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North American Mega-Droughts and Global Mega-Warming
Routson, C.C., Woodhouse, C.A. and Overpeck, J.T. 2011. Second century megadrought in the Rio Grande headwaters, Colorado: How unusual was medieval drought? Geophysical Research Letters 38: 10.1029/2011GL050015.

The authors write that "many southwestern United States high-resolution proxy records show numerous droughts over the past millennium, including droughts far more severe than we have experienced during the historical period (e.g., Woodhouse and Overpeck, 1998; Cook et al., 2004, 2010; Meko et al., 2007)," adding that (1) "the medieval interval (ca. AD 900 to 1400), a period with relatively warm Northern Hemisphere temperatures, has been highlighted as a period in western North America with increased drought severity, duration and extent (e.g., Stine, 1994; Cook et al., 2004, 2010; Meko et al., 2007; Woodhouse et al., 2010)," and that (2) "the mid-12th century drought associated with dramatic decreases in Colorado River flow (Meko et al., 2007), and the 'Great Drought' associated with the abandonment of Ancient Pueblo civilization in the Colorado Plateau region (Douglass, 1929), all occurred during the medieval period," which observations would appear to suggest that significant Northern Hemispheric warmth tends to produce western North America megadroughts.

What was done
Routson et al. used a new tree-ring record derived from living and remnant bristlecone pine wood from the headwaters region of the Rio Grande River in Colorado (USA), along with other regional records, to evaluate what they describe as "periods of unusually severe drought over the past two millennia (268 BC to AD 2009)."

What was learned
The three researchers report that the record they derived "reveals two periods of enhanced drought frequency and severity relative to the rest of the record," and that "the later period, AD ~1050-1330, corresponds with medieval aridity well documented in other records," while "the earlier period is more persistent (AD ~1-400), and includes the most pronounced event in the ... chronology: a multi-decadal-length drought during the 2nd century," which "includes the unsmoothed record's driest 25-year interval (AD 148-173) as well as a longer 51-year period, AD 122-172, that has only two years with ring width slightly above the long-term mean," and where "the smoothed chronology shows the periods AD 77-282 and AD 301-400 are the longest (206 and 100 years, respectively, below the long-term average) droughts of the entire 2276-year record." And they note that this 2nd-century drought "impacted a region that extends from southern New Mexico north and west into Idaho."

What it means
Noting that "reconstructed Colorado Plateau temperature suggests warmer than average temperature could have influenced both 2nd century and medieval drought severity," and that "available data also suggest that the Northern Hemisphere may have been warm during both intervals," Routson et al. go on to suggest that the southwestern United States could well experience similar or even more severe megadroughts in the future, as they suspect it will continue to warm in response to continued anthropogenic CO2 emissions. We would caution, however, that studies from all around the globe - which depict both a Medieval Warm Period and a Roman Warm Period that were equally as warm or even warmer than the Current Warm Period has been to date, and at times when there was way less CO2 in the atmosphere than there is today (see both of these items in our Subject Index) - suggest that there is nothing unusual, unnatural or unprecedented about earth's current level of warmth, and, in fact, that it must be significantly cooler now than it was during those two prior multi-century warm periods, since we have not yet experienced droughts of anywhere near the severity or duration of those that were experienced in the Roman and Medieval Warm Periods, which further suggests that the planet's current level of warmth is likely not a result of historical anthropogenic CO2 emissions, but rather a result of a milder expression of whatever was the cause of those two earlier stellar warm periods.

Cook, E.R., Seager, R., Heim Jr., R.R., Vose, R.S., Herweijer, C. and Woodhouse, C. 2010. Megadroughts in North America: Placing IPCC projections of hydroclimatic change in a long-term paleoclimate context. Journal of Quaternary Science 25: 48-61.

Cook, E.R., Woodhouse, C., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Douglass, A.E. 1929. The Secret of the Southwest Solved with Talkative Tree Rings. Judd and Detweiler, Washington, DC, USA, pp. 736-770.

Meko, D.M., Woodhouse, C.A., Baisan, C.H., Knight, T., Lukas, J.J., Hughes, M.K. and Salzer, W. 2007. Medieval drought in the Upper Colorado River Basin. Geophysical Research Letters 34: 10.1029/2007GL029988.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during mediaeval time. Nature 369: 546-549.

Woodhouse, C.A., Meko, D.M., MacDonald, G.M., Stahle, D.W. and Cook, E.R. 2010. A 1,200-year perspective of 21st century drought in southwestern North America. Proceedings of the National Academy of Sciences USA 107: 21,283-21,288.

Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought variability in the central United States. Bulletin of the American Meteorological Society 79: 2693-2714.

Reviewed 22 February 2012