SCOPE 35 - Scales and Global Change

INTRODUCTION

 Many of the most pressing environmental problems facing mankind today are global in nature. An understanding of global environmental processes, a prerequisite for finding scientific solutions to these problems, will be found only by combining many disciplines, including ecology, oceanography, meteorology, geomorphology, and geology. As a result of the widespread recognition of the need for an international research programme on geo sphere-biosphere interactions, the concept of the International Geo sphere- Biosphere Program was developed (Malone and Roederer, 1985; National Research Council, 1983, 1986). The plan for this ambitious global research programme includes the following steps:

(a) designing specific studies of the most important processes in global environmental change

(b) developing models of interfacial problems incorporating existing and newly derived data

(c) performing carefully designed tests of these models

(d) making observations based on these process studies models, and experimental tests.

At each step, integrating data from different disciplines and from different spatial and temporal scales will be important components of the research designs (Risser, 1986). This book is based on a planning workshop designed to assist in the formulation of appropriate research approaches to be used in the coherent investigation of geospheric-biospheric processes.

Except for cumulonimbus thunderstorm clouds which reach heights greater than 10 km above the earth's surface, most of the clouds and humidity are found in the lower few kilometres of the global atmosphere. The most rapid and turbulent fluxes of atmospheric materials are found in the planetary boundary layer at heights of one to several kilometres although the stratosphere, which extends to about 50 km, has significant photochemical connections to the lower atmosphere layers. However, these processes, and the resulting chemical fluxes, demonstrate marked vertical and horizontal variation, and, in addition, this variation occurs on time scales from seconds to decades or longer (Bolin and Cook, 1983).

Vegetation, as an indicator of terrestrial ecosystems on the earth's surface, responds to changes in atmospheric conditions, but it also influences atmospheric processes by affecting fluxes of energy, momentum and water. Similarly, oceans play equally interactive roles by influencing not only atmospheric conditions, but also terrestrial ecosystems. Geological processes, especially those relating to topography and soils, cause certain characteristic responses in climate and vegetation, and, in turn, geologic processes are influenced by vegetation and climate. Thus, the interactions among the biospheric components are complicated, and operate over a broad array of spatial and temporal scales. This initial chapter will amply describe this complexity, largely from the point of view of each discipline. In this introductory chapter, the most important themes and challenges will be presented and distilled in order to focus on common components of an integrated geosphere-biosphere research programme.

ATMOSPHERE

In the 1960s the first mathematical attempts were made at describing the  atmospheric patterns of the globe. The general circulation models (GCMs), began with equations of existing weather prediction models, to which were added equations for the dynamics of water vapour, including evaporation, precipitation, and movement, and equations for atmospheric radiation (Dickinson, 1988). These models describe processes in the atmosphere and the oceans on scales larger than a few hundred kilometres in the horizontal directions and one to a few in the vertical. Thus, long term integrations will not permit horizontal spatial resolutions of smaller than about 100 km (Bolin, 1988).

Though there are meoscale climate models and smaller scale models which focus on a localized set of conditions, it is now clear that some components of this finer scale resolution must be incorporated into general circulation type models. The necessity of this model enhancement can be easily demonstrated by recognizing the need to extend the current numerical weather prediction approaches to periods of several weeks or longer. This improvement in general circulation models must incorporate fluctuations in the hydrological cycle, radiation, and surface fluxes (Dickinson, 1988). Furthermore, as noted below, understanding the behaviour of atmospheric gases involves the expansion of circulation models to include interactions among the physical, chemical, and biological processes found in the lower atmosphere, top layer of soils, uppermost layers of ocean sediments, oceans and terrestrial ecosystems' portions of the biosphere (Bolin, 1988).

Improvement in modelling the atmosphere will involve the need to characterize many atmospheric processes at scales smaller than 100 x 100 km. Current models will require greater definition in the description of the radiative properties of clouds, and the vertical transfer of heat, moisture, and momentum by moist and dry convection. In linking small scale atmospheric process models to larger circulation models, the following issues are of particular importance (Dickinson, 1988):

(a) identifying appropriate parameterizations for the energy-transfer processes within vegetation canopies that yield satisfactory descriptions of the vegetation-atmosphere exchanges of heat and moisture.

(b) providing adequate descriptions of the soil and plant root resistance processes that limit the rate at which water can be transferred from the soil to the leaves and then to the atmosphere (Dickinson, 1984; Bolin, 1988).

The challenge to describing the fluxes does not only concern the development of methods to statistically characterize these terms over heterogeneous surfaces of the globe, but also to capture the importance of long-term changes of the biota and soils. These changes are often obscured by variations occurring on relatively short time scales such as daily cycles, weather variations, and various annual cycles. Thus, the crucial issues for models in the realm of atmospheric-terrestrial and marine ecosystem interactions involve the interplay between abiotic factors that describe the atmosphere and inorganic substrate with the characteristics of the biota including the organic component of the soil (Bolin, 1988).

TRACE GASES

It is now possible to state unequivocally that the atmospheric constituents are changing dramatically, and that, in some cases, these changes involve magnitudes that lead to rates approximately doubling on time scales of decades. Carbon dioxide has increased from concentrations of less than 280 ppm a few hundred years ago to present values of about 345 ppm (Neftel et al., 1985). Notable and perhaps deleterious increases have also been measured for chlorofluorocarbons (CCl₂F₂ and CCl₃F), methane (CH₄), tropospheric ozone (O₃) and nitrous oxide (N₂O). The sources of these increases are not completely understood, but some are generated from human activities such as combustion of fuel, agricultural practices, and land use changes.

Various models have been used to predict the probable warming of the atmosphere caused by increases in CO₂ and other trace gases. Most suitable advanced global circulation models which include reasonable continuity and a seasonal cycle for solar radiation (Manabe and Stouffer, 1980; Hansen et al., 1984; Washington and Meehl, 1984) indicate a steady state global warming of from 2.0 to 4.8 ^oC average temperature for a doubling of CO₂. However, virtually all model simulations predict greater temperature increases at high latitudes, especially in the winter, because of more stable temperature stratification and ice-snow feedbacks.

Other compounds produced at the earth's surface by biological and anthropogenic processes are not uniformly distributed in the atmosphere. Oxides of nitrogen (NO and NO₂) are primarily produced from specific sources (e.g. power plants, industrial centres) and since the lifetime of NO_x is only a few days, the dispersion of these molecules in the atmosphere is uneven. In addition, however, physical-chemical processes of the interactions between the biospheric and atmospheric processes also lead to a non-homogeneous distribution of trace gases. For example, although only about ten percent of all atmospheric ozone is found in the troposphere, this small amount of ozone is extremely important because with ultraviolet light it produces hydroxyl radicals (OH). In the background troposphere, two-thirds of the OH react   with CO, one-third with CH₄, and small fractions with other atmospheric gases. The concentration of NO, which as just noted is short-lived and spatially heterogeneous, plays a significant role in determining the oxidation pathways of methane and carbon dioxide. It is likely that global increases in CH₄ have contributed to increases in O₃ concentration in NO-rich environments (particularly in the highly industrialized regions of the northern hemisphere) and decreases in hydroxyl concentrations in NO-poor environments elsewhere. Thus, lesser quantities of industrial and natural gases may be oxidized in the tropics by reactions with OH which, over time, may lead to an increase in atmospheric trace gases in this region (Crutzen, 1988).

Understanding these atmospheric-biospheric chemical systems demands resolution of the temporal and spatial heterogeneous distribution and sources of short-lived, reactive compounds. Indeed, the discontinuity of both emission sources and atmospheric chemical interactions is a major challenge to the formulation of photochemical-meteorological models describing the transport and photochemistry of these reactive compounds. Given this complexity, it may be necessary to use other approaches, such as the incorporation of stochastic simulation of boundary layer and convective processes which may be derived from statistical analysis of results obtained by large scale meteorological models (Crutzen, 1988).

OCEANS

Oceans play major roles in numerous biospheric processes, but the magnitude and even the direction of these roles is just now being recognized because of the recent availability of adequate instrumentation. Approximately 30% of the global annual plant carbon fixation comes from the seas which also form an integral part of the N flux of the earth (Lewis et al., 1986; Walsh and Dieterle, 1988). The ocean is also the main sink for CO₂ produced by human activities, and because of its size and heat capacity, the ocean dampens changes in the earth's energy balance (Oeschger, 1988). However, the general patterns have not been measured nor understood specifically enough to predict the response of the biosphere to future modifications of the environment. For example, uncertainties in the magnitudes of the annual uptake of both carbon and nitrogen have been caused by the inability to accurately specify temporal and spatial heterogeneities of phytoplankton biomass and associated productivities in the ocean shelf regions. Under some conditions the Nimbus-7 coastal zone scanner can now measure oceanic chlorophyll concentration to ± 30% . Significant improvements in productivity estimates will be achieved when satellite measurements of ocean colour can be combined with in situ observations. In fact, the applicability of more sophisticated regional productivity models to the oceans is still restricted by scarcity of information about the coupling of physical dynamics with biological processes on the appropriate time and space scales (Walsh and Dieterle, 1988).

GEOLOGY AND SOILS

Much of the attention on geosphere-biosphere issues has focused on the atmosphere and oceans where the global nature of the process is so obvious because of the broad expanses and the lack of conspicuous boundaries. For the reverse reasons, global processes in groundwater systems have proven particularly challenging. Spatial scales range from less than a nanometre when considering interactions between water molecules and dissolved chemicals to hundreds of kilometres when assessing and managing regional groundwater systems. As in atmospheric and oceanic systems, expanding the spatial and temporal scales of groundwater systems demands consideration of different characteristics. For example, larger spatial scales mean increasing importance of geological heterogeneities and anisotropy, and of the effects of long-term recharge variations on the water balance of a system for longer time periods (van der Heijde, 1988).

The complexity of the groundwater system offers many challenges to the construction of models, especially as groundwater processes relate to climatic conditions and geomorphology. A major problem is how to distinguish among variables that can be considered as constants or as being uniform across discrete intervals of time and space dimensions, and those non-uniform variables that must be included in models. An example involves the modelling of infiltration into the soil and subsequent percolation toward the saturated zone (van der Heijde, 1988). Runoff from precipitation is divided into a single horizontal segment and infiltration into a one- or two-dimensional vertical component. The infiltrated water percolates to the groundwater where a two-dimensional horizontal or three-dimensional model is used. For each of these sub models different timesteps are invoked, from hourly for surface runoff and daily for percolation, to weekly or monthly for the flow in the saturated zone. To address global issues it will eventually be necessary to develop integrated models which account for the relevant variables on medium-scale drainage basins and physiographically defined spatial units. Since at any given moment some portions of the geomorphic system are eroding and some are aggrading, the definition of the spatial scale and the recognition of the importance of episodic events, are critical in the connections between superficial and groundwater processes (Schumm, 1988).

VEGETATION

The vegetation plays a dynamic role in numerous of these processes, e.g. moderating surface flows of water and sediments, producing gases to the atmosphere, modifying soil processes, and responding to various environmental conditions. These interactions occur over a wide range of space and time scales (Delcourt et al:, 1983; Emanuel et al., 1985). The successional response of vegetation to disturbance and changing environmental conditions occurs in a number of ways, such as changes in productivity, species composition, and rates of material cycling. These successional processes have been modelled under a variety of conditions, but it is now clear that these successional models must be made more responsive to changes in atmospheric conditions. A significant need appears to be the realistic input of synoptic climatological variables that are associated with vegetation successional processes (Shugart et al., 1988).

Successional models now incorporate the consequences of the natural history characteristics of plants as well as their physiology and demography. Transect models include both the mechanistic formulation of important population processes and the realism of relatively small-scale spatial heterogeneity. Competition among individuals is usually modelled as a function of the proximity and size of neighbouring individuals. Despite the realism of these vegetation models, atmospheric perturbations have traditionally been treated in a simplistic fashion and furthermore, the relevant atmospheric variables have been assumed to exert primarily first-order effects rather than acting through intermediate agents such as defoliators, herbivores, or disease (Shugart et al., 1988). Thus, the next generation of terrestrial ecosystem or vegetation models must recognize the importance of climatic history in explaining the present vegetation patterns, must realistically couple vegetation responses to changing climatic conditions, and must describe how the spatial heterogeneity of vegetation affects biospheric processes.

ANALYTICAL CHALLENGES

Inherent in the studies described and proposed in this volume is the need to relate measurements of geospheric-biospheric processes made in several different scientific disciplines. A major challenge to these data interrelations is that collections of many of these previous measurements have not followed the formulation of multidisciplinary a priori hypotheses and clear statements about the relevant inferences (Jeffers, 1988). Furthermore, the necessary measurements to test the most important interdisciplinary ideas range over broad spatial and temporal scales. Thus, the analytical challenge is to adopt valid methods of sampling within appropriate experimental designs in order to measure the relevant spatial and temporal heterogeneity. A portion of the success in these interdisciplinary studies will depend upon the selection of coherent experimental variables which bridge the procedures used and the measurements made by two or more disciplines involved in the analysis (Allen and Starr, 1982; O'Neill, 1988).

In some cases the challenge of linking space and temporal scales will be met by new methodology such as remote sensing techniques capable of refined measurements of the chemical constituents of the soil, vegetation, and atmosphere. Other experimental procedures will involve the careful construction of models for systems in which the models are specifically designed to simulate the dynamics of several common variables crucial to the biospheric system in question. In virtually all approaches, however, invoking detailed mechanistic information about the processes will be essential to designing appropriate experiments and building reasonable models (Woodmansee, 1988).

An example of a successful experimental and analytical approach involves the measurement of fluxes of gaseous nitrogen compounds from terrestrial ecosystems. As noted earlier, these gases influence the climate as greenhouse gases, participate in the formation and destruction of ozone, contribute to atmospheric acidity, and are significant vectors for loss and gain of nitrogen from terrestrial ecosystems (Lacis et al., 1981; Crutzen, 1983; Bolin and Cook, 1983). Nitrogen-gas fluxes typically display a high degree of spatial and temporal variability (Schimel et al., 1986). In evaluating this variability, Schimel et al. ( 1988) noted that in a Swedish meadow, a significant amount of the fine scale variation in N₂O production could be explained by spatial autocorrelation and by correlation with soil water. In the shortgrass prairie, differences in N₂O production seasonally and between sites were closely coupled to soil mineral nitrogen dynamics (Schimel et al., 1985). The data suggested that nitrification was the dominant vector for N₂O production, and that patterns of temperature and water availability affect N₂O production by controlling nitrogen mineralization and nitrification. From an experimental and analytical viewpoint, the common technique employed in these studies was that the difficult-to-measure gas flux rates were related to more readily measured soil and landscape properties using statistical or modelling techniques (Schimel et al., 1987). Moreover, the choice of the appropriate predictor values depends upon a thorough knowledge of the processes involved in the phenomenon under study.

This book describes an astonishingly broad array of scientific issues that must be studied to understand the global geospheric-biospheric processes that will control the habitability of the earth. These issues irreverently cross the boundaries of typical disciplines, and, thus, mandate that experimental and analytical procedures must do the same. This challenge demands new measurements, more insightful mathematical models, and broader collaborative experimental approaches. The chapters in this volume carefully outline many of the required scientific challenges, and both individually and collectively suggest experimental approaches for the solution of these issues which are of such great global significance. 

REFERENCES

Allen, T .F. H. and Starr, T. B. (1982). Hierarchy: Perspectives for Ecological Complexity. University of Chicago Press, Chicago, Illinois.

Bolin, B. (1988). Linking terrestrial ecosystem process models to climate models. (Chapter 7, this volume.).

Bolin, B., and Cook, R. B. (Eds.) (1983). The Major Biogeochemical Cycles and their Interactions. SCOPE 21. John Wiley and Sons, Chichester .

Crutzen, P. J. (1983). Atmospheric interactions-homogeneous gas reactions of C, N, and S containing compounds. In Bolin, B., and Cook, R. B. (Eds.) The Major Biogeochemical Cycles and Their Interactions, John Wiley and Sons, New York.

Crutzen, P. J. (1988). Variability in atmospheric-chemical systems (Chapter 6, this volume).

Delcourt, H. R., Delcourt, P. A., and Webb III, T. (1983). Dynamic plant ecology: the spectrum of vegetation change in space and time. Quaternary Science Review, 1, 153-175.

Dickinson, R. E. (1984). Modelling evapotranspiration for three-dimensional global climate models. In Hansen, J. E., Takahashi, T. (Eds.), Climate Processes and Climate Sensitivity, pp. 58-72. M. Ewing Series 5, American Geophysical Union, Washington, DC.

Dickinson, R. E. (1988). Atmospheric systems and global change. (Chapter 5, this volume.) .

Emanuel, W. R., Shugart, H. H., and Stevenson, M. P. (1985). Climatic change and the broadscale distribution of terrestrial ecosystem complexes. Climate Change, 7, 29-43.

Hansen, J., Lacis, A., Rind, D., Russell, G., Stone, P., Fung, I., Ruedy, R., and Lerner , J. (1984). Climate sensitivity: analysis of feedback mechanisms. In Hansen, J. E.,   and Takahashi, T. (Eds.), Climate Processes and Climate Sensitivity., M. Ewing Series 5. American Geophysical Union, Washington, DC.

van der Heijde, P. K. M. (1988). Spatial and temporal scales in groundwater modelling. (Chapter 11, this volume.)

Jeffers, J. N. R. (1988). Statistical and mathematical approaches to issues of scales in ecology. (Chapter 4, this volume.)

Lacis, A., Hanson, G., Lee, P., Mitchell, T., and Lebedeff, S. (1981). Greenhouse effect of trace gases, 1970-1980. Geophys. Res. Lett., 8, 1035-1038.

Lewis, M. R., Harrison, W. G., Oakey, N. S., Hebert, D., and Platt, T., (1986). Vertical nitrate fluxes in the oligotrophic ocean. Science, 234, 870-873.

Malone, T. F., and Roederer, J. G. (Eds.) (1985). Global Change: Proceedings of a Symposium Sponsored by ICSU during its 20th General Assembly in Ottawa, Canada. ICSU Press Symposium Series No.5. Cambridge University Press.

Manabe, S., and Stouffer, R. J. (1980). Sensitivity of a global climate model to an increase of CO₂ concentration in the atmosphere. J. Geophys. Res., 85,5529-5554.

National Research Council. (1983). Toward an International Geosphere-Biosphere Program: a Study of Global Change. Commission on Physical Sciences, Mathematics, and Resources. National Academy Press, Washington, DC.

National Research Council. (1986). Global Change in the Geosphere-Biosphere: Initial Priorities for IGBP. Commission on Physical Sciences, Mathematics, and Resources. National Academy Press, Washington, DC.

Neftel, A., Moor, E., Oeschger, H., and Stauffer, B. (1985). Evidence from polar ice cores for the increase of atmospheric CO₂ in the last two centuries. Nature, 315, 45-57.

Oeschger, H. (1988). The ocean system-ocean/climate and ocean CO₂ interactions. (Chapter 15, this volume.)

O'Neill, R. V. (1988). Hierarchy theory and global change. (Chapter 3, this volume.) Risser, P. G. (Compiler .) (1986). Spatial and Temporal Variability of Biospheric and   Geospheric Processes: Research Needed to Determine Interactions with Global Environmental Change. ICSU Press, Paris.

Schimel, D., Stillwell, M. A., and Woodmansee, R. G. (1985). Biogeochemistry of C, N, and P in a soil catena of the shortgrass steppe. Ecology, 66, 276-282.

Schimel, D. S., Parton, W. J., Adamsen, F. J. Woodmansee, R. G., Senft, R. L. and Stillwell, M. A. (1986). The role of cattle in the volatile loss of nitrogen from a shortgrass steppe. Biogeochemistry, 2, 39-52.

Schimel, D. S., Simkins, S., Rosswall, T., Mosier, A. R., and Parton, W. J ., (1988). Scale and the measurement of nitrogen-gas fluxes from terrestrial ecosystems. (Chapter 10, this volume.)

Schumm, S. A. (1988). Variability of the flu vial system in space and time. (Chapter 12, this volume.)

Shugart, H. H., Michaels, P. J., Smith, T. M., Weinstein, D. A., and Rastetter, E. B. (1988). Simulation models of forest succession (Chapter 8, this volume.)

Walsh, J. J., and Dieterle, D. A. (1988). Use of satellite ocean colour observations to refine understanding of global geochemical cycles. (Chapter 14, this volume.)

Washington, W. M., and Meehl, G. A. (1984). Seasonal cycle experiment on the   climate sensitivity due to a doubling of CO₂ with an atmospheric general circulation model coupled to a simple mixed layer ocean model. J. Geophys. Res., 89, 9475-9503.

Woodmansee, R. G. (1988). Ecosystem processes and global change. (Chapter 2, this volume.)