Present climate and biological change - Climate Change: Biological and Human Aspects - page 240

these interpretations are subject to human psychological factors that make it addition-
ally difficult to assess how reliable the IPCC's conclusions may be. For instance, to
take our computer model case further, it is possible to begin to estimate how robust the
IPCC's conclusions are against the types of computer model the IPCC uses. Follow-
ing the publication of the IPCC's 2001 assessment, researchers from the University
of Bern, Switzerland, attempted to do just this (Knutti et al., 2002). They took a
simple climate model (therefore, one with an output subject to relatively large error
margins) but calibrated it to the same three-dimensional ocean/atmosphere models
used by the IPCC in their third assessment. Other than ensuring that the model's
responses reflected known past climate change (i.e. that the model's responses to
established historic circumstances were 'realistic'), they varied the model's paramet-
ers by the estimated uncertainties of its various climate forcing factors. In this way
they obtained a number of climate forecasts for the 21st century. The conclusions
were clear. Just 5% suggested that the IPCC's 2001 high - low estimates were too
high. In other words, there was a 5% chance that the IPCC was being too alarmist in
predicting greater warming than is likely. Conversely, the researchers concluded that
there is a 40% probability that the increase in global mean surface temperatures will
exceed the range predicted by the IPCC. In short, they estimated that the balance of
probability would be that global warming in the 21st century will be greater than the
IPCC forecasted in 2001. If the researchers' conclusions were valid, it means that the
consequences of the warming discussed in subsequent chapters will be greater. One
literature review in 2005 on aerosol cooling, by Meinrat Andreae, Chris Jones and
Peter Cox, suggests that warming forcing since the Industrial Revolution up to the
end of the 21st century might be between 2 and 8 ◦ C, although a mid-estimate of 6 ◦ C
is unlikely because there will probably be some natural background aerosol cooling
in addition to that from human activity.
With research like this, it was not surprising that the 2007 IPCC analogous scenario
estimates for 21st-century warming were a little greater than in 2001. The best models
(including the ones the IPCC used) may not be accurate on a sub-regional basis but
they do broadly paint similar pictures to each other on a global scale. Getting similar
results by different computer methods lends a certain credibility to the results, hence
the models. This is worth noting, as it is model development and the increase in their
sophistication that will one day surely prove most useful for determining change at
increasingly local levels in numerous ways.
A second instance of concern that the IPCC's earlier 2001 forecast may have under-
estimated possible future climatic change was presented by a European research team
led by Hans von Storch in 2004. They examined the validity of previous interpreta-
tions of the palaeoclimatic proxy record for the northern hemisphere that were in turn
used by the IPCC. Their conclusions were that such past analyses of noisy (highly
variable) proxy data may have underestimated past climate change. If the 2001 IPCC
had been relying on underestimates of past change to make their future forecasts,
then their future forecasts would also be underestimates. Past variations may have
been at least a factor of two larger (see section 2.5).
The above gives just two examples of studies that have examined the reliability
of the 2001 IPCC scenarios. These examples are scientific attempts to quantify
confidence in scenario conclusions that, in addition to the previous subsection, provide
further reason to treat the climate models on which the IPCC rely with caution. New