A Two-Millennia Record of the South American Summer Monsoon
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.
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.
Based on their analysis, 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."
Commenting on their findings, 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.