Sometimes AGW dismissives attack calculations and/or use of climate sensitivity measures. They're actually using an argument of impossible expectations. What counts is whether such measures are useful.
In responding to dismissives, people might find the following source useful.
Knutti (2015) "Feedbacks, climate sensitivity and the limits of linear models" gives a useful review of usefulness of climate sensitivities:
"Are the current concepts of feedbacks and climate sensitivity still useful? Describing a complex system like the climate with a very simple model inevitably means that many factors are ignored, or assumed to be constant. The results above show that the global temperature response to different forcing magnitudes and timescales cannot be fully described with the assumption of a constant feedback parameter λ even in models that ignore long-term Earth system feedbacks (ice sheets, dynamic vegetation, permafrost), non-CO2 forcings, chemistry and land-use change. In our models, the feedback parameter varies by about 50% or more between different forcing magnitudes and over time as the system approaches equilibrium...
Does this imply the zero-order linear energy balance model is useless? A model is always wrong with regard to reality in a strict sense, but the constant feedback parameter model may still be an adequate approximation for some purposes...
We argue that the quote ‘modelling for insight, not numbers’ makes an essential point here .We have to conclude that the global linear forcing feedback model may be of limited value to estimate quantities like the ECS of the real world, or at least we have to be more careful in understanding and quantifying in which range of forcings, timescales and climate states a simple model with a constant feedback parameter can be adequately used...
[for example] Recent evidence from observations and models that the climate system will continue to warm for a constant forcing, the commitment warming [65,66], can be traced back to Siegenthaler & Oeschger , and Wigley & Schlesinger , who noted that ‘at any given time, the climate system may be quite far removed from its equilibrium with the prevailing CO2 level’, and Schlesinger , who wrote that ‘sequestering of heat into the ocean’s interior is responsible for the concomitant warming being only about half that which would have occurred in the absence of the ocean. These studies also indicate that the climate system will continue to warm towards its yet unrealized equilibrium temperature change, even if there is no further increase in the CO2 concentration’...
All comprehensive climate models indicate sensitivities above 2◦C, and those that simulate the present-day climate best [54–57] even point to a best estimate of ECS in the range of 3–4.5◦C."
Attached, from the Knutti paper, shows various climate sensitivity measures. Not perfect, but still very useful.
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