Turned up to eleven: Fair and Balanced

Thursday, February 07, 2002


There is a very interesting article about Global Warming on Reason Online. The article sites some research by an MIT climate scientist about the effects of tropical cumulus clouds as a feedback control reducing the effect of increased CO2 in the atmosphere. If he is right, the global warming effect may be much reduced from current estimates. It is an important area for researchers in the field, as well as outside observers, to become cognizant of. More generally, the concept of feedback needs some attention. In the best Den Besteian tradition, I will attempt to explain a little of this. If math bores you, too bad!
When climatologists, biologists, chemists and physicists try to model things that happen in the world, they will often use systems of differential equations. These systems are often devilishly complex, and usually use a lot of approximations for the various variables (ha!) that are part of the system. A way to simplify the models is to use a systems engineering approach (I can almost hear the groans, but I can't help myself...I went to an Engineering school!!). This approach takes a lot of the complicated workings out, and replaces them with a "black box". The modeler can define the inputs and outputs, and then use a set of relatively simple "transfer functions" to represent them. The transfer function is simply defined as the operation that turns the inputs into the outputs. It is usually represented as an arbitrarily complex differential equation. I realize this is getting a bit out of hand, so let me bring it back down a little. A DE is just an equation which describes the rates of change of variables as a function of other variables (like time, for example). They can be simple, like the logarithmic growth equation N=N(0)e^kt or very complex (I won't get into it). An important facet of engineering design, however, is feedback control. Suppose you want to have a bucket of water, and you want the temperature to always be 50 degrees (Celcius). You could put it on a hot plate, and heat it up, using a thermometer to find the temperature. When it gets to 50 degrees, you can turn the heat down, adjusting it to keep the temp at 50. In this example, you turning the knob are the feedback controller. Of course, electronics can do this as well or better than we can, and in many of the objects we use everyday they do. What is really interesting, though, is how prevalent these type of feedback loops are in nature. Feedback loops are absolutely ubiquitous in biological systems, from temperature control in mammals, to insulin production to control glucose levels, to all sorts of systems inside cells. Feedback, in general, comes in two forms, positive and negative. A positive feedback loop can result in an explosive growth of the output, where a negative feedback can limit the output. This brings us to the question of clouds in climate models. Simply put, it is not clear whether cloud cover is a net positive or negative feedback on global warming. Convincing evidence is present for clouds as reflectors of incident sunlight, as well as an insulator keeping the light and heat in. Those of us in the audience need to understand that this is a reflection of the difficulty of modeling something as big as the global climate, and also need to understand the questions being asked. I think that some level of both feedback systems is at play in the global climate, and that probably Earth will not change as much as a result of human CO2 production as many think, but maybe more than some expect. I hope that this helps some to understand the processes involved. I don't think there is a concept more important in natural science than the power of a feedback loop. Anyway, back to your regularly scheduled programming.

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