This paper presents a pioneering attempt to evaluate the impact of long-term seasonal wave energy trends on hydrodynamic capture by wave energy converters (WECs) over the 20th century. The ERA20c reanalysis generated by the European Centre for Medium-Range Weather Forecasts is calibrated against the ERA-Interim reanalysis via the quantile matching technique, and validated against buoy measurements across the Northeast Atlantic Ocean. The study focus is the seasonal variation of wave resources over the 20th century, so the calibration is performed using seasonally classified reanalysis and measured data. Results show that wave energy flux increased to 3 and 2 kW/m per decade in winter and spring/autumn, respectively, and that the frequency of off-limit events, defined as sea-states with significant wave height of over 5 m, has doubled over the 20th century. The impact of such wave energy trends is analysed in this paper using an oscillating wave surge converter, which shows steadily increasing power absorption over the 20th century. However, as a result of higher decadal trends and the increase in off-limit events, the hydrodynamic efficiency of the WEC, referred to as the capture width ratio, decreases up to 20%.