What was done
Noting that recent studies suggest the Indian summer monsoon may be sensitive to small changes in solar forcing of as little as 0.25% (Overpeck et al., 1996; Neff et al., 2001; Fleitmann et al., 2003), the authors set out to test this hypothesis by comparing trends in the Indian summer monsoon with trends in solar variability across the Holocene.  In this endeavor, trends in the Indian summer monsoon were inferred from relative abundances of fossil shells of the planktic foraminifer Globigerina bulloides in sediments of the Oman margin, while trends in solar variability were inferred from relative abundances of ¹⁴C, ¹⁰Be and haematite-stained grains.  Time resolutions in the latter data only allowed the authors to test for solar variability at multidecadal to centennial timescales.

What was learned
Spectral analysis revealed statistically significant periodicities in the G. bulloides time series centered at 1550, 152, 137, 114, 101, 89, 83 and 79 years, all but the first of which periodicities (representative of a millennial-scale oscillation) closely matched the periodicities of sunspot numbers centered at 150, 132, 117, 104, 87, 82 and 75 years.  This close correspondence, in the words of the authors, provides strong evidence for a "century-scale relation between solar and summer monsoon variability."  In addition, they report that intervals of monsoon minima correspond to intervals of low sunspot numbers, increased production rates of the cosmogenic nuclides ¹⁴C and ¹⁰Be, and increased advection of drift ice in the North Atlantic, such that over the past 11,100 years "almost every multi-decadal to centennial scale decrease in summer monsoon strength is tied to a distinct interval of reduced solar output," and nearly every increase "coincides with elevated solar output," including a stronger monsoon (high solar activity) during the Medieval Warm Period and a weaker monsoon (low solar activity) during the Little Ice Age.  As for the presence of the 1550-year cycle in the Indian monsoon, they consider it to be "remarkable," as it has been identified in numerous climate records of both the last glacial epoch and the present interglacial, including Dansgaard/Oeschger cycles in the North Atlantic, strengthening the case for a sun-monsoon-North Atlantic link.

What it means
Given the remarkable findings of this study, it is no wonder the authors say they are "convinced" there is a direct solar influence on the Indian summer monsoon in which small changes in solar output bring about pronounced changes in the tropical climate, adding there is "no need to hypothesize an indirect link," as they did in an earlier paper on the same subject (Gupta et al., 2003).  Consequently, if minimal changes in solar output can significantly influence this large atmospheric circulation, they likely can influence others, making it all the more important to thoroughly and rigorously investigate sun-climate connections throughout the earth-ocean-atmosphere system before accepting the climate-alarmist claim that the historical rise in the air's CO2 content was the major driver of 20th-century climate change, for it likely was not.

References
Fleitmann, D., Burns, S.J., Mudelsee, M., Neff, U., Kramers, J., Mangini, A. and Matter, A.  2003.  Holocene forcing of the Indian monsoon recorded in a stalagmite from southern Oman.  Science 300: 1737-1739.

Gupta, A.K., Anderson, D.M. and Overpeck, J.T.  2003.  Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean.  Nature 421: 354-357.

Neff, U., Burns, S.J., Mangini, A., Mudelsee, M., Fleitmann, D. and Matter, A.  2001.  Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago.  Nature 411: 290-293.

Overpeck, J.T., Anderson, D.M., Trumbore, S. and Prell, W.L.  1996.  The southwest monsoon over the last 18,000 years.  Climate Dynamics 12: 213-225.