Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 334

What is interesting is that those modelers with high values of seem to choose

low values of g and vice versa. This keeps D T tot typically in the range 3 1.5 C.
Huybers (2010) provided a very insightful review of climate models. He
suggested that:

''Inter-model compensation between climate sensitivity and radiative
forcing underscores that the models are not based purely on theory but are also
conditional upon observations and, possibly, expectations.''

Huybers (2010) showed that the treatment of clouds was the ''principal source
of uncertainty in models''. Indeed, his Table I shows that the variation of net feed-
back varied only from 0.49 to 0.73 (a narrow range) whereas the response of the
climate system to clouds by various models varied from 0.04 to 0.37 (a wider
spread). He then examined several possible sources of compensation between
climate sensitivity and radiative forcing. He concluded:

''Model conditioning need not be restricted to calibration of parameters
against observations, but could also include more nebulous adjustment of
parameters, for example, to fit expectations, maintain accepted conventions,
or increase accord with other model results. These more nebulous adjustments
are referred to as 'tuning'.''

He gave an example of possible tuning: ''reported values of climate sensitivity
are anchored near the 3 1.5 C range initially suggested by the Ad Hoc Study
Group on Carbon Dioxide and Climate (1979) and that these were not changed
because of a lack of compelling reason to do so.''
He went on to say:

''More recently reported values of climate sensitivity have not deviated
substantially. The implication is that the reported values of climate sensitivity
are, in a sense, tuned to maintain accepted convention.
Convergence between model results, if not truly driven by a decrease in
model uncertainty or clearly understood as a result of calibration, could have
the unfortunate consequence of lulling us into too great a confidence in model
predictions or inferences of too narrow a range of future climates. To the extent
that it occurs, tuning the models based on expectation or convention renders the
modeling process a partially subjective exercise from which it is very complicated
to derive a statistical interpretation.''

Translated into simple terms, the implication is that climate modelers have
been heavily influenced by the early (1979) estimate that doubling of CO 2 from
pre-industrial levels would raise global temperatures 3 1.5 C. Modelers have
chosen to compensate their widely varying estimates of climate sensitivity by
adopting cloud feedback values countering the effect of climate sensitivity, thus
keeping the final estimate of temperature rise due to doubling within the limits