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A 2300-Year History of the South American Summer Monsoon

Reference
Bird, B.W., Abbott, M.B., Vuille, M., Rodbell, D.T., Stansell, N.D. and Rosenmeier, M.F. 2011. A 2,300-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.
Working with an annually-resolved authigenic calcite record of precipitation δ18O obtained from a varved lake in the Central Peruvian Andes -- Laguna Pumacocha (10.70°S, 76.06°W, 4300 m asl) -- Bird et al. (2011) developed a 2300-year history of the South American Summer Monsoon (SASM) that shows, in their words, that "δ18O peaked during the Medieval Climate Anomaly (MCA) from AD 900 to 1100, providing evidence that the SASM weakened considerably during this period." Thereafter, they found that "minimum δ18O values occurred during the Little Ice Age (LIA) between AD 1400 and 1820, reflecting a prolonged intensification of the SASM," after which they found that "δ18O increased rapidly, particularly during the Current Warm Period (CWP; AD 1900 to present), indicating a return to reduced SASM precipitation."

The six scientists also note that the Pumacocha record tracks the 900-year-long Cascayunga Cave δ18O record (6.09°S, 77.23°W, 930 m asl), which they say "is interpreted as a record of South American rainfall (Reuter et al., 2009)," and they report that it shares many features with the annually-resolved Quelccaya Ice Cap δ18O record (13.93°S, 70.83°W, 5670 m asl), which was derived by Thompson et al. (1986). Thus, they conclude that "the close agreement in the timing, direction, and magnitude of mean state changes in δ18O during the MCA, LIA, and CWP from lake sediment, speleothem, and ice core records supports the idea that a common large-scale mechanism influenced δ18O reaching these central Andean sites spanning 11° latitude and 4,740 meters of elevation." And they say that based on the above observations, "the most likely cause of these documented shifts in δ18Oprecip is a change in SASM intensity, as all three sites receive the majority of their annual precipitation during the monsoon season."

Most interesting of all, however, is what Bird et al. describe as the "remarkable correspondence" that exists between the Pumacocha δ18O record of SASM rainfall and the 2000-year Northern Hemispheric temperature reconstruction of Moberg et al. (2005), plus the similar relationship that both records share with the somewhat shorter North Atlantic temperature reconstruction of Mann et al. (2009). More specifically, they say "the two greatest reductions in SASM intensity in the Pumacocha δ18O record were coincident with Northern Hemisphere temperature maxima during the MCA and CWP," and that "the SASM was stronger than at any other point in the last 2,300 years when Northern Hemisphere temperatures were at a 2,000-year low during the LIA." And as noted above, their data show that the same relationships exist between the Pumacocha δ18O history and the North Atlantic temperature history.

What is especially interesting about these several observations is the fact that Bird et al.'s graphical representations of the Northern Hemisphere and North Atlantic temperature histories of Moberg et al. and Mann et al. both show the peak warmth of the MCA to be at least as great as, and possibly a bit greater than, the peak warmth of the CWP, plus the fact that the δ18O data of Bird et al. suggest much the same thing, based on the "remarkable correspondence" among the three data sets, which can readily be seen in the figure below.


(A) The reconstructed Northern Hemispheric temperature history of Moberg et al. (2005), (B) the reconstructed North Atlantic temperature history of Mann et al. (2009), and (C) the Cariaco Basin %Ti data of Haug et al. (2001) that represent the degree of northward migration of the Intertropical Convergence Zone, each plotted together with the δ18O data (grey lines) of Bird et al. (2011), from whose paper this figure is adapted.

As can be seen in the figure above, the correspondence among the four data sets is nothing short of astounding; and, therefore, the equivalent or slightly greater warmth of the MCA (known also as the Medieval Warm Period or MWP) compared to that of the CWP would appear to be well established for the North Atlantic Ocean, the Northern Hemisphere, and a good portion of South America, in light of the fact that Bird et al. note that the diminished SASM precipitation (higher δ18O data) during the MWP and CWP also tracks the northward migration of the Intertropical Convergence Zone over the Atlantic, since "the Pumacocha record shows that the SASM was considerably reduced during the MCA when peak %Ti in the Cariaco Basin record indicates that the Intertropical Convergence Zone was persistently northward," as demonstrated by Haug et al. (2001).

In conclusion, there is yet more evidence pointing to the likelihood that the Medieval Warm Period of a thousand or so years ago was equally as warm as, or maybe even a little warmer than, the Current Warm Period has been to date. And since the air's CO2 concentration has risen by some 40% since the days of the MWP, and since it is no warmer now than it was back then, there is no compelling reason to believe that the modern rise in atmosphere's CO2 is driving that warmth.

Additional References
Haug, G.H., Hughen, K., Sigman, D.M., Peterson, L.C. and Rohl, U. 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293: 1304-1308.

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.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.

Reuter, J., Stott, L., Khidir, D., Sinha, A., Cheng, H. and Edwards, R.L. 2009. A new perspective on the hydroclimate variability in northern South America during the Little Ice Age. Geophysical Research Letters 36: 10.1029/2009GL041051.

Thompson, L.G., Thompson, E.M., Dansgaard, W. and Groots, P.M. 1986. The Little Ice Age as recorded in the stratigraphy of the tropical Quielccaya Ice Cap. Science 234: 361-364.

Archived 21 September 2011