Current climate models are summarized by the IPCC 2007 report (Section 8.1 and following), which provides references to individual studies. In section 8.3.1, the authors flatly contradict Dr. Mitchell’s claim, noting that:
Simply put, Dr. Mitchell’s claim that “past increases in atmospheric carbon dioxide…have produced only a fraction of the predicted change” is false. For the sake of argument, however, let us entertain the possibility that Dr. Mitchell has access to models not cited by the IPCC—models that project more ‘catastrophic changes’. In this case, she would be obligated to cite those models and offer reasonable evidence to reject the currently accepted models, which dominate the peer-reviewed scientific literature. Since she has neglected to cite supporting evidence and instead misrepresented available literature, she is guilty of misleading her readers—many of whom presume that her career experience as an obstetrician qualifies her to critique paradigms in climatology.
So as not to mislead you, I have reposted the IPCC graphic summary of 14 climate models (produced independently by various research groups). I will let you judge whether observed warming (black) constitutes only a fraction of predicted warming (yellow).
When recent additions to atmospheric CO2 are removed from the models, all of them diverge significantly from the warming trend and predict a stability in global temperatures since 1960. These model results are not the final word on climate change, but they do provide significant evidence in support of the current understanding.
Dr. Mitchell continues her critique by addressing paleoclimate data supposedly used to support model projections—namely, ice-core records of the Younger Dryas (YD) event. The YD cold period was first hypothesized from an abrupt shift in post-glacial, European vegetation patterns (specifically, a flower species called Dryas octopetala). Only much later was it identified in oxygen isotope records from the Greenland ice core. The current interpretation of the event as a short-lived cold period, which lasted from ~13–11.5 thousand years ago, has been corroborated by global vegetation records, lake-core isotope and pollen records, marine isotope records, speleothem records, and more.
But what does all this have to do with climate models? According to prevailing interpretations, the YD ‘cold snap’ had less to do with greenhouse gases and more to do with oceanic circulation (as an aside, the movie The Day After Tomorrow cites an embellished version of this mechanism). Nonetheless, Dr. Mitchell writes:
In actuality, oxygen-isotope data are not used to construct climate models, which instead rely on meteorological and geophysical parameters. Rather, we use past climatic trends to verify the accuracy of these models. The figure above provides one example, where each model was ‘asked’ to predict 20th-century warming trends in response to atmospheric perturbations. Whether or not we have rightly interpreted ice-core records is simply not relevant to the integrity of climate models, which predict 0.4–3.6°C warming by 2100 AD (depending on future trends in atmospheric CO2).
I would love to have the opportunity to discuss personally the above citation with Dr. Mitchell. It is unfortunate that her misperceptions about paleoclimatology have caused her inadvertently to suppress the academic freedom she ostensibly defends. Academic freedom allows us to examine critically claims made by the scientific community, as well as to challenge interpretations of data via the peer-review process. But as long as relevant data are suppressed through ‘strawman’ arguments presented by public figures (whether Dr. Mitchell, university administrators, or presidential candidates), investigators (such as yourselves) are forced to draw conclusions from limited, biased datasets. In other words, misrepresenting the current science only inhibits critical thinking and academic freedom.