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A Second Review of Drought in Morocco
Esper, J., Frank, D., Buntgen, U., Verstege, A., Luterbacher, J. and Xoplaki, E. 2007. Long-term drought severity variations in Morocco. Geophysical Research Letters 34: 10.1029/2007GL030844.

Every once in a while we inadvertently review the same paper twice, as it is difficult to remember what we have written in the past when dealing with so many different subjects and when reading and writing about them nearly every day of the week. Whenever this happens, we generally give a deep sigh and send one of the reviews to the circular file. In the present case, however, the two reviews were different enough that we decided to post both of them. So here is the second of the two.

What was done
In the words of the authors, "Cedrus atlantica ring width data [were] used to reconstruct long-term changes in the Palmer Drought Severity Index (PDSI) over the past 953 years in Morocco, Northwest Africa."

What was learned
Esper et al. report that "the long-term PDSI reconstruction indicates generally drier conditions before ~1350, a transition period until ~1450, and generally wetter conditions until the 1970s," after which there were "dry conditions since the 1980s." In addition, they determined that "the driest 20-year period reconstructed is 1237-1256 (PDSI = -4.2)," adding that "1981-2000 conditions are in line with this historical extreme (-3.9)." Also of significance, the six researchers note that "millennium-long temperature reconstructions from Europe (Buntgen et al., 2006) and the Northern Hemisphere (Esper et al., 2002) indicate that Moroccan drought changes are broadly coherent with well-documented temperature fluctuations including warmth during medieval times, cold in the Little Ice Age, and recent anthropogenic warming."

What it means
The latter coherency would tend to suggest that the peak warmth of the Medieval Warm Period was at least as great as that of the last two decades of the 20th century throughout the entire Northern Hemisphere; and, if the coherency is strictly interpreted, it suggests that the warmth of the MWP was likely even greater than that of the late 20th century.

Buntgen, U., Frank, D.C., Nievergelt, D. and Esper, J. 2006. Summer temperature variations in the European Alps, A.D. 755-2004. Journal of Climate 19: 5606-5623.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Reviewed 2 January 2008