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Notes from the Field

Aahh, the feedback…

August 1st, 2010 by Matteo Ottaviani

In the very beginning of this blog, I had promised to reveal why noise created by electric guitars pedals&amps reminded me of the kind of science we faced during CARES. So, after the closing of the campaign, I want to keep that promise.

A microphone in front of an amplifier can generate an endless reverb, as much as getting a haircut in a shop with mirrors both in front and behind you can echo the image of your face. Very embarrassing if you don’t like the haircut. Or your own face.

The outcome of a feedback process is difficult to predict, because it is the result of a self-amplifying effect.

It is a non-linear process, meaning that it is not simply proportional to the change brought to a system. All in all, it is a logic less predictable than the simple causality of “one thing leads to another”, because in this case the “another thing” ends up in turn influencing the “one thing”. You can see an example in the figure below, to get a glint of how intertwined the dependences can be. And this is most certainly not all of it.


Some of these effects are positive and some are negative.  “Positive” and “negative” here mean “increasing” or “decreasing” and not “good” or “bad”.  In this sense, positive feedback enhances the direct effect (warming in the case of climate), and negative feedback offsets it. A classic example of positive feedback: water vapor concentration increases under warmer climate, and more water vapor generates more warming, leading to more water vapor, and so on and so forth. Repercussions can be so intense that, past a tipping point, they may can cause a runaway feedback effect spinning beyond our prediction (and possibly mitigation) capabilities.

Without all the complications aerosols entail, we haven’t mentioned their “indirect effects”, like that on clouds (follow the link for very illustrative clips!). Humidity in the air spontaneously condenses to form clouds, but it would rather condense around solid grains if it finds them around, and aerosols can very effectively act as “cloud condensation nuclei” therefore helping clouds to form. Interestingly, the clouds they help to form are whiter and with smaller droplets.

Any effect on climate? You bet! Clouds are like gigantic mirrors for Sun beams and just imagine the repercussions of even a tiny variation on cloud cover. Even determining the concentration of cloud droplets, as simple a challenge as it might sound, is driving scientists crazy.

Our knowledge of how aerosols respond to climate change is certainly better than in 1985, when the figure above was published on the Bulletin of the American Meteorological Society, but is still limited. For example, aerosol perturbations to cloud microphysics may involve feedbacks; after all smaller droplets should collide less often. Less coalescence prevents the growth of droplets, therefore suppressing precipitation, and making a cloud live longer and exert an even stronger cooling effect. Where’s the end of the loop? Any suggestion?

We appreciate your feedback. Even if it rips our eardrums apart.

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