How does rising atmospheric CO2 affect marine organisms?

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High, Middle and Rif Atlas Mountains, Morocco, Northwest Africa
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.

Esper et al. (2007) used Cedrus atlantica tree-ring width data to reconstruct long-term changes of the Palmer Drought Severity Index (PDSI) over nearly a full millennium in the part of Morocco (northwest Africa) centered on approximately 34N, 5W. This analysis revealed that "millennium-long temperature reconstructions from Europe (Bungten 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." In addition, they report that the driest 20-year period of their reconstruction was 1237-1256 (with a PDSI of -4.2), while the driest 20-year period of the 20th century was 1981-2000 (with a less extreme PDSI of -3.9). Hence, a strict interpretation of the coherence that exists between Esper et al.'s (2007) PDSI history and European and Northern Hemispheric temperatures suggests that the peak warmth of the Medieval Warm Period was likely greater than the peak warmth of the 20th century over the entire Northern Hemisphere.

Additional References
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.