role of the biological pump in the ocean carbon cycle, discussed only briefly in Houghton et al.
From page 507...
... Global Biogeochemical Cycles 9:621-636. Banse, K., and D.C.
From page 508...
... Comparing global models of terrestrial net primary productivity (NPP) : Analysis of the seasonal behaviour of NPP, LAI, FPAR along climatic gradients across ecotones.
From page 509...
... Comparing global models of terrestrial net primary productivity (NPP) : Overview and key results.
From page 510...
... Global Biogeochemical Cycles. Henderson-Sellers, A
From page 511...
... In press. Comparing global models of terrestrial net primary productivity (NPP)
From page 512...
... Global Biogeochemical Cycles 6:101-124. McGuire, A.D., L.A.
From page 513...
... In press. Comparing global models of terrestrial net primary productivity (NPP)
From page 514...
... Comparing global models of terrestrial net primary productivity (NPP) : Analysis of differences in light absorption, light-use efficiency, and whole respiration cost.
From page 515...
... Comparing global models of terrestrial net primary productivity (NPP) : Comparison of annual NPP to spatial climatic drivers and the normalized difference vegetation index.
From page 516...
... In Asian Change in the Context of Global Change: Impacts of Natural and Anthropogenic Changes in Asia on Global Biogeochemical Cycles, J Galloway and J


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