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A Hot Time in the Medieval Warm Period in Africa
Reference
Tyson, P.D., Karlen, W., Holmgren, K. and Heiss, G.A. 2000. The Little Ice Age and medieval warming in South Africa. South African Journal of Science 96: 121-126.

What was done
The authors obtained a quasi-decadal-resolution record of oxygen and carbon-stable isotope data from a well-dated stalagmite recovered from Cold Air Cave in the Makapansgat Valley, located approximately 30 km southwest of Pietersburg, South Africa, which they augmented with 5-year-resolution temperature data reconstructed from color variations in banded growth-layer laminations of the stalagmite that were derived from a relationship calibrated against actual air temperatures obtained from a surrounding 49-station climatological network over the period 1981-1995, which had a correlation of +0.78 that was significant at the 99% confidence level.

What was learned
The Little Ice Age (prevailing from about AD 1300 to 1800) and the Medieval Warm Period (prevailing from before AD 1000 to around 1300) were found to be distinctive features of the climate of the last millennium. Relative to the period 1961-1990, in fact, the Little Ice Age, which the authors say "was a widespread event in South Africa specifically and southern Africa generally," was characterized by a mean annual temperature depression of about 1C at its coolest point. The Medieval Warm Period, on the other hand, was as much as 3-4C warmer at its warmest point. The authors also note that the coolest point of the Little Ice Age corresponded in time with the Maunder Minimum of sunspot activity and that the Medieval Warm Period corresponded with the Medieval Maximum in solar activity.

What it means
Contrary to the attempts of some to rewrite climatic history and relegate the Little Ice Age and the Medieval Warm Period to minor localized events that become lost in the blending of many data sets from across the globe, this paper demonstrates - with many citations to similar works from around the world - that that endeavor is doomed to fail. There are just too many well-calibrated high-quality data sets that tell a dramatically different story. The Little Ice Age and the Medieval Warm Period were real planet-wide events; and the Medieval Warm Period was indeed warmer than the present. In this particular study, in fact, the authors state that "maximum warming at Makapansgat at around 1250 produced conditions up to 3-4C hotter than those of the present." And we feel that their use of the word "hotter" in place of "warmer" is entirely appropriate. The Medieval Warm Period was indeed a hot time in Africa.

The importance of these facts, of course, resides in the demonstration that the warming of the earth since the termination of the Little Ice Age is not at all unusual or different from other climate changes of the past millennium, when atmospheric CO2 concentrations were quite stable, much lower than present, and obviously not responsible for the observed variations in climate, which suggests that the warming of the past century or so need not be due to the contemporaneous increase in atmospheric CO2. In this regard the authors make a point of noting that the Little Ice Age coincided with a period of low solar activity, while the Medieval Warm Period coincided with a period of high solar activity, suggesting that there may be a solar forcing involved in the development and sustaining of these climatic regimes.


Reviewed 4 October 2000