Background
The authors write that "analysis of the PDSI [Palmer Drought Severity Index], a standardized measure of surface moisture conditions, revealed distinct 20th century aridity changes in vulnerable NW Africa, including a sharp downward trend towards drier conditions in the 1980s (Luterbacher et al., 2006)," but that "a high-resolution long-term reconstruction that could place current conditions in the context of the past millennium is missing for N Africa."

What was done
Esper et al. "re-use Cedrus atlantica tree-ring data generated in the 1980s (Glueck and Stockton, 2001) and combine these measurements with a major update collected in 2002," which "allows analysis of tree growth and instrumental data during the current drought episode in comparison to PDSI estimates back to AD 1049."

What was learned
The six scientists report that "PDSI values were above average for most of the 1450-1980 period, which let recent drought appear exceptional." However, they say that the long-term results they obtained indicate that the "pluvial episode of the past millennium was preceded by generally drier conditions back to 1049," leading them to state that the late 20th-century drought "appears more typical when associated with conditions before 1400." In addition, they conclude their paper by stating that the "ultimate drivers" for the medieval hydroclimate pattern that led to the earlier drought conditions in Morocco "seemed to be high solar irradiance and low volcanic forcings (Emile-Geay et al., 2007)."

What it means
This study provides another good example of the need to have long-term (millennial-scale) records of various climatic and meteorological phenomena in order to determine how exceptional 20th-century changes in their characteristics might be, which can help to determine whether there is compelling reason to attribute these changes to historical increases in the atmospheric concentrations of various greenhouse gases.

References
Emile-Geay, J., Cane, M., Seager, R., Kaplan, A. and Almasi, P. 2007. El Niņo as a mediator of the solar influence on climate. Paleoceanography 22: 10.1029/2006PA001304.

Glueck, M.F. and Stockton, C.W. 2001. Reconstruction of the North Atlantic Oscillation, 1429-1983. International Journal of Climatology 21: 1453-1465.

Luterbacher, J., et al. 2006. Mediterranean climate variability over the last centuries: A review. In: Lionello, P., et al., Eds. The Mediterranean Climate, Elsevier, Amsterdam, The Netherlands, pp. 27-148.