The color of the Earth’s oceans has changed significantly over the past two decades, and these changes are a result of human-driven climate change. They cannot be explained by natural variation alone and they affect more than 56 percent of our planet’s oceans, an area larger than the entire land area of our planet.
The color of the Earth’s oceans is a reflection of the organisms and minerals within its waters. This means that although these color variations may appear subtle to the human eye, they definitely indicate that marine ecosystems are in flux. Although the specific changes that are occurring in these ecosystems are not yet completely clear, the team behind the results is certain human activity and the effects on the climate that are causing it.
“I’ve been running simulations that tell me for years that these changes in ocean color are happening,” Stephanie Dutkiewicz, study co-author and senior research scientist from the Massachusetts Institute of Technology Department of Earth, Atmospheric and Planetary Sciences and the Center for Global Change Science, said in a statement. “It’s really not surprising to see it happening, but it’s alarming. And these changes are consistent with human-caused changes in our climate.”
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How does the ocean get its color?
The color of the ocean can be used as a measure of what lives in its upper layers. Deep blue waters, for example, indicate a lack of life while green waters indicate the presence of plant-like microbes called phytoplankton that contain green pigment chlorophyll.
Phytoplankton capture sunlight and use carbon dioxide to create sugars through photosynthesis, which forms the basis of the oceanic food web. They are fed by small creatures, such as krill, which feed on larger fish, which feed on seabirds and marine mammals.
Apart from feeding the oceans, this process of photosynthesis also means that phytoplankton are vital in capturing and storing carbon dioxide from the atmosphere. Because carbon dioxide is a major greenhouse gas, scientists closely monitor phytoplankton over ocean surfaces to see how colonies of this microorganism respond to climate change.
This is traditionally done by monitoring that green pigment, chlorophyll, which helps phytoplankton and plants harvest sunlight. Chlorophyll changes can be seen in the ratio of blue light to green light reflected on the ocean surface, a balance that can be tracked by space-borne satellites.
However, it would take about 30 years, experts say, to determine a trend based on climate change through changes in chlorophyll alone. So in 2019, scientists decided they could cut a chunk of time off that figure to find signs of climate change by monitoring just 20 years by tracking smaller changes in the ocean’s other colors.
“I thought, doesn’t it make sense to look for a trend in all these other colors, rather than just chlorophyll?” BB Cael, the lead author of the research and a scientist at the National Oceanographic Centre, said. “It’s worth looking at the whole spectrum rather than trying to estimate one number from bits of the spectrum.
“This provides further evidence of how human activities are affecting life on Earth over vast spatial distances.”
Looking at the color of the sea from space
To reach their current findings, Cael and co-researchers analyzed ocean color measurements collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Aqua satellite.
For 21 years, MODIS has been observing the oceans in seven wavelengths of light, including two wavelengths used to track chlorophyll in particular. The instrument sees the ocean as a mixture of subtle wavelengths, from blue to green and even red, in contrast to the remote azure hue that our eyes have changed. This means he can see changes far too subtle to be seen by human vision alone.
Cael assessed the seven ocean colors measured by MODIS between 2002 and 2022, looking at how they changed in individual regions over the year to get an idea of natural variability.
Zooming out on this year’s data allowed the researcher to see how changes had progressed over a total of 20 years. This showed Cael a clear trend that only existed in annual variation. To determine if the trend was a result of climate change, Cael compared it to two ocean-color models. One considers adding greenhouse gases and the other does not.
Ultimately, the satellite data matched the greenhouse gas model’s prediction of a 20-year trend in ocean color changes in about half of the world’s surface oceans. This showed that the trend observed in MODIS was more than random variation, and suggested a new and faster way to detect changes in marine ecosystems affected by climate change.
“The color of the oceans has changed, and we can’t say how. But we can say that changes in color indicate changes in plankton communities that will affect everything that feeds on plankton,” said Dutkiewicz. “It will also change the amount of carbon that the ocean will take up because different types of plankton have different abilities to do that.
“So we hope people take this seriously. These models are not just predicting these changes will happen. We can see it happening now, and the ocean is changing.”
The team’s research was published this month in the journal Nature.