Deuterium-excess ( d-excess ) in Antarctic ice-cores has long been used to reconstruct glacial–interglacial changes in moisture-source temperature (ΔT source ) or relative-humidity. Yet temperature, relative-humidity, and wind-speed jointly and inseparably influence d-excess , an ambiguity that has undermined single-variable readings. Here we resolve it by reframing d-excess as a proxy for the oceanic evaporation rate (E): all three controls drive both evaporation and its d-excess in the same direction. We calibrate the modern spatial E– d-excess relationship against reanalysis and satellite evaporation products (R² = 0.83–0.97) and apply it to the EPICA Dome C record to produce the first continuous 800,000-year history of moisture-source evaporation. Reconstructed E couples tightly with ΔT source during cold glacial periods (≤ −3.5 °C; R² = 0.57) but decouples through the transitional (−3.5 to −1 °C; R² = 0.10) and warm (≥ −1 °C; R² = 0.23) regimes, where wind, proxied by EDC dust-flux, becomes the dominant control. Jointly, source-temperature and wind explain 81% of evaporation variance across 800 kyr, against 55% for temperature alone. The control on evaporation shifts from thermodynamic to dynamic as climate warms, implying that the response of the water cycle to future warming is governed by atmospheric-circulation as much as by temperature.