Surface Ocean Chlorophyll: Do the Green Get Greener and the Blue Bluer?

Abstract Long‐term marine ecosystem changes are uncertain, partially because detecting them is challenging. Recent evidence suggests a “green‐get‐greener‐and‐blue‐bluer” phenomenon characterizes these changes for surface ocean chlorophyll concentrations (Chl) over the 21st century, particularly in the Northern Hemisphere, driven by ocean warming. Here we treat the “green‐get‐greener‐and‐blue‐bluer” as a compelling hypothesis, and test it by investigating whether higher long‐term mean Chl is associated with more positive long‐term trends in Chl over the global ocean and within seasons and broad regions. We do not find strong evidence for this hypothesis; globally, long‐term mean Chl is weakly related to long‐term Chl trends, and the sign of this relationship varies over regions and seasons. Conversely, Chl trends are more strongly (negatively) related to sea surface temperature (SST) warming trends by more than an order of magnitude, and consistently so over regions and seasons. Hence ocean warming is associated with decreasing surface chlorophyll concentrations almost everywhere.