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The Little Ice Age in Central Mexico
Cuna, E., Zawisza, E., Caballero, M., Ruiz-Fernandez, A.C., Lozano-Garcia, S. and Alcocer, J. 2014. Environmental impacts of Little Ice Age cooling in central Mexico recorded in the sediments of a tropical alpine lake. Journal of Paleolimnology 51: 1-14.

The authors write that "late Holocene paleoclimate is characterized by a warm interval known as the Medieval Climate Anomaly (MCA), ~AD 1000-1300, which was followed by a highly variable, but generally cold period known as the Little Ice Age (LIA), ~AD 1350-1850," citing Crowley and Lowery (2000) and Mann et al. (2009). And they say that "many authors have related cooling during the LIA with solar forcing, specifically with the lower solar irradiance during the Sporer (1450-1540) and Maunder (1645-1715) solar minima," citing Bond et al. (2001) and Lozano-Garcia et al. (2007), while noting that "during the Maunder Minimum, solar activity and UV irradiance reached particularly low levels," citing Lean et al. (1995) and Lean and Rind (1999).

What was done
Cuna et al. developed "new information about the nature of the LIA in central Mexico based on a decadal-resolution sediment sequence from high-altitude tropical Lake La Luna, in the Nevado de Toluca volcano," which they did via analyses of "magnetic susceptibility, charcoal particles, palynomorphs [organic-walled microfossils], diatoms, cladoceran [small crustacean] remains and multivariate statistics."

What was learned
The six scientists say that the coldest period of the LIA occurred "between 1660 and 1760, an interval that broadly corresponds with the Maunder Minimum in solar activity," which they say "is also consistent with the timing of the coldest northern hemisphere temperatures during the last millennium," citing Jones and Mann (2004) and Matthews and Briffa (2005). As for the more recent past, they say that "biological assemblages that showed rapid changes during the LIA have remained relatively stable during the last few decades." They also report that these modern assemblages "resemble those in the lake ~500 years ago, during the MCA." And they say that "no clear evidence of modern, human-induced environmental change was recorded."

What it means
In light of Cuna et al.'s several findings regarding the climate of central Mexico, we have ever more reason to accept the fact that there is nothing unusual, unnatural or unprecedented about Earth's current level of warmth.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Crowley, T.J. and Lowery,T.S. 2000. How warm was the medieval warm period? Ambio 29: 51-54.

Jones, P.D. and Mann, M.E. 2004. Climate over past millennia. Reviews of Geophysics 42: 10.1029/2003RG000143.

Lean, J., Beer, J. and Bradley, R. 1995. Reconstruction for solar irradiance since 1610: implications for climate change. Geophysical Research Letters 22: 3195-3198.

Lean, J. and Rind, D. 1999. Evaluating sun-climate relationships since the Little Ice Age. Journal of Atmospheric and Solar-Terrestrial Physics 61: 25-36.

Lozano-Garcia, M.S., Caballero, M., Ortega, B., Rodriguez, A. and Sosa, S. 2007. Tracing the effects of the little ice age in the tropical lowlands of Eastern Mesoamerica. Proceedings of the National Academy of Sciences USA 104: 16,200-16,203.

Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G. and Ni, F. 2009. Global signatures and dynamical origins of the little ice age and medieval climate anomaly. Science 326: 1256-1260.

Matthews, J.A. and Briffa, K.R. 2005. The "Little Ice Age," reevaluation of an evolving concept. Geografiska Annaler. Series A, Physical Geography 87: 17-36.

Reviewed 23 April 2014