Is Earth's Thermal Feedback Loop Unstable?

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Discussion Overview

The discussion revolves around the stability of Earth's thermal feedback loop in relation to CO2 levels and their impact on temperature changes. Participants explore the implications of feedback mechanisms, potential tipping points, and the linear versus non-linear nature of the system.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that the direct greenhouse heating from CO2 is approximately linear, with a doubling of CO2 resulting in a 1°C change, leading to a potential unstable thermal feedback loop.
  • Another participant challenges the assumption of linearity, proposing that if a tipping point exists, the system would exhibit non-linear behavior.
  • A different viewpoint argues that a loop gain exceeding 1 could lead to exponential increases, indicating a tipping point, but questions what specific non-linearities might be involved.
  • One participant defines a "tipping point" as a change in the system's gain, which may not necessarily exceed +1 but could alter the system's trajectory.
  • Another participant notes that observed temperature and CO2 changes suggest a thermal loop gain close to 1, necessary to explain the observed temperature increase from CO2 forcing.
  • A later reply acknowledges a mistake regarding feedback gain calculations, clarifying that feedback generates a geometric series, which affects the interpretation of gain.
  • Links to external resources, including the IPCC report and a summary by CarbonBrief, are provided for further reading on the concepts discussed.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the feedback loop, with some supporting the idea of linearity while others argue for non-linear dynamics and the existence of tipping points. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Participants acknowledge potential flaws in their arguments and the complexity of the feedback mechanisms involved, indicating that assumptions about linearity and feedback gain may influence their conclusions.

stuartmacg
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TL;DR
Published CO2 and T data seem to imply unstability.
Looking at some apparently widely accepted data, I understand:

- CO2 mass change => direct "greenhouse" heating is approx linear over ranges considered
- Doubling CO2 (i.e. adding same again) would give 1C change for direct greenhouse.
- CO2 change x1.5 since pre-industrial times (half added), hence direct greenhouse would be 0.5C
- delta T has been observed to be 1C in same period, and attributed to CO2 increase.
- this suggests feedback gain around 1 => nearly unstable heat in-> heat out loop
- heat may generate some net CO2 directly e.g. by warmed sea out gassing
- this would suggest the thermal loop (even without any external forcing) is unstable, gain>1

An unstable Earth might change between 2 saturating temperatures, with little external "forcing". Saturating regions could have loop gains just below 1.
I expect there are flaws in the above, but it would be interesting to find out where.
 
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You are assuming the system is linear.
If there is a tipping point, the system will be non-linear.
 
I am not clear that that is so. A loop gain becoming >1 would generate exponential increases i.e. a "tipping point", in an otherwise linear system (e.g. microphone howl).
Non linear, second order, effects are what would change the loop gain, and create stable "saturation" levels.
What non linearities are you suggesting?
 
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I see a "tipping point" as being a point where the gain of the system changes. It does not need to become greater than +1, it could just change sign, inducing a change in the trajectory of the system state.
 
Our replies crossed. The observed T and CO2 changes and (I am told) agreed direct CO2 heating effect together suggest that there is a purely thermal loop gain close to 1, required to amplify the CO2 "forcing" change from 0.5C to the observed 1C.
 
Last edited:
I made a foolish mistake: feedback effectively generates a geometric series, so if the result is to double the input, the feedback gain is 1/2, not 1. i.e. factor=1/(1-gain).
 
Thanks for the links. I'll work my way through this stuff.
 

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