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Positive feedbacks and negative feedbacks (global warming)

  1. Oct 15, 2015 #1
    1. The problem statement, all variables and given/known data
    Consider the feedback loop involving Clouds, Rainfall and forest area.
    All the relationships represented in this figure show positive couplings between the components:
    Forest area is positively coupled to rainfall, plant evapotranspiration is positively coupled to forest area, and rainfall is positively coupled to plant evapotranspiration.

    Is this a positive or negative feedback loop
    2. Relevant equations
    Forest Area-> Plant evapotranspiration and rain cloud formation
    Plant evapotranspiration and rain cloud formation->Rainfall
    Rainfall->Forest Area

    3. The attempt at a solution
    I don't actually understand that loop because I cant see the invariant. In my class we are analyzing how those feedbacks influence the variation of the temperature. In this case, I cant see the invariant or I dont know if this loop is related to the temperature. Can someone explain me how this loop works?
     
  2. jcsd
  3. Oct 15, 2015 #2
    Hi MathNoob:

    I confess I also find this loop a bit puzzling.

    I added number in brackets to your quote for ease of reference.

    First I would think that [1] involved 2 distinct steps:
    [1a] Forest Area-> Plant evapotranspiration
    [1b] Plant evapotranspiration -> cloud formation​

    Next, [3] has a limit.
    [3x] If rain fall rate grows too much, flooding will reduce forest areas.​
    Also,
    [3y] If rain fall rate diminishes too much, drought will reduce forest areas.​
    Therefore, rainfall rate has an optimum range, and less or more is bad for the forest growth. I think this is sufficient to break the loop.

    Also, the is another link in the opposite direction, that is a negative feedback effect..
    [1bx] Cloud formation -> Plant evapotranspiration​
    If cloud formation increases, Plant evapotranspiration will decrease due to reduced sunlight.
    So now there is a negative feedback loop:
    Increasing Plant evapotranspiration -> Increasing cloud formation -> Decreasing Plant evapotranspiration​

    All in all I think this "All the relationships represented in this figure show positive couplings between the components" is seriously flawed.

    I also don't understand the following:
    3. The attempt at a solution
    I cant see the invariant or I dont know if this loop is related to the temperature. Can someone explain me how this loop works?​
    I don't understand what you mean by "invariant", or what "the solution" is supposed to solve. I am guessing the solution is supposed to determine the trend of something, and that is some variable lioke temperature.

    So, as I discussed above, I don't think this "loop" works. I am guessing that:
    (1) the student is supposed to ignore reality facts that contradict the premises of the "loop", and
    (2a) that the (unrealistic) never ending increase in forests, clouds and rain causes a never ending increase in water vapor in the atmosphere, and
    (2b) the increase of water vapor (a greenhouse gas) causes an increase in temperature.

    Hope this is helpful.

    Regards,
    Buzz
     
    Last edited: Oct 15, 2015
  4. Oct 15, 2015 #3
    Never mind what I said. I understand now what is going on, but consider just the positive feedbacks. The problem is telling us that there is just positive feedbacks between the events, so it would be a positive or a negative feedback loop?
     
  5. Oct 15, 2015 #4
    Hi MathNoob:

    I edited some addition to my previous post while you were posting #3. Your #3 seems to confirm some of my guesses.

    Regards,
    Buzz
     
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