Arctic lake ecosystems are highly biodiverse sites that play an important role in the carbon cycle, but the impacts of emerging warmer and wetter conditions on the ecology of these lakes are poorly understood, in part due to a lack of long-term data. Using a 10-year dataset, we report an abrupt, coherent, climate-driven transformation of Arctic lakes in Greenland, demonstrating how a season of record warmth and precipitation caused a change in state in these systems. This shift from a “blue” to a “brown” lake state altered numerous physical, chemical, and biological lake properties. The coherent changes in lake state quantified here are unprecedented and may represent changes that can be expected in Arctic lakes more broadly as the hydrological cycle continues to intensify. (Jasmine E. Saros, Václava Hazuková, Robert M. Northington, Suzanne McGowan, more at pnas.org)