Abstract: We used subfossil mosses and peats to document changes in regional climate, cryosphere, and terrestrial ecosystems in the western Antarctic Peninsula at ~65S latitude. We find that most peat-forming ecosystems have initiated since 2800 cal BP, in response to warmer summers and increasing summer insolation. The period at 900-600 cal BP was coldest as indicated by ice advance, abundance of kill ages from ice-entombed mosses exposed recently from retreating glacial ice, and apparent gap in peatbank initiation. Furthermore, the discovery of a novel Antarctic hairgrass (Deschampsia antarctica) peatland at 2300-1200 cal BP from the mainland Antarctic Peninsula suggests a much warmer climate than the present. A warming and wetting climate in the 1980s caused very high carbon accumulation in a Polytrichum strictum moss peatbank. Our results document dramatic transformations of landscape and ecosystems in response to past warmer climate, providing a telltale sign for what may come in the future.