A sub-centennial-resolution record of lacustrine carbonate oxygen isotopes (delta O-18(C)) from the closed-basin Lake Qinghai on the NE Tibetan Plateau shows pronounced variability over the past 1500 years. Changes in delta O-18(C) in hydrologically closed lakes are often interpreted in terms of changing effective moisture. Under this interpretation our record would imply increasing effective moisture during the Little Ice Age (LIA) compared to the Medieval Warm Period (MWP). However, independent evidence from other archives strongly suggests the Asian summer monsoon was stronger during the MWP and weakened during the LIA. Controls other than effective moisture (the balance of water inputs over evaporative loss) must therefore have contributed to the delta O-18(C) values. We propose the LIA signal in Lake Qinghai resulted from a reduction in evaporation caused by colder air temperatures, coupled with a decrease in oxygen isotope composition of input waters as a result of an increase in the relative importance of westerly-derived precipitation. Our results caution against simplistic interpretations of carbonate oxygen isotope records from hydrologically closed lakes and suggest all possible controlling factors must be taken into account in order to avoid misleading palaeoclimatic reconstructions. (C) 2010 Elsevier Ltd. All rights reserved.