More than one-fifth of the global ocean – an area spanning more than 75 million square kilometres – has been the subject of ‘ocean darkening’ over the past two decades, a process researchers suggest could affect huge numbers of marine species and ecosystems. Ocean darkening occurs when changes in the optical properties of the ocean reduce the depth of its photic zones, home to 90% of all marine life and places where sunlight and moonlight drive ecological interactions. Conducted by researchers from the University of Plymouth and Plymouth Marine Labs, a new study has now combined satellite data and numerical modelling to analyse the annual changes in the depth of photic zones all over the planet. It’s found that between 2003 and 2022, 21% of the global ocean – including large expanses of both coastal regions and the open ocean – had become darker.