Feedback Analysis: 3 links

1) On MetaSD Brian Eno, meet Stafford Beer

Brian Eno reflects on feedback and self-organization in musical composition, influenced by the organization of complex systems in Stafford Beer’s The Brain of the Firm.

2) RealClimate: Introduction to feedbacks.

Feedbacks are components of the climate system that are constrained by the background climate itself; they don’t cause it to depart from its reference norm on their own, but rather may amplify or dampen some other initial push. These original “pushes” are forcings which are typically radiative in nature (such as adding CO2 to the air) and manifest themselves as a climate change when they are large enough or persistent enough to overcome the large heat capacity of the oceans, and thus change the annual mean radiative energy balance of the Earth. In a broad sense, a feedback means that some fraction of the output is fed back into the input, so the radiative perturbation gets an additional nudge (amplifying the forcing, a positive feedback or damping the forcing, a negative feedback). The major examples such as decline in ice extent in a warmer world, thereby reducing the reflected fraction of incident surface radiation are pretty well known at this point.

3) Gerard Roe in Annual Review of Earth and Planetary Sciences writes about Feedbacks, Timescales, and Seeing Red (doi:10.1146/ writes:

The history of the recognition of feedbacks is perhaps best described as an emerging awareness. Adam Smith, for instance, had a clear understanding of the feedbacks inherent in the operation of the invisible hand—the set of natural and mutual interactions that govern commerce (Smith 1776). In practical applications, the use of feedback principles to regulate mechanical devices goes back much further. Centrifugal governors, which act to automatically maintain the distance between the bed and runner stones, have been employed in wind- and water mills since the seventeenth century (e.g., Maxwell 1867), and float valves were used by the Greeks and Romans in water clocks. However, the abstract idea of a feedback was first conceived of and formalized by Harold S. Black in 1927. Black was searching for a way to isolate and cancel distortion in telephone relay systems. He describes a sudden flash of inspiration while on his commute into Manhattan on the Lackawanna Ferry. The original copy of the page of the New York Times on which he scribbled down the details of his brain wave a few days later still has pride of place at the Bell Labs museum, where it is regarded with great reverence (Figure 1). Some of the concepts and consequences of feedbacks are counterintuitive, so much so that it took Black more than nine years to get his patent granted—the U.K. patent office would not countenance it until a fully working model was delivered to them. Only after being convinced that seventy negative-feedback amplifiers were already in operational use were they finally persuaded to issue a patent. Black (1977) writes that “[o]ur patent application was treated in the same manner one would a perpetual motion machine.” Since the initial skepticism, the principles of feedback analysis have become widely disseminated in the fields of electrical engineering and control systems. For the latter, in fact, they are the foundational theory.

The notion that internal, mutually interacting processes in nature may act to amplify or damp the response to a forcing goes back at least as far as Croll (1864), who invoked the interaction between temperature, reflectivity, and ice cover in his theory of the ice ages. Arrhenius (1896), in his original estimate of the temperature response to a doubling of carbon dioxide, takes careful and quantitative account of the water vapor feedback that amplifies the response to the radiative forcing. The explicit mention of feedbacks seems to enter the Earth sciences via the climate literature starting in the mid 1960s (e.g., Manabe & Wetherald 1967, Schneider 1972, Cess 1975), and in the popular imagination through the concept of Gaia (Lovelock & Margulis 1974). At first, it appears mainly as a conceptual description of physical processes relating to climate sensitivity.  Hansen et al. (1984) and Schlesinger (1985) contributed groundbreaking papers, making quantitative comparisons of different feedbacks in a climate model (but see footnote 4). Since then, there has been a thin but steady stream of studies quantifying climate system feedbacks (e.g., Manabe and Wetherald 1988, Schlesinger 1988, Cess et al. 1990, Zhang et al. 1994, Colman et al. 1997, Colman 2003, Soden & Held 2006).

From Roe 2009

2 thoughts on “Feedback Analysis: 3 links”

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