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The World's Oscillatory Climate as Experienced in Northern Iberia
Martin-Chivelet, J., Munoz-Garcia, M.B., Edwards, R.L., Turrero, M.J. and Ortega, A.I. 2011. Land surface temperature changes in Northern Iberia since 4000 yr BP, based on δ13C of speleothems. Global and Planetary Change 77: 1-12.

What was done
Based on δ13C data that they obtained from stalagmites recovered from three different caves in the northern part of Castilla-Leon in northern Spain -- each of which were situated approximately 50 km from a common central point (~42°40'N, 4°W) -- and with the help of a good correlation between the mean annual temperatures of the past 125 years (measured at a site located 14 km from one of the caves) and the corresponding δ13C data, Martin-Chivelet et al. developed a 4000-year temperature history of the region.

What was learned
The five researchers report that their δ13C record began with "an initial interval of broad warm conditions between 4000 and 3000 yr BP." Then came "a prolonged time during which thermal conditions become permanently cold," with the coldest conditions occurring between 2850 and 2550 yr BP, an interval that they describe as "the 'first cold phase' of the Subatlantic period, also called in Europe the Iron Age Cold Period." Next came another warm period when "maximum temperatures were probably reached in the three hundred years interval between 2150 and 1750 yr BP," which corresponds, in their words, "to the well-known Roman Warm Period, an interval which has been correlated with a phase of relatively high solar flux."

Thereafter came "another relatively cold episode, which lasted about 250 years and reached its minimum at ~1500 yr BP," which "correlated with the Dark Ages Cold Period described in other areas of Europe." Then, "a rapid trend of warming led to a new, prolonged interval of warmth" that lasted from 1400 to 700 yr BP. Known as the Medieval Warm Period, Martin-Chivelet et al. say that it is "probably the most robust climatic feature in our records, perfectly outlined in the series of the three stalagmites." And, as might well be expected by now, they write that "the end of the Medieval Warm Period was marked by a progressive and rapid ... transition into the Little Ice Age, a relatively cold period broadly reported from all Europe and also from other areas in the world as far as South Africa or South America."

Last of all, a graph of the researchers' data reveals the development of the Current Warm Period; and it suggests that temperatures at the end of the 20th century were about a quarter of a degree Centigrade warmer than the peak warmth of the Medieval Warm Period. However, they note that studies in Northern Spain based on peat bog proxies "suggest that the temperatures during both the Roman Warm Period and the Medieval Warm Period were higher than present-day ones," citing the work of Martinez-Cortizas et al. (1999)."

What it means
Irrespective of which of the four multi-century warm periods of the last four millennia was the warmest of the bunch, it is clear that there is nothing unusual about the current level of warmth of northern Iberia, of Europe, or of the globe as a whole, especially when it is realized that the atmosphere's CO2 concentration is about 40% greater today than it was during the prior three warm periods of the last four millennia.

Martinez-Cortizas, A., Pontevedra-Pombal, X., Garcia-Rodeja, E., Novoa-Munoz, J.C. and Shotyk, W. 1999. Mercury in a Spanish peat bog: archive of climate change and atmospheric metal deposition. Science 284: 939-942.

Reviewed 20 July 2011