An analysis using satellite remote sensing shows that from 2003 to 2022—about twenty years—approximately 21 percent of the global ocean (roughly 75 million km², equivalent in area to Europe + Africa + China + North America) has “darkened.

 An analysis using satellite remote sensing shows that from 2003 to 2022—about twenty years—approximately 21 percent of the global ocean (roughly 75 million km², equivalent in area to Europe + Africa + China + North America) has “darkened.” Here, “darkening” means a shoaling of the euphotic zone’s upper boundary (the depth to which enough light penetrates for photosynthesis). In about 3 percent of surveyed waters, that boundary has risen by more than 100 m, and in about 9 percent by more than 50 m. Notable hotspots include offshore of the U.S. East Coast, the Labrador Sea in the North Atlantic, and parts of the East China Sea.

The drivers differ by region. Along coasts, increased light attenuation stems from elevated inputs of nutrients (nitrogen and phosphorus), colored dissolved organic matter (CDOM) from soils and forests, and suspended sediments from agricultural runoff. Offshore, ocean warming intensifies the thermal stratification, suppressing upward nutrient flux from depth and altering plankton community structure. As a result, primary production declines, compressing energy flow through marine food webs; fish and invertebrate habitats become constrained, raising risks of shifts in fisheries distributions; and the ocean’s biological carbon pump weakens, reducing CO₂ uptake efficiency.

Urgent responses include sustained monitoring with high-resolution satellites and ocean buoys; integrated modeling and field observations to untangle coastal versus open-ocean processes; watershed management to curb terrestrial runoff; greenhouse-gas emissions reductions; and coordinated marine conservation policies. Darkening joins sea-surface warming and acidification as a key climate-change indicator, demanding international data sharing and policy alignment with coastal development and agricultural management—and immediate action.