The North Sea became much less clear during the 20th century, with potential consequences for primary production. This study analyses the hypothesis that changes in wave regime were a key driver of this change. We hindcast bed shear stress over the 20th century using a long-term wave reanalysis. Shear stress increased by over 20 % in large parts of the southern and central North Sea during the 20th century. An increase of this magnitude would have caused a large decline in water clarity.