Natural feedback

Positive feedback nature of clot formation. Thrombin promotes clot formation at several points in the coagulation cascade through positive feedback. Activities of thrombin include ... 

V.K. Vanag, L.F. Yang, M. Dolnik, A.M. Zhabotinsky, and l.R. Epstein. Oscillatory cluster patterns in a homogeneous chemical system with global feedback. Nature, 406(6794) 389-391, 2000. 

Heimann M, Reichstein M. Terrestrial ecosystem carbon dynamics and climate feedbacks. Nature. 2008 451(7176) 289-92. 

Bains S., Norris R. D., Corfield R. M., and Paul K. L. (2000) Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback. Nature 407, 171-174. 

Are the outer loops in the MRAC and STR configurations of feedforward or feedback nature ... 

Chao, J., R. C. Wilhoit and B. J. Zwolinski (1974) Gas phase chemical equihbriumin dinitro-gen trioxide and dinitrogen tetroxide. Thermochimica Acta 10, 359-371 Charlson, R. I, I E. Lovelock, M. O. Andreae and S. G. Warren (1987) Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate A geophysiological feedback. Nature... 

However, the reaction between zirconium and water which gives zirconium dioxide and hydrogen as products, generates heat (i.e. is exothermic) and so, if initiated, is likely to proceed rapidly. The rate of reaction will increase as the temperature of the components increases. An exothermic reaction, once started, will tend to raise the system temperature and increase the rate of the reaction. Such reactions can lead to instabilities of a positive feedback nature. Endothermic reactions, however, which require energy to be supplied—the reverse of exothermic—are therefore of a negative feed-back and inherently stable type. 

Some of the inherent advantages of the feedback control strategy are as follows regardless of the source or nature of the disturbance, the manipulated variable(s) adjusts to correct for the deviation from the setpoint when the deviation is detected the proper values of the manipulated variables are continually sought to balance the system by a trial-and-error approach no mathematical model of the process is required and the most often used feedback control algorithm (some form of proportional—integral—derivative control) is both robust and versatile. 

DMS has been observed in the marine atmosphere since the early 1970s, but it was not until the mid-1980s that there was interest in this gas as being a natural source for sulfate CCN. Sulfate aerosols are, in number terms, the dominant source of CCN. The major role clouds play in the climate system leads to possible climatic implications if changes to DMS production occurred. Furthermore, the dependence of this production on environment conditions means that scope for a feedback process arises this feedback is called the Charlson hypothesis. ... 

The role of oceanic physical chemistry and biochemistry in the enhanced greenhouse future is still uncertain. We have discussed the mechanisms generating a number of potential feedbacks, both positive and negative in their impact. However, new interactions are constantly being discovered in nature, and model representation of them is a rapidly evolving science. At present what we can say is that this is a young field of much intellectual and practical promise. 

In SVCs Ihe number of switchings is of no relevance, as they arc free from inrush currents. Switching is performed at the instant when the cuiTcnt wave is passing through its natural zero. Static devices in various combinations and feedback control systems, which may be computer-aided, can tilmost instantaneously (< I cycle) generate or absorb reactive power, as may be demanded by the system. Correction... 

Such feedback, from anyone who interacts with the system by providing input, using information, or receiving reports, can greatly enhance the effectiveness of the new PSM system. System users may have important suggestions about how to improve or streamline a system, or may not clearly understand its importance or the part they play in its implementation. At the same time, soliciting—and acting on—user feedback helps underscore the collaborative nature of PSM cmd the fact that its success depends on the user. 

Against the backdrop of an Earth warmed by its own greenhouse effect, other forces operate that can increase or decrease the retention of heat by the atmosphere. Some of these forces are of human origin, some are produced by nature, and some are produced by mutual feedback reactions. 

Coordinated by the positive feedback these simple rules induce between robots and their environment, the result, over time, is a seemingly intelligent, coordinated sorting activity. Clusters of randomly distributed objects spontaneously and quite naturally emerge out of a simple set of autonomous local actions having nothing at all to do with clustering per se. 

Climate is often viewed as the aggregate of all of the elements of weather, with quantitative definitions being purely physical. However, because of couplings of carbon dioxide and many other atmospheric species to both physical climate and to the biosphere, the stability of the climate system depends in principle on the nature of feedbacks involving the biosphere. For example, the notion that sulfate particles originating from the oxidation of dimethylsulfide emitted by marine phytoplankton can affect the albedo (reflectivity) of clouds (Charlson et ai, 1987). At this point these feedbacks are mostly unidentified, and poorly quantified. 

Mountain building usually involves compres-sional deformation of the crust. Studies of the physics of orogeny suggest that there is a feedback between the nature of the building process and denudation rates. Suppe (1981), Davis et al. (1983), and Dahlen et al. (1984) have modeled the effects of brittle deformation in accretionary fold-thrust mountain belts such as Taiwan and the Andes. The basis of their model is the hypothesis that rock deformation is governed... 

Kutzbach, J., Bonan, G., Foley, J. and Harrison, S. P. (1996). Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing in the early to middle Holocene, Nature 384, 623-626. 

Taken together. Figs 17-2, 4-13, 17-3, and 1-2 constitute a complex image of the Earth s climate system, including most of the factors that are known to be involved. However, such diagrams fail to adequately represent the dynamical nature of the totality of interactions of all of the parts. In order to explore these interactions, the natural variability of climate, and changes due to external perturbations, we must now introduce the key notions of forcings, feedbacks, and responses. 

The nature of such processes can be depicted as a feedback loop, as shown in Fig. 17-4. Using the nomenclature in this figure and continuing with enhanced evaporation of water vapor as our physical example of a feedback that is completely internal to the climate system, we... 

While this feedback may or may not be climatically relevant, it does serve to illustrate the nature of biogeochemical feedbacks. It seems likely that many such complex systems exist, and that they may indeed be factors that influence climate. To return to the introduction to this chapter, it is not possible to rule out biogeochemical feedbacks as factors that have stabilized climate over the past ca. 10 years. 

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