Climate alarmists warn of all sorts of weather disasters, including droughts, as the earth recovers from the debilitating chill of the Little Ice Age and begins to experience the more benign temperatures of the Current Warm Period. Does history vindicate them? We here explore this question via a brief review of recent papers that are relevant to this question as it pertains to the occurrence of droughts in Mexico.
Stahle et al. (2000) developed a long-term history of drought over much of North America from reconstructions of the Palmer Drought Severity Index, based on analyses of many lengthy tree-ring records. This history reveals the occurrence of a 16th-century drought in Mexico that persisted from the 1540s to the 1580s. Writing of this anomalous period of much reduced precipitation, they say that "the 'megadrought' of the 16th century far exceeded any drought of the 20th century," when climate alarmists claim the planet experienced a level of warmth that was unprecedented over the past two millennia and should therefore, according to their reasoning, have been host to the most extreme lack of moisture of the studied period.
Diaz et al. (2002) constructed a history of winter-spring (November-April) precipitation -- which accounts for one-third of the yearly total -- for the Mexican state of Chihuahua for the period 1647-1992, based on earlywood width chronologies of over 300 Douglas fir trees growing at four locations along the western and southern borders of Chihuahua and at two locations in the United States just above Chihuahua's northeast border. On the basis of these reconstructions, they correctly note that "three of the 5 worst winter-spring drought years in the past three-and-a-half centuries are estimated to have occurred during the 20th century." Although this observation tends to make the 20th century look highly anomalous in this regard, it is not; for two of those three worst drought years occurred during a period of average to slightly-above-average precipitation.
Diaz et al. also correctly note that "the longest drought indicated by the smoothed reconstruction lasted 17 years (1948-1964)," which is again correct and seemingly indicative of abnormally dry conditions during the 20th century. However, for several of the 17 years of that below-normal-precipitation interval, precipitation values were only slightly below normal. For all practical purposes, therefore, there were four very similar dry periods interspersed throughout the preceding two and a half centuries: one in the late 1850s and early 1860s, one in the late 1790s and early 1800s, one in the late 1720s and early 1730s, and one in the late 1660s and early 1670s.
With respect to the 20th century alone, there was also a long period of high winter-spring precipitation that stretched from 1905 to 1932; and following the major drought of the 1950s, precipitation remained at or just slightly above normal for the remainder of the record. Finally, with respect to the entire 346 years, there is no long-term trend in the data, nor is there any evidence of any sustained departure from that trend over the course of the 20th century, indicating that neither 20th century anthropogenic CO2 emissions nor 20th century warming have significantly impacted rainfall in the Mexican state of Chihuahua.
Cleaveland et al. (2003) constructed a winter-spring (November-March) precipitation history for the period 1386-1993 for Durango, Mexico, based on earlywood width chronologies of Douglas-fir tree rings collected at two sites in the Sierra Madre Occidental. They report that this record "shows droughts of greater magnitude and longer duration than the worst historical drought that occurred in the 1950s and 1960s." These earlier dramatic droughts include the long dry spell of the 1850s-1860s and what they call the megadrought of the mid- to late-16th century. Hence, their work clearly demonstrates that the worst droughts of the past 600 years did not occur during the period of greatest warmth, as the world's climate alarmists are fond of saying they should. Instead, they occurred during the Little Ice Age, which was perhaps the coldest period of the current interglacial.
Investigating the same approximate time period, Hodell et al. (2005b) analyzed a 5.1-m sediment core they retrieved from Aguada X'caamal, a small sinkhole lake in northwest Yucatan, Mexico, finding that an important hydrologic change occurred there during the 15th century AD, as documented by the appearance of A. beccarii in the sediment profile, a decline in the abundance of charophytes, and an increase in the δ18O of gastropods and ostracods. In addition, they report that "the salinity and 18O content of the lake water increased as a result of reduced precipitation and/or increased evaporation in the mid- to late 1500s." These several changes, as well as many others they cite, were, as they describe it, "part of a larger pattern of oceanic and atmospheric change associated with the Little Ice Age that included cooling throughout the subtropical gyre (Lund and Curry, 2004)." Their assessment of the situation was that the "climate became drier on the Yucatan Peninsula in the 15th century AD near the onset of the Little Ice Age," as is also suggested by Maya and Aztec chronicles that "contain references to cold, drought and famine in the period AD 1441-1460."
Going back even further in time, Hodell et al. (1995) had provided evidence for a protracted drought during the Terminal Classic Period of Mayan civilization (AD 800-1000), based on their analysis of a single sediment core retrieved in 1993 from Lake Chichanacanab in the center of the northern Yucatan Peninsula of Mexico. Subsequently, based on two additional sediment cores retrieved from the same location in 2000, Hodell et al. (2001) determined that the massive drought likely occurred in two distinct phases (750-875 and 1000-1075). Reconstructing the climatic history of the region over the past 2600 years and applying spectral analysis to the data also revealed a significant recurrent drought periodicity of 208 years that matched well with a cosmic ray-produced 14C record preserved in tree rings, which is believed to reflect variations in solar activity; and because of the good correspondence between the two data sets, they concluded that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing."
In a still-later study, Hodell et al. (2005a) returned to Lake Chichanacanab in March of 2004 and retrieved a number of additional sediment cores in some of the deeper parts in the lake, with multiple cores being taken from its deepest point, from which depth profiles of bulk density were obtained by means of gamma-ray attenuation, as were profiles of reflected red, green and blue light via a digital color line-scan camera. As they describe their findings, "the data reveal in great detail the climatic events that comprised the Terminal Classic Drought and coincided with the demise of Classic Maya civilization." In this regard, they again report that "the Terminal Classic Drought was not a single, two-century-long megadrought, but rather consisted of a series of dry events separated by intervening periods of relatively moister conditions," and that it "included an early phase (ca 770-870) and late phase (ca 920-1100)." Last of all, they say that "the bipartite drought history inferred from Chichancanab is supported by oxygen isotope records from nearby Punta Laguna," and that "the general pattern is also consistent with findings from the Cariaco Basin off northern Venezuela (Haug et al., 2003), suggesting that the Terminal Classic Drought was a widespread phenomenon and not limited to north-central Yucatan."
Concurrent with the study of Hodell et al. (2005a), Almeida-Lenero et al. (2005) analyzed pollen profiles derived from sediment cores retrieved from Lake Zempoala and nearby Lake Quila in the central Mexican highlands about 65 km southwest of Mexico City, determining that it was generally more humid than at present in the central Mexican highlands during the mid-Holocene. Thereafter, however, there was a gradual drying of the climate; and their data from Lake Zempoala indicate that "the interval from 1300 to 1100 cal yr BP was driest and represents an extreme since the mid-Holocene," noting further that this interval of 200 years "coincides with the collapse of the Maya civilization." Likewise, they report that their data from Lake Quila are also "indicative of the most arid period reported during the middle to late Holocene from c. 1300 to 1100 cal yr BP." In addition, they note that "climatic aridity during this time was also noted by Metcalfe et al. (1991) for the Lerma Basin [central Mexico]," that "dry climatic conditions were also reported from Lake Patzcuaro, central Mexico by Watts and Bradbury (1982)," and that "dry conditions were also reported for [Mexico's] Zacapu Basin (Metcalfe, 1995) and for [Mexico's] Yucatan Peninsula (Curtis et al., 1996, 1998; Hodell et al., 1995, 2001)."
Based on the many results described above, it is evident that throughout much of Mexico some of the driest conditions and worst droughts of the Late Holocene occurred during the Little Ice Age and the latter part of the Dark Ages Cold Period. These observations do much to discredit the climate-alarmist claim that droughts will only get worse as air temperatures rise, especially when it is realized that all of the Mexican droughts of the 20th century (when climate alarmists claim the planet warmed at a rate and to a level that were both unprecedented over the past two millennia) were much milder than many of the droughts that occurred during much colder centuries.
References
Almeida-Lenero, L., Hooghiemstra, H., Cleef, A.M. and Van Geel, B. 2005. Holocene climatic and environmental change from pollen records of Lakes Zempoala and Quila, central Mexican highlands. Review of Palaeobotany and Palynology 136: 63-92.
Cleaveland, M.K., Stahle, D.W., Therrell, M.D., Villanueva-Diaz, J. and Burns, B.T. 2003. Tree-ring reconstructed winter precipitation and tropical teleconnections in Durango, Mexico. Climatic Change 59: 369-388.
Curtis, J., Hodell, D. and Brenner, M. 1996. Climate variability on the Yucatan Peninsula (Mexico) during the past 3500 years, and implications for Maya cultural evolution. Quaternary Research 46: 37-47.
Curtis, J., Brenner, M., Hodell, D. Balser, R., Islebe, G.A. and Hooghiemstra, H. 1998. A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten Guatemala. Journal of Paleolimnology 19: 139-159.
Diaz, S.C., Therrell, M.D., Stahle, D.W. and Cleaveland, M.K. 2002. Chihuahua (Mexico) winter-spring precipitation reconstructed from tree-rings, 1647-1992. Climate Research 22: 237-244.
Haug, G.H., Gunther, D., Peterson, L.C., Sigman, D.M., Hughen, K.A. and Aeschlimann, B. 2003. Climate and the collapse of Maya civilization. Science 299: 1731-1735.
Hodell, D.A., Brenner, M. and Curtis, J.H. 2005a. Terminal Classic drought in the northern Maya lowlands inferred from multiple sediment cores in Lake Chichancanab (Mexico). Quaternary Science Reviews 24: 1413-1427.
Hodell, D.A., Brenner, M., Curtis, J.H. and Guilderson, T. 2001. Solar forcing of drought frequency in the Maya lowlands. Science 292: 1367-1370.
Hodell, D.A., Brenner, M., Curtis, J.H., Medina-Gonzalez, R., Can, E. I.-C., Albornaz-Pat, A. and Guilderson, T.P. 2005b. Climate change on the Yucatan Peninsula during the Little Ice Age. Quaternary Research 63: 109-121.
Hodell, D.A., Curtis, J.H. and Brenner, M. 1995. Possible role of climate in the collapse of Classic Maya civilization. Nature 375: 391-394.
Lund, D.C. and Curry, W.B. 2004. Late Holocene variability in Florida Current surface density: patterns and possible causes. Paleoceanography 19: 10.1029/2004PA001008.
Metcalfe, S.E. 1995. Holocene environmental change in the Zacapu Basin, Mexico: a diatom based record. The Holocene 5: 196-208.
Metcalfe, S.E., Street-Perrott, F.A., Perrott, R.A. and Harkness, D.D. 1991. Palaeolimnology of the Upper Lerma Basin, central Mexico: a record of climatic change and anthropogenic disturbance since 11,600 yr B.P. Journal of Paleolimnology 5: 197-218.
Stahle, D.W., Cook, E.R., Cleaveland, M.K, Therrell, M.D., Meko, D.M., Grissino-Mayer, H.D., Watson, E. and Luckman, B.H. 2000. Tree-ring data document 16th century megadrought over North America. EOS, Transactions, American Geophysical Union 81: 121, 125.
Watts, W.A. and Bradbury, J.P. 1982. Paleoecological studies at Lake Patzcuaro on the West Central Mexican plateau and at Chalco in the Basin of Mexico. Quaternary Research 17: 56-70.
Last updated 27 September 2006