How changing ocean colors could impact California

San Francisco Chronicle​

19-06-2025

Earth's oceans have been getting greener at the poles and becoming bluer closer to the equator, according to a study published Thursday in Science. The shift reflects changes in marine ecosystems, which experts say could affect fish populations and create problems for fisheries, including in California.
'It has lots of potential implications for the way we use the ocean,' said Raphael Kudela, a professor of ocean sciences at UC Santa Cruz, who wasn't part of the new study.
The scientists analyzed satellite data from 2003 to 2022 to track ocean concentrations of chlorophyll, a green pigment that phytoplankton use to absorb sunlight and produce sugars. While phytoplankton are often associated with harmful algal blooms, they are also the base of the marine food web, serving as food for fish and other sea creatures.
Ocean regions with the highest concentrations of chlorophyll, or the greenest areas, were at higher latitudes, toward the poles. Mid-latitudes had relatively low levels of chlorophyll, or were more blue.
The authors tracked global shifts in chlorophyll concentrations, a proxy for phytoplankton populations, over recent decades. The authors associated the poleward 'greening' with increasing sea-surface temperatures.
'This uneven distribution of chlorophyll (has been) intensifying over the past 20 years,' said lead author Haipeng Zhao, a postdoctoral researcher at the Georgia Institute of Technology. Zhao performed the research while at Duke University.
The scientists quantified the trend using the Gini index, a measure typically used for studying wealth disparities. Kudela, of UC Santa Cruz, described the approach as 'a very elegant way' to address the question of whether ocean waters have been getting greener toward the poles.
The new study analyzed open ocean waters; researchers didn't directly address coastal regions, like the Pacific Ocean waters offshore of California. Sediment in shallow coastal waters complicates satellite data: 'We don't think there's an effective algorithm that can accurately gather the phytoplankton concentrations in those coastal regions,' Zhao said. Coastal waters and the open ocean also experience distinct physical processes, Zhao explained.
Kudela expects that the poleward shift in chlorophyll and phytoplankton described by the authors extends to the California coast, though the data could be noisier. Much of California lines up with latitudes associated with ocean waters that have gotten bluer over recent decades. Latitudes north of roughly Humboldt Bay have gotten greener.
Scientists have already observed changes in marine environments: 'We're seeing organisms in California and Oregon and Washington moving northward because they're basically trying to follow their preferred temperature,' said Kudela, who authored a perspective accompanying the new study.
Kudela noted in the perspective that the new results contrast with those of a 2023 study, which reported that oceans have become greener at low latitudes in recent decades and that the trend wasn't associated with sea-surface temperatures. That analysis, however, used a different type of satellite observation.
El Niño conditions, associated with warmer-than-average sea surface temperatures, have provided short glimpses into what California waters could be like if global oceans warm, Kudela said.
'We oftentimes see the peak anchovy abundance shifts from Central California (and) Monterey up into Northern California,' Kudela said.
Yellowfin tuna and dorado, normally found off Mexico, are more common off Southern California during El Niño years.
The authors write that additional decades of satellite data are needed to determine whether poleward greening is a product of natural variability or driven by climate change.