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Climate Models Inch Towards Acknowledging the Reality of the Medieval Warm Period and Little Ice Age
Goosse, H., Renssen, H., Timmermann, A. and Bradley, R.S.  2005.  Internal and forced climate variability during the last millennium: a model-data comparison using ensemble simulations.  Quaternary Science Reviews 24: 1345-1360.

The Northern Hemispheric temperature reconstruction of Mann et al. (1999) and the global temperature reconstruction of Mann and Jones (2003) fail to clearly portray the existence of the Medieval Warm Period (MWP) and Little Ice Age (LIA), which failure has led many proponents of CO2-induced global warming to conclude that these multi-century intervals of relative warmth and cold were but regional temperature anomalies that occurred in certain lands surrounding the North Atlantic Ocean, as opposed to true Northern Hemisphere-wide or global phenomena forced by variations in some factor or factors external to earth's climate system.

What was done
In the present paper, the authors "address the question as to whether the MWP and the LIA are robust features which were forced by solar and volcanic activity or whether they are representations of internal climate noise."  This is done via a three-dimensional climate model that was used to perform 25 simulations over the last millennium driven by what the authors believe to be "the main natural and anthropogenic forcing."  The results of these model runs were then "compared to available reconstructions in order to evaluate the relative contribution of internal and forced variability during this period."

What was learned
The results of this "model-data comparison," in the words of Goosse et al., was that the MWP and LIA were found to be "hemispheric-scale phenomena, since the temperature averaged over the Northern Hemisphere was, respectively, generally higher/lower during those periods because of a stronger/weaker external forcing at that time."

With respect to the former of these periods, they say "the MWP was a hemispheric-scale phenomenon, at least [our italics], since the temperature averaged over the Northern Hemisphere was generally higher during the period 1000-1200 AD than during the following centuries," and they state that "this is the consequence of a global [our italics] forcing, external to the climate system itself."

Natural internal variability of the system still exerts itself, however, and in some places and at some times it masks the external global signal; and "because of this role of internal variability," as they put it, they conclude that "synchronous peak temperatures during the MWP between different locations are unlikely to have occurred."  Nevertheless, they find that on local and regional scales, the external forcing results in "a higher probability for any location to have warm conditions than cold ones during the MWP."

What it means
These findings are very significant, for they support the likely global-scale reach of the MWP and LIA.  They are not yet the end of the story, however, for as noted by Goosse et al., they were "only able to test the response of the physical processes well represented in the model and processes not included might imply a different behavior," as well, we would add, as a different magnitude of temperature change.  Many of the papers whose archived reviews may be found in our Subject Index, for example, portray a much warmer MWP than is suggested by the data employed by Mann et al. and Mann and Jones in developing their temperature histories.  Hence, we believe it will not be too much longer before it will be impossible to deny that the MWP was at least as warm as it has been in recent decades, the significance of which conclusion resides in the fact that whatever caused the warming of the Medieval Warm Period (which we know was not CO2) could well be responsible for creating the equivalent (or possibly even lesser) warmth of the Modern Warm Period.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

Reviewed 9 November 2005