Queshque Glacier of Peru's tropical Andes has retreated rapidly since the mid-20th century. Using a glacier model, we show that this has primarily been driven by steady warming despite the counteracting effect of recent snowfall amounts. Independently from climatic trends, we find that the formation of a new lake at the glacier's base has further accelerated ice loss. This research demonstrates the utility of empirical glacier models for interpreting past and future changes in the tropics.