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Assessing the Sensitivity and Accuracy of State-of-the Art Climate Models
Alley, R.B. and Agustsdottir, A.M.  2005.  The 8k event: cause and consequences of a major Holocene abrupt climate change.  Quaternary Science Reviews 24: 1123-1149.

What was done
The authors review what is known about the abrupt climatic event of approximately 8200 years ago that "brought generally cold and dry conditions to broad northern-hemisphere regions especially in wintertime, in response to a very large outburst flood that freshened the North Atlantic."

What was learned
With respect to the consequences of this "8k event," Alley and Agustsdottir say "some simple models produce shutdowns of overturning circulation and coolings around the North Atlantic."  On the other hand, they say that "complex models generally slow the overturning circulation without shutdown or net cooling."  On yet another hand, they say that "because of shortcomings in understanding of North Atlantic processes, one cannot exclude the possibility of a shutdown in the future."  On yet still another hand, they say that "most modelers seem to consider this to be a low-probability event."

As if this wasn't enough back-and-forth equivocation, they go on to say that "8k climate reconstructions might provide a useful guide to possible anomalies."  On the other hand, they say that "because the 8k climate anomalies never reached a plateau, and because the forcing was not maintained whereas future greenhouse forcing presumably will be maintained for centuries or longer, the 8k anomalies might serve as lower limits [our italics] on the size of possible changes should a complete shutdown happen."  On yet another hand, they say that "with continued warming, the sea-ice amplifier will become less potent, and possible anomalies will shrink."

What it means
So what does all the dizzying turning from hand to hand to hand to hand to hand to hand add up to?  In their closing paragraph, Alley and Agustsdottir give their answer: "the 8k [event] offers perhaps the best chance we have for a well-characterized, rigorous test of Earth-system-model response to a large perturbation, to assess the sensitivity and accuracy of the models."

If this be true, all we can say is "Heaven help us!"

Reviewed 25 May 2005