Terrestrial ecosystems and climatic change

The structure and function of terrestrial ecosystems depend on climate, and in turn, ecosystems influence atmospheric composition and climate. A comprehensive, global model of terrestrial ecosystem dynamics is needed. A hierarchical approach appears advisable given currently available concepts, data, and formalisms. The organization of models can be based on the temporal scales involved. A rapidly responding model describes the processes associated with photosynthesis, including carbon, moisture, and heat exchange with the atmosphere. An intermediate model handles subannual variations that are closely associated with allocation and seasonal changes in productivity and decomposition. A slow response model describes plant growth and succession with associated element cycling over decades and centuries. These three levels of terrestrial models are linked through common specifications of environmental conditions and constrain each other. 58 refs

Saturn Atmospheric Structure and Dynamics

2 Saturn inhabits a dynamical regime of rapidly rotating, internally heated atmospheres similar to Jupiter. Zonal winds have remained fairly steady since the time of Voyager except in the equatorial zone and slightly stronger winds occur at deeper levels. Eddies supply energy to the jets at a rate somewhat less than on Jupiter and mix potential vorticity near westward jets. Convective clouds exist preferentially in cyclonic shear regions as on Jupiter but also near jets, including major outbreaks near 35°S associated with Saturn electrostatic discharges, and in sporadic giant equatorial storms perhaps generated from frequent events at depth. The implied meridional circulation at and below the visible cloud tops consists of upwelling (downwelling) at cyclonic (anti-cyclonic) shear latitudes. Thermal winds decay upward above the clouds, implying a reversal of the circulation there. Warm-core vortices with associated cyclonic circulations exist at both poles, including surrounding thick high clouds at the south pole. Disequilibrium gas concentrations in the tropical upper troposphere imply rising motion there. The radiative-convective boundary and tropopause occur at higher pressure in the southern (summer) hemisphere due to greater penetration of solar heating there. A temperature “knee ” of warm air below the tropopause, perhaps due to haze heating, is stronger in the summer hemisphere as well. Saturn’s south polar stratosphere is warmer than predicted by radiative models and enhanced in ethane, suggesting subsidence-driven adiabatic warming there. Recent modeling advances suggest that shallow weather laye

Simulation of land-use patterns affecting the global carbon cycle. [Reconstruction and projection of CO/sub 2/ scenarios from 1860 to 2460]

Past increase of atmospheric CO/sub 2/ involves significant ntributions from both fossil and nonfossil (biospheric) sources. A simulation model was used to reconstruct changes since 1860 and project four hypothetical future scenarios of CO/sub 2/ injection to 2460. Nineteen compartments and their exchanges of carbon were considered. Areal extent of tropical forests, other wooded ecosystems, and nonforests were incorporated into the model. Rapidly and slowly exchanging pools of carbon per unit area, and net primary production for each pool and exosystem group, were projected by integrating income-loss differential equations numerically using CSMP programming language. Estimated cumulative releases of CO/sub 2/ from fossil fuels (plus cement) near 120 Gtons of carbon (1 Gton = 10/sup 9/ metric tons) from 1860 to 1970 were assumed to equal prompt and delayed releases from forest clearing. Limits of exploitable forest area and biomass were evaluated and found to contribute much less future CO/sub 2/ than the usable coal, oil, gas, and oil shale. Ultimate release from the latter (7500 +- 2500 x 10/sup 9/ tons of C) could increase atmospheric CO/sub 2/ manyfold: doubling the assumed 1860 levels as early as (1) year 2025 for assumed nominal scenario (expanding releases slightly less rapidly than at present), (2) year 2033 for a delayed expansion scenario that would prolong use of fossil reserves (lowering peak carbon release rate from approx. 43 to approx. 28 Gtons/year), (3) year 2087 for a slow burner scenario (increasing very slowly from present levels), and (4) year 2290 for a combination scenario (which assumes low fossil-fuel use, high carbon storage, and high net primary production of forested exosystems)

Calibration and testing or models of the global carbon cycle

A ten-compartment model of the global biogeochemical cycle of carbon is presented. The two less-abundant isotopes of carbon, /sup 13/C and /sup 14/C, as well as total carbon, are considered. The cycling of carbon in the ocean is represented by two well-mixed compartments and in the world's terrestrial ecosystems by seven compartments, five which are dynamic and two with instantaneous transfer. An internally consistent procedure for calibrating this model against an assumed initial steady state is discussed. In particular, the constraint that the average /sup 13/C//sup 12/C ratio in the total flux from the terrestrial component of the model to the atmosphere be equal to that of the steady-state atmosphere is investigated. With this additional constraint, the model provides a more accurate representation of the influence of the terrestrial system on the /sup 13/C//sup 12/C ratio of the atmosphere and provides an improved basis for interpreting records, such as tree rings, reflecting historical changes in this ratio

Simulation in the service of environmental research: experience with a stream ecosystem project

A perspective on the use of simulation modeling as one working component of a complex ecosystem research project is presented. Experiences with modeling in the context of a research program dealing with nutrient dynamics in stream ecosystems are documented. As the project progressed, the role of modeling changed, resulting in four distinct models, each developed for a specific purpose. The models have been used to screen initial hypotheses, set up details of the experimental design, analyze results, and serve as a logical framework for investigating mechanistic details of the ecosystem. Through time, individual models were discarded, not because they were inadequate, but because they were irrelevant to the next stage of the project. It was particularly interesting to note how the importance of validation, as well as its characteristics, have changed during this process

Spectral analysis and forest dynamics: the effects of perturbations on long-term dynamics

Long-term dynamics of forest growth and succession and the effects of environmental modification can be studied using stochastic stand growth models. The result of simulations using these models is a set of stochastic time series for stand characteristics such as total biomass. Additional analysis is generally required to clarify behavior present in the time series. In this paper spectral analysis is used to elucidate differences in dynamics of perturbed and unperturbed forest stands as evidenced by comparison of peaks in the spectral density representing various types of cyclic behavior in the total biomass of the stand. A typical Appalachian deciduous forest stand area is considered. The consequences of three types of environmental perturbations are analyzed: a blight resulting in the elimination of a dominant species, a change in average annual temperature representing the greenhouse effect of gases released to the atmosphere, and a change in the growth rate of selected tree species due to increases in the concentration of atmospheric pollutants or to acid precipitation. The stand growth model was run for each type of environmental change as well as for unperturbed conditions. Simulations were made for a 1000 year time period. The power spectral density was estimated for the total stand biomass time series using the direct method for the perturbed and unperturbed cases. The general forms of the spectral densities were different. Differences were also noted in the numbers and frequencies of specific peaks in the spectral density. An explanation is given for these differences based on tolerance characteristics, gap replacement processes and the roles of dominant tree species

Modeling the role of terrestrial ecosystems in the global carbon cycle

A model for the global biogeochemical cycle of carbon which includes a five-compartment submodel for circulation in terrestrial ecosystems of the world is presented. Although this terrestrial submodel divides carbon into compartments with more functional detail than previous models, the variability in carbon dynamics among ecosystem types and in different climatic zones is not adequately treated. A new model construct which specifically treats this variability by modeling the distribution of ecosystem types as a function of climate on a 0.5/sup 0/ latitude by 0.5/sup 0/ longitude scale of resolution is proposed

Functional complexity and ecosystem stability: an experimental approach

The complexity-stability hypothesis was experimentally tested using intact terrestrial microcosms. Functional complexity was defined as the number and significance of component interactions (i.e., population interactions, physical-chemical reactions, biological turnover rates) influenced by nonlinearities, feedbacks, and time delays. It was postulated that functional complexity could be nondestructively measured through analysis of a signal generated from the system. Power spectral analysis of hourly CO/sub 2/ efflux, from eleven old-field microcosms, was analyzed for the number of low frequency peaks and used to rank the functional complexity of each system. Ranking of ecosystem stability was based on the capacity of the system to retain essential nutrients and was measured by net loss of Ca after the system was stressed. Rank correlation supported the hypothesis that increasing ecosystem functional complexity leads to increasing ecosystem stability. The results indicated that complex functional dynamics can serve to stabilize the system. The results also demonstrated that microcosms are useful tools for system-level investigations

Models of Clouds, Precipitation, and Storms: Atmosphere and Precipitation, Ice and Glaciers, Oceans and Coasts, Soils and Mineral‐Water InterfaceAtmosphere and Precipitation

International audienceClouds play an important role in life on Earth. Apart from influencing the radiative balance of the atmosphere and the lifetime of atmospheric trace constituents, they are the essential element in the hydrological cycle.This article provides an introduction to the complex subject of cloud modeling, from their formation up to the production of precipitation, and the development of cloud and storm systems. The elements intervening in cloud modeling are exposed, starting from a description of the physical phenomena. On the basis of the occurring scale problem, a number of approaches for simplification are presented. These simplifications concern the dynamics as well as the microphysics. Bulk and bin modeling approaches as well as cumulus parameterizations are explained. Some numerical problems are discussed. This approach gives an insight into the current state‐of‐the‐art cloud modeling and the necessary balance between the degree of parameterization, the number of physical and chemical processes relevant to a particular problem, and the available computing resources