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Causal loop diagram from system dynamics methodology showing the solar-earth-space energy flow system (the dominant flow system driving earth's surficial systems, including the biosphere) and interactive natural earthly processes that influence it. Discussion follows figure. (See How to Read a Causal Loop Diagram).
The solar-earth-space energy flow system is shown at the top of the figure. Earth's biosphere (earth's life of earth and its environment) is an intermediate system in contact with a hot energy source (the sun), and a cold heat sink (outer space). Solar energy is the dominant energy flux for biological organization, and the organizing factor for ecology (Morowitz, 1979).
The solar-earth-space energy flow rate is influenced by greenhouse gas composition of earth's atmosphere; H2O and CO2 are the dominant greenhouse gases. Greenhouse gases trap heat from the sun via the greenhouse effect, raising earth's surface temperature by about 30°K, to 288°K (Levine, 1986). Water vapor accounts for about two thirds of the greenhouse effect. If not for greenhouse warming, earth's surface would be too cold to support life.
Solar-earth-space energy flow is influenced by many variable processes: release of mantle CO2 onto earth's surface, marine transgressions and regressions, areal coverage of terrestrial flora, weathering rates, abundance of marine algae, impact events, and orbital dynamics, etc. The Deccan Traps mantle plume perturbation was part of the core-mantle system shown on the lower right of the figure. Mantle plume volcanism can extrude vast amounts of basaltic lavas onto earth's surface in a geologically short duration. Most of the Deccan Traps lavas erupted in 100,000 to 200,000 years.
Over long time periods, surficial systems and sinks become adjusted to existing mantle CO2 flux rates. The sudden additional CO2 released onto earth's surface by mantle plume volcanism overwhelms surficial systems, disrupting the carbon cycle and the solar-earth-space energy flow system, and forcing elements of the biosphere into new domains.
Causal loop diagrams, the first step in evaluating system behavior, show interrelations between system variables and expose feedback loops within individual systems, and between adjoining systems. They are developed by gathering variables, and then correlating the variables with one another as causally-related pairs where one member acts as an independent variable, and the other as a dependent variable.
Independent and dependent variable pairs are connected to one another by an arrow which is interpreted to read "affects," "influences," or "causes," and which expresses asymmetrical and irreversible relationships between the variables. The independent variable is at the base of the arrow, and the dependent variable is at the arrowhead. Correlation between variables is either positive or negative and is expressed by a plus or minus sign placed by the arrowhead. Correlation is positive if the independent and dependent variables both either increase or decrease in value, and negative if they respond differently from one another. The causal pairs are next integrated with other variable pairs, exposing relationships that might otherwise not be apparent. Often, causal pairs will close upon themselves, exposing feedback loops.
Feedback is either positive or negative, and is expressed as the polarity of a feedback loop. The polarity is indicated by a (+) or (-) sign within a curved, partially closed arrow placed within the feedback loop. Polarity is determined by counting the minus signs at the arrowheads within a feedback loop. An even number indicates positive polarity (positive feedback), and an odd number negative polarity (negative feedback). Positive feedback produces exponential growth or collapse. Conversely, negative feedback expresses goal-seeking behavior, and stability. Causal loop diagrams are used as a basis for developing flow diagrams which allow mathematical analysis of the behavior of systems.
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Copyright © 1995 Dewey M. McLean