Darkening of the global ocean

The photic zones of the oceans – where sunlight and moonlight drive ecological interactions - are one of the most productive habitats on the planet, and fundamental to the maintenance of healthy global biogeochemical cycles. Ocean darkening occurs when changes in the optical properties of the oceans reduces the depth to which sufficient light penetrates to facilitate biological processes guided by sunlight and moonlight. We analysed a 9km resolution annual time series of MODIS Aqua’s diffuse attenuation coefficient of light at 490nm [Kd(490)] to quantify whether the oceans have darkened over the last twenty years, and the impact of this on the depth of photic zones around the world. Kd(490) increased across 75,341,181 km2 (21%) of the global ocean between 2003 and 2022, resulting in photic zone depths reducing by more than 50m across 32,449,129 km2, (9%) by area. The depth of the photic zone has reduced by more than 10% across 32,446,942 km2 (9%) of the global ocean. Our analysis indicates that ocean darkening is not restricted to coastal regions, but affecting large swathes of the open ocean. A combination of nutrient, organic material and sediment loading near the coasts, and changes in global ocean circulation are probable causes of increases in primary and secondary productivity that have reduced light penetration into surface waters. The implications of ocean darkening for marine ecology, and the ecosystem services provided by the surface oceans are currently unknown, but likely to be severe.