Vuille, M., Burns, S.J., Taylor, B.L., Cruz, F.W., Bird, B.W., Abbott, M.B., Kanner, L.C., Cheng, H. and Novello, V.F. 2012. A review of the South American monsoon history as recorded in stable isotopic proxies over the past two millennia. Climate of the Past 8: 1309-1321.
The authors write that "the South American summer monsoon (SASM) is one of the major monsoon systems in the Southern Hemisphere, yet it has received relatively little attention, due to the fact that it has only been considered a proper monsoon system for little more than a decade." They also report that the "monsoon characteristics on which society relies today have undergone considerable fluctuations in the past," citing Bird et al. (2011); and they say "there is considerable concern that the SASM dynamics will be significantly affected by increasing greenhouse gas concentrations in the 21st century."
What was done
In light of this concern, Vuille et al. review the history of the SASM over the past two millennia, based on information obtained from high-resolution stable isotopes derived from speleothems, ice cores and lake sediments acquired from the monsoon belt of the tropical Andes and Southeast Brazil.
What was learned
The nine researchers report that these data show "a very coherent behavior over the past two millennia with significant decadal to multi-decadal variability superimposed on large excursions during three key periods: the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA) and the current warm period (CWP)," which they interpret as "times when the SASM's mean state was significantly weakened (MCA and CWP) and strengthened (LIA), respectively."
What it means
Vuille et al. hypothesize that "these centennial-scale climate anomalies were at least partially driven by temperature changes in the Northern Hemisphere and in particular over the North Atlantic, leading to a latitudinal displacement of the Intertropical Convergence Zone and a change in monsoon intensity (amount of rainfall upstream over the Amazon Basin)." And with their noting that the intensity of the SASM "today appears on par with conditions during the MCA," it can logically be concluded that the peak temperatures of the MCA and the CWP over the North Atlantic Ocean are likely on a par with each other as well, which suggests that (1) there is nothing unusual, unnatural or unprecedented about today's current level of warmth over the North Atlantic, and that (2) today's level of warmth there need not have been caused by the 40% greater atmospheric CO2 concentration of today.
Bird, B.W., Abbott, M.B., Vuillle, M., Rodbell, D.T., Rosenmeier, M.F. and Stansell, N.D. 2011. A 2300-year-long annually resolved record of the South American summer monsoon from the Peruvian Andes. Proceedings of the National Academy of Sciences USA 108: 8583-8588.