How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

20th-Century and Medieval Droughts: Differences, Similarities and Implications
Volume 10, Number 45: 7 November 2007

Using Palmer Drought Severity Index data found in the North American Drought Atlas prepared by Cook and Krusic (2004), which were derived from a network of drought-sensitive tree-ring chronologies (some stretching all the way back to AD 800 and encompassing the Medieval Warm Period), Herweijer et al. (2007) were able to put into longer perspective "the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts)."

With respect to differences between 20th-century and medieval droughts, this work revealed, in the words of the four researchers who conducted it, that "the famous droughts of the instrumental era are dwarfed [our italics] by the successive occurrence of multidecade-long 'megadroughts' in the period of elevated aridity between the eleventh and fourteenth centuries AD," while with respect to similarities, they note that medieval megadroughts, although more extreme in terms of persistence, "share the severity and spatial distribution characteristics of their modern-day counterparts," which led them to conclude that the mechanism responsible for major North American droughts of the 20th century "is synonymous with that underlying the megadroughts of the medieval period," the only difference being the degree of persistence of the forcing that caused them.

So what is the common denominator that is shared by the major North American droughts of both the modern and medieval periods?

"With ENSO showing a pronounced signal in the gridded drought reconstructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability," Herweijer et al. conclude that "medieval megadroughts were forced by protracted La Niņa-like tropical Pacific sea surface temperatures." In addition, they demonstrate there is "a global [our italics] hydroclimatic 'footprint' of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions," and they say that "this global pattern matches that observed for modern-day persistent North American drought," namely, "a La Niņa-like tropical Pacific."

Interestingly, we have identified a large number of paleoclimate studies that demonstrate that when the earth was significantly warmer than it is currently, such as during the Medieval Warm Period, ENSO events were often substantially reduced and sometimes even absent (see ENSO (Relationship to Global Warming) in our Subject Index). Consequently, since the North American droughts of the Medieval Warm Period literally dwarfed those of the Current Warm Period - with both produced by La Niņa-like conditions (which are more prevalent during times of greater warmth) - it should follow as surely as night follows day that the Medieval Warm Period was significantly warmer than the Current Warm Period has been to date. And this implication begets a further implication: since the Medieval Warm Period was likely warmer than the Current Warm Period has been, and since it achieved such warmth when there was approximately 25% less CO2 in the air than there is today, there is no compelling reason to believe that the historical increase in the atmosphere's CO2 concentration has had anything whatsoever to do with the production of our current warmth.

Sherwood, Keith and Craig Idso

Cook, E.R. and Krusic, P.J. 2004. North American Summer PDSI Reconstructions. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series No. 2004-045, NOAA/NGDC Paleoclimatology Program, Boulder, Colorado, USA, 24 pp.

Herweijer, C., Seager, R., Cook, E.R. and Emile-Geay, J. 2007. North American droughts of the last millennium from a gridded network of tree-ring data. Journal of Climate 20: 1353-1376.