Answer: Because the Climate Models Have All Been Fudged

The Question, as asked (surprisingly) by a global warming believer:

One curious aspect of this result is that it
is also well known [Houghton et al., 2001] that the same models that
agree in simulating the anomaly in surface air temperature differ
significantly in their predicted climate sensitivity. The cited range
in climate sensitivity from a wide collection of models is usually 1.5
to 4.5 deg C for a doubling of CO2, where most global climate models
used for climate change studies vary by at least a factor of two in
equilibrium sensitivity.

The question is: if climate models differ by a factor of 2
to 3 in their climate sensitivity, how can they all simulate the global
temperature record with a reasonable degree of accuracy
. Kerr
[2007] and S. E. Schwartz et al. (Quantifying climate change"“too rosy a
picture?, available at www.nature.com/reports/climatechange, 2007)
recently pointed out the importance of understanding the answer to this
question. Indeed, Kerr [2007] referred to the present work and the
current paper provides the "˜"˜widely circulated analysis'' referred to
by Kerr [2007]. This report investigates the most probable explanation
for such an agreement. It uses published results from a wide variety of
model simulations to understand this apparent paradox between model
climate responses for the 20th century, but diverse climate model
sensitivity.

Much more here at Climate Skeptic

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