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#27
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"Martin Dixon" wrote:
...the thing that I was trying to illustrate was the remarkable stability of the climate over a very long time. This is not the behaviour of a system with predominantly positive feedback mechanisms. There is a fourth power relationship between temperature and IR emission, which is a strong stabilising influence on climate, but when climate scientists talk about positive and negative feedbacks, they mean with respect to that baseline. Roy Spencer explains it thus: =============== Although not usually considered a feedback per se, the most fundamental component of the net feedback parameter λ is the direct dependence of the rate of IR emission on temperature, estimated to be about 3.3W/m² in the global average [8]. This ‘Planck’ or ‘Stefan-Boltzmann’ response stabilizes the climate system against runaway temperature changes, and represents a baseline from which feedbacks are traditionally referenced. Positive feedbacks in the climate system reduce the net feedback parameter below 3.3, while negative feedbacks increase it above 3.3. Here we will deal with the net feedback parameter exclusively, as it includes the combined influence of all climate feedbacks, as well as the Planck effect. The larger the net feedback parameter λ, the smaller the temperature response to an imposed energy imbalance N will be; the smaller λ is, the greater the temperature response will be. A negative value for λ would indicate a climate system whose temperature is unstable to radiative forcing. The coupled ocean-atmosphere climate models tracked by the IPCC have diagnosed long-term net feedback parameters ranging from λ = 0.89 for the most sensitive model, MIROC-Hires, to λ = 1.89 for the least sensitive model, FGOALS [8]. Since this range is below the Planck response of 3.3W/m², all of the IPCC models therefore exhibit net positive feedbacks. Also, since all climate models have net feedback parameters greater than zero, none of the climate models are inherently unstable to perturbations. =============== So when scientists talk of climate sensitivity being a positive feedback of (say) a factor of three, they mean with respect to the fundamental Planck response. It doesn't mean the system is unstable, but just that the feedbacks amplify any initial forcing by a factor of three. |
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