The oceans are changing colour: Greener at the poles, darker at the equator

A subtle yet significant transformation is underway in the waters of our planet’s oceans, according to a new scientific study: their colours are shifting, a consequence of climate change and global warming.

This trend sees waters near the poles deepening to a rich green, while those near the equator intensify to a vibrant blue. This shift is tied to the warming of our seas and, if it persists, could have substantial effects on both ecosystems and human activities.

The research, published in the journal Science, utilised satellite data from a NASA instrument that scans the Earth every two days, capturing the spectrum of light within the surface layers of the oceans. The team of scientists analysed measurements taken between 2003 and 2022, with a focus on detecting chlorophyll – the green pigment produced by phytoplankton.

“Warming waters are causing the ocean’s colours to alter,” explained Haipeng Zhao, a postdoctoral researcher and lead author of the study, in an interview with ABC News. As he further clarified, this change mirrors fluctuations in the concentration of phytoplankton, a photosynthetic marine organism fundamental to the marine food chain.

“As a type of algae, phytoplankton contain photosynthetic pigments that reflect green light, giving the waters a greenish hue,” added Susan Lozier, dean of the College of Science at Georgia Institute of Technology and co-author of the study. “In areas where phytoplankton is absent, the water appears blue.”

What’s behind the change in ocean colour?

The geographical distribution of these changes reveals a dual trend: subtropical latitudes are experiencing a loss of chlorophyll, while polar zones – especially in the northern hemisphere – are becoming greener. As stated in a press release by Duke University, “green areas are becoming greener, and blue areas are becoming even bluer.”

The researchers employed tools from the economic theory of inequality to comprehend the distribution of chlorophyll in the ocean. “We borrowed concepts from economics such as the Lorenz curve and the Gini index, which together illustrate how wealth is distributed in a society,” explained Nicolas Cassar, lead professor at Duke’s Nicholas School of the Environment. “We thought we could apply them to see if the proportion of the ocean containing the most chlorophyll has changed over time.”

In the course of their analysis, factors such as sea surface temperature, wind intensity, light availability, and the depth of mixed layers were taken into consideration.

Among all the factors examined, only the warming of water seemed to consistently correlate with changes in chlorophyll concentration.

However, the researchers underscored that the limited duration of observations prevents a definitive link between these changes and human-induced climate change. “Satellite technology is relatively new, and we haven’t been observing the oceans for decades,” Lozier pointed out, highlighting the limitations of the methodology.

Zhao, whose doctoral research concentrated on regional studies in high-latitude oceans, expressed his motivation to delve deeper and explore whether the oceans are undergoing color changes on a global scale.

“The ocean is warming,” Lozier observed. “The biological implications of this warming pose a significant question.”

While an increase in terrestrial greenness has been documented since the 1990s, largely attributed to enhanced photosynthetic activity from rising temperatures, identifying similar trends in the ocean is challenging. Satellite imagery only offers surface data on chlorophyll production, leaving many gaps in the overall understanding.

Scientists postulate that if this trend persists, it could have repercussions on marine ecosystems. Given that phytoplankton forms the base of the food chain, a shift in its concentration could influence the distribution of fish and, consequently, fishing activities.

“A sustained decline in phytoplankton near the equator could trigger a redistribution of fishing grounds,” the authors of the study suggested. This could especially impact low- and middle-income countries, like Pacific island nations, which depend on commercial fishing for food and economic growth.

Information sourced from ABC News

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