Color of Earth‘s oceans has changed significantly in the last two decades. These changes are a result of human-caused climate change and cannot be solely explained by natural variability. They have impacted over 56% of our planet’s oceans, an expanse larger than the total area of land.
Color of Earth’s oceans reflects the organisms and minerals present in its waters. While these color variations might appear imperceptible to the human eye, they undoubtedly signify evolving marine ecosystems. The specific changes occurring in these ecosystems are not yet fully clear.
“I’ve run simulations that have been telling me for years that these ocean color changes would occur” stated Stephanie Dutkiewicz, co-author of the study published in Nature (ref.). “To see it actually happening is not surprising, but frightening. These changes are consistent with human-induced changes to our climate.”
What gives the ocean its color?
The color of the oceans can be influenced by what is present in their upper layers. Intense blue waters, for example, indicate a lack of life, while green waters indicate the presence of plant-like microorganisms called phytoplankton that contain the green pigment chlorophyll.
Phytoplankton gather sunlight and use carbon dioxide (CO2) to create sugars through photosynthesis, forming the foundation of the oceanic food chain. Phytoplankton nourish small creatures like krill, which in turn feed larger fish, feeding marine birds and mammals.
Beyond sustaining ocean life, the photosynthesis process of phytoplankton is vital for capturing and storing carbon dioxide from the atmosphere. Since carbon dioxide is a major greenhouse gas, scientists closely monitor phytoplankton on ocean surfaces to see how colonies of this microorganism respond to climate changes. The change in ocean color can be evaluated by observing the ratio of reflected blue to green light a balance that can only be monitored using space satellites.
The MODIS satellite
Determining a climate change trend solely through changes in ocean chlorophyll would require about 30 years of study, experts say. Therefore, in 2019, scientists established that by monitoring even small variations in ocean chlorophyll, they could reduce the time to detect signals to just 20 years of monitoring.
“I thought, why look for a trend in all these other colors rather than just chlorophyll?” said BB Cael, lead author of the research. “Is it worth looking at the whole spectrum rather than trying to estimate a number from parts of the spectrum? This provides further evidence of how human activities are influencing life on Earth over a vast spatial extent”.
To reach their current findings, Cael and fellow researchers analyzed ocean color measurements collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite.
Analysis of Colors from Space
For 21 years, MODIS has observed the oceans at seven wavelengths of light, including two specific wavelengths used to track chlorophyll. This means it can detect changes invisible to the human eye. Cael evaluated the seven oceanic colors measured by MODIS between 2002 and 2022, observing how they shifted in individual regions each year to get an idea of the natural variation.
Zooming back on this data year by year then allowed the researcher to see how the changes progressed over a total of 20 years. This revealed a clear trend that wasn’t present in simple yearly variability. To determine if the trend was the result of climate change, Cael compared it with two models. One considered the addition of greenhouse gases and the other did not.
Satellite data aligns with the greenhouse gas model’s prediction. This demonstrates that the trend observed in MODIS was more than random variation. It also suggests a new and rapid way to detect disturbances caused by climate change in marine ecosystems. “The color of the oceans has changed and we cannot say how. But we can say that color changes reflect changes in plankton communities that will impact everything that feeds on plankton,” said Dutkiewicz.”