Our planet’s oceans are becoming less green, a concerning change linked to global heating. A recent study reveals that this shift is due to a decrease in phytoplankton, tiny marine organisms vital for life on Earth. Phytoplankton are responsible for almost half of the planet’s biological productivity, and their decline has serious implications for oxygen levels and marine food chains.
The study analysed daily chlorophyll concentrations in oceans from 2001 to 2023. Chlorophyll is the green pigment that allows plants, algae, and phytoplankton to perform photosynthesis. This process converts sunlight, water, and carbon dioxide into oxygen and essential sugars.
Using advanced deep-learning technology and data from satellites and monitoring ships, researchers found a significant yearly drop in ocean greenness. This reduction averaged about 0.35 micrograms per cubic metre. The effect was even more pronounced in coastal areas and near river mouths, where the decline was higher.
This change suggests a weakening of the ocean’s ecological functions. The study estimates an annual decrease of 0.088% in the ocean’s ability to absorb carbon dioxide. This is equivalent to about 32 million tons of carbon dioxide per year.
“The decline in surface phytoplankton’s carbon sequestration capacity has profound implications for the carbon cycle,” stated Dr. Di Long from Tsinghua University, one of the study’s authors. “This is the first study to robustly demonstrate that, yes, we can confidently conclude that we are seeing a decrease in the greenness of the ocean, indicating a lowering of marine productivity, constituting yet another threat to humanity associated with fossil fuel burning and human-caused planetary warming,” added co-author Professor Michael Mann from the University of Pennsylvania.
Researchers believe rising ocean temperatures are the primary cause. As the surface waters warm, they become more distinct from the colder, deeper waters. This warmer surface layer acts like a barrier, preventing the upward movement of nutrients from the deep ocean. Phytoplankton rely on these nutrients to thrive, so their reduced availability leads to a decrease in phytoplankton populations.
This finding contradicts some earlier studies that suggested increasing algal blooms. However, the new research highlights that those previous studies were less comprehensive. While regional variations exist, influenced by factors like agricultural runoffs and pollution, the overall trend of declining phytoplankton in low and mid-latitude oceans is clear.
“These changes will profoundly affect the magnitude and distribution of marine ecosystem functioning,” the paper warns. The authors advise policymakers to carefully examine marine environments, especially in coastal zones. They recommend developing strategies to manage agricultural fertiliser use, sewage discharge, deforestation, and water pollution.
However, the biggest challenge remains addressing the climate crisis itself. As the ocean, our largest natural carbon absorber, weakens, the pressure to reduce global emissions increases. The study raises concerns about the effectiveness of current climate agreements.
“Based on the findings, we have concerns about future global emissions reduction. The decline in ocean carbon sequestration capacity means that we may face greater emissions reduction pressure than expected,” Dr. Long concluded. This research underscores the urgent need for global action to combat climate change and protect vital ocean ecosystems.