Long-term ocean climate effects on wave energy are often analysed from the viewpoint of the well-known increment of wave height over the decades. However, this increment associated with the increase of wave energy flux and absorbed power does not consider the influence of variations in the wave period, whose contribution is more important according to an adimensional performance analysis given by the capture width ratio. This study identifies significant past variations in wave periods during the 20th and 21st centuries using the reanalysis ERA-20C globally and at specific locations, such as Ireland, via calibration with ERA5. A more specific analysis developed in this area shows very significant performance variations (up to 20%) for two types of wave energy converters: oscillating water column devices and a floating body, in which laboratory empirical equations have been used to compute their performance loss due to the deviation from its natural resonance frequency or optimum working wave period. Thus, the performance measured as capture width ratio is highly sensitive to wave period changes, even losing productivity for regions where the wave energy potential is being incremented during the last decades.