Reference
Xu, H., Liu, X. and Hou, Z. 2008. Temperature variations at Lake Qinghai on decadal scales and the possible relation to solar activities. Journal of Atmospheric and Solar-Terrestrial Physics 70: 138-144.
What was done
Working with sediments retrieved from Lake Qinghai on the northeastern Qinghai-Tibet Plateau, the authors reconstructed a 600-year temperature history of the region based on high-resolution series of the sediment's total carbonate content, the δ18O and δ13C of its carbonates, and the detrended δ15N of organic matter contained in the sediment cores, which they compared against total solar irradiance histories reconstructed from sunspot numbers, and tree-ring 14C and ice-core 10Be data that are also indicative of solar activity.
What was learned
Xu et al. report that their temperature history indicates that "the intervals of 1430-1470, 1650-1715, and 1770-1820 are consistent with the three coldest intervals of the Little Ice Age," and that "these obvious cold intervals are also synchronous with the minimums of the sunspot numbers during the past 600 yeas," namely, "the Sporer, the Maunder, and the Dalton minimums," which facts strongly suggest, in their words, "that solar activities may dominate temperature variations on decadal scales at the northeastern Qinghai-Tibet plateau." In addition, their observation that "more and more evidence indicates that the Little Ice Age occurred worldwide," provides more and more evidence that it was likely solar-induced.
What it means
If the development of the significant cold of the worldwide Little Ice Age was driven by a concomitant change in some type of solar activity (which seems fairly well proven by a wealth of real-world data, of which the study of Xu et al. is but one example), it logically follows that the "undevelopment" of the Little Ice Age (i.e., the global warming of the 20th century) was primarily driven by the reversal of that change in solar activity, and not by the historical rise in the air's CO2 content. However, as also noted by Xu et al., how small perturbations of solar activity have led "to the observed global warming, what is the mechanism behind it, etc., are still open questions." Nevertheless, everything else about the matter seems fairly well established.