Feedbacks

One tricky part about modeling climate is the way that different parts of the climate system interact with each other. Positive feedbacks tend to amplify the variability of climate whereas negative feedbacks provide stability. Water evaporates into warmer air, acting as a greenhouse gas to amplify the initial warming. Melting ice changes the albedo of the Earth, acting as a positive feedback to temperature variations. Clouds have a huge influence on climate by interacting with both visible and IR light. We can point to present-day human influence on cloudiness, but it is difficult to predict how clouds will feed back to long-term climate change. Ocean circulation has feedback interactions with climate, in the equatorial Pacific and in the North Atlantic. In general, real climate variations appear to be larger than our models predict because it is difficult to capture all of the feedbacks in models.

Positive and negative feedbacks

The challenging thing about modeling climate is that there are feedbacks. A feedback is a loop of cause and effect (Fig. 7.1). It will be easier to wade through this particular thicket if we make a few abstruse-sounding definitions here. The temperature is the quantity that gets fed-back in many examples, although feedbacks also control populations, and chemistry and water vapor lots of other things. So we can be general: let's call the quantity that is being fed-back the state variable. The loop of causality is called the feedback loop. The arrow coming in from stage left is an input perturbation.

One example of a feedback loop is the ice albedo feedback, operating on the state variable of temperature. An input perturbation, such as a rise in greenhouse gases, drives temperature up a bit. Ice melts, reducing the albedo (the reflectivity of the Earth: Chapter 3), which drives temperature up a bit more. This is called a positive feedback because the directions of the input perturbation and the feedback loop agree with each other. A positive feedback can work in the opposite direction as well, during glacial time for example: colder, more ice, colder still. The point is that the feedback loop, in this case, reinforces the direction of change in the input. A positive feedback loop is an amplifier.

Negative feedbacks tend to stabilize things. One example we have already discussed is the way that the Earth regulates its heat flux to space, according to the Stefan-Boltzmann rule a T4. If the Earth is too warm, the outgoing energy flux will be greater than the incoming solar energy flux, and the Earth will cool. The water vapor concentration in the atmosphere at a particular temperature, which we will run into in the next section, is another example.

Hydrological cycle

Hydrological cycle

Rain evaporation

Fig. 7.1 Feedback diagrams. (a) Ice albedo feedback: an example of a positive feedback. Some external perturbation increases the temperature. The increase in temperature causes ice to melt, allowing the land to absorb more of the incoming solar radiation (by decreasing the albedo). The melting ice drives temperature up further. (b) Stefan-Boltzmann feedback: an example of a negative feedback, resulting from the Stefan-Boltzmann IR energy flux a T4. (c) is the water vapor feedback, which amplifies the temperature effect of rising CO2. (d) Hydrological cycle: the negative feedback that controls atmospheric water vapor, given an atmospheric temperature.

Rain evaporation

Fig. 7.1 Feedback diagrams. (a) Ice albedo feedback: an example of a positive feedback. Some external perturbation increases the temperature. The increase in temperature causes ice to melt, allowing the land to absorb more of the incoming solar radiation (by decreasing the albedo). The melting ice drives temperature up further. (b) Stefan-Boltzmann feedback: an example of a negative feedback, resulting from the Stefan-Boltzmann IR energy flux a T4. (c) is the water vapor feedback, which amplifies the temperature effect of rising CO2. (d) Hydrological cycle: the negative feedback that controls atmospheric water vapor, given an atmospheric temperature.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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