Gaby
Over the past two decades, satellite images show the Earth’s oceans changing color, and researchers say climate change is the likely cause. U.S. and U.K. scientists analyzed images from this period and reached a troubling conclusion: the world’s waters are becoming greener, a change that likely reflects shifts in phytoplankton populations driven by global warming. The shift in ocean color is substantial. The research team emphasized this is not merely natural year-to-year fluctuation. They found that these color changes, though imperceptible to the human eye, have already affected more than half of the world’s oceans — an area larger than Earth’s total landmass.
Researchers explain that ocean color reflects the organisms and materials in the water. So a change in color signals that surface ecosystems are also changing. Scientists cannot yet precisely determine how marine ecosystems are responding to these color shifts, but they strongly suspect human-caused climate change is the main driver.
Color Change Linked to Phytoplankton
According to the researchers, the greening of the oceans is being driven in part by increases in phytoplankton tied to warming. These tiny microbes, including plant-like algae, use green chlorophyll for photosynthesis, so more phytoplankton makes surface waters appear greener. While the green hue might look harmless, a rising phytoplankton population can cause a range of ecological consequences. Short-term effects are already being observed: sudden population booms can deplete oxygen in surrounding waters, creating hypoxic dead zones that many marine species cannot escape.
ScienceAlert notes that the long-term consequences of changing ocean color are not yet well studied. It remains unclear how much data is needed to detect these shifts reliably; preliminary estimates suggest about three decades of observations are required to identify changes in ocean ecosystems. Studying 20 years of satellite data is enough to show that climate change demands a faster response, and processing those images is somewhat easier than measuring phytoplankton directly through chlorophyll sampling. Researchers also acknowledge that phytoplankton is not the only cause of greening. Still, they say most changes in ocean ecosystems are interconnected with climate change and need to be addressed promptly.
Phytoplankton: Friend or Foe?
The greening is particularly noticeable around the equator, researchers say. Because phytoplankton absorb CO2, their increase can act as a carbon sink, which complicates the picture: more phytoplankton could help remove carbon dioxide from the atmosphere. At the same time, phytoplankton can strongly alter local conditions, changing temperature, nutrient availability, and light penetration in the water. Phytoplankton also form the base of the marine food web, so their increase could lead to widespread, significant changes to resources such as conservation areas and fisheries.
The study does not explore all of these consequences in depth. But regardless of the exact meaning of the greening, it has been occurring for at least the past ten years. Overall, the results indicate that the impacts of climate change are already being felt in surface marine microbial ecosystems, even if those impacts are not yet fully identified. Monitoring phytoplankton at the ocean surface and understanding how it responds to climate change is therefore a priority for scientists.
