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Of Grapes and Hockeysticks
Volume 7, Number 48: 1 December 2004

In an intriguing paper published in Nature, Chuine et al. (2004) used recorded dates of grape harvests in Burgundy, France to reconstruct mean spring-summer (April-August) air temperatures for that location on a yearly basis from 1370 to 2003, using what they call "a process-based phenology model developed for the Pinot Noir grape."  The result, which they describe as being "without chronological uncertainties," is also significantly correlated with mean summer air temperatures deduced from tree rings in central France, the Burgundy portion of a spatial multi-proxy temperature reconstruction, as well as observed summer air temperatures in Paris, central England and the Alps, the thermal interconnectedness of which sites gives the new temperature history an important regional significance.  So what does this unique record show?

The most important aspect of the 633-year temperature history, in our estimation, is the fact that it looks nothing at all like the infamous "hockeystick" temperature history of Mann et al. (1998, 1999), which underpins the climate-alarmist claim that 20th century warming is without precedent over the past thousand years.

For starters, the temperature track left by the 30-year Gaussian filter that was employed by the six scientists to smooth the data depicts but a minor 20th-century recovery from the cold temperatures of the Little Ice Age.  It should perhaps be noted, however, that the final year of the record (2003) is wildly anomalous in terms of its great spring-summer warmth; and many people are attempting to make much of that fact.  Apparently, these folks are unaware of some other important facts, namely, that one year does not a climate make and that a temperature anomaly (comprised in this case of but a single very aberrant data point) is in no way indicative of the true thermal state of the regional climate, which is more properly defined by the many closely-clustered data points that precede the aberrant point.

Perhaps the most significant difference between the temperature histories of Chuine et al. and Mann et al. is the existence of much warmer-than-present air temperatures at various times in the past in the grape-graph record (most notably from the late 1300s through the early 1400s and over a large portion of the 1600s), where there are not even any hints of elevated warmth in the hockeystick record.  In this respect, the Chuine et al. temperature history forcefully rebuts the hockeystick-derived claim of the world's climate alarmists that air temperatures of the last two decades of the 20th century were unprecedented over the past millennium and possibly over the past two millennia (Mann and Jones, 2003).

The great importance of the Chuine et al. findings is that if it was warmer than it is now during parts of the 1300s, 1400s and 1600s, when there was far less CO2 in the air than there is currently (perhaps as much as 100 ppm less), it is reasonable to believe that whatever was responsible for the warmth of those earlier times may be responsible for much of the warmth of the present era (or perhaps even all of it), which leaves rising atmospheric CO2 concentrations pretty much "out in the cold" with respect to their being responsible for earth's recovery from the inclement climate of the Little Ice Age.

Sherwood, Keith and Craig Idso

Chuine, I., Yiou, P., Viovy, N., Seguin, B., Daux, V. and Le Roy Ladurie, E.  2004.  Grape ripening as a past climate indicator.  Nature 432: 289-290.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

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.