"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.