Scientists warn that the ocean is growing greener at poles

(CN) — The ocean is turning greener at the poles and bluer at the equator, and researchers say the small but mighty change in color spells danger, and fisheries will likely take the hit.

A tracked increase in vegetation cover, also known as greening, has been consistently recorded on land by scientists since the 1990s — meaning the average leaf cover across the planet’s surface has grown steadily, likely due to rising temperatures.

Using satellite images, the phenomenon has also been seen in the ocean, but inconsistent data on chlorophyll production thanks to the ocean’s sheer depth means studying greening rates sea-side is harder.

From 2003 to 2022, NASA satellite MODIS-Aqua traveled the entire Earth every two days, measuring light wavelength and gathering data nonstop.

Researchers at Duke University in Durham, North Carolina, conducted an ocean greening analysis, published June 19 in the journal Science, on the data in search of changes in the amount of chlorophyll, which signals the presence of aquatic ecosystem fundamental phytoplankton.

To finally record consistent rates of greening, the satellite data had to be narrowed down to an analysis of only open ocean, excluding coastal waters, explained Haipeng Zhao, a postdoctoral researcher at Duke University’s Cassar Lab and the study’s first author. Coastal waters have more suspended sediments, said Zhao, “so optical properties are different than in the open ocean.”

Zhao led the team of researchers in discovering that the planet has slowly grown less green and more blue near the equator, particularly in subtropical and tropical regions, while the planet’s poles have only experienced more greening.

Supported by the U.S. National Science Foundation, the research was inspired by the figures in economic concepts related to wealth distribution, utilizing similar curves to examine the change of chlorophyll concentration at different latitudes.

“It’s like rich people getting richer and the poor getting poorer,” Zhao said.

The researchers considered how the rates of greening were affected by variables like sea surface temperature, wind speed, light availability and depth. Over a two-decade period, a drastic shift was observed in the satellite measurements — polar regions were greening, particularly the north pole, while tropics and subtropics were doing just the opposite.

Global carbon cycles are likely to be affected by the “poleward shifts in phytoplankton,” researchers say in the study. The organisms, which absorb carbon dioxide from the atmosphere during photosynthesis, could cause an increased carbon store at the bottom of the ocean and possible climate warming in turn.

The cause of the greening? The authors can’t guarantee it’s climate change yet.

“The study period was too short to rule out the influence of recurring climate phenomena such as El Niño,” Susan Lozier, dean of the College of Sciences and Betsy Middleton and John Sutherland Chair at Georgia Tech, said in a statement. “Having measurements for the next several decades will be important for determining influences beyond climate oscillations.”

Either way, fisheries will likely see most of the repercussions for this global ecological shift. As phytoplankton populations decline and marine food webs change, a major fluctuation could occur in coming decades for fisheries located in equatorial regions like the Pacific Islands.

“Phytoplankton are at the base of the marine food chain,” said Nicolas Cassar, the Lee Hill Snowdon Bass Chair at Duke University’s Nicholas School of the Environment in a statement. “If they are reduced, then the upper levels of the food chain could also be impacted, which could mean a potential redistribution of fisheries.”