Tropical oceans are changing color, according to new research. Over the past 2 decades, 56% of ocean surfaces have become greener, and that means microorganisms living close to the surface are changing as well. The study, recently published in Nature, points to climate change as a possible cause for the shift.
The world’s oceans get their color from sunlight bouncing off water molecules and whatever else is floating near the surface. That includes tiny phytoplankton, which contain abundant chlorophyll—a pigment that reflects green light.
Scientists typically use satellites to evaluate ocean color by measuring blue and green wavelengths of light reflected from the surface and determining the amount of chlorophyll needed to make the specific blue-green hue, explained coauthor Stephanie Dutkiewicz, a marine scientist at the Massachusetts Institute of Technology in Cambridge.
But phytoplankton—and thus chlorophyll—abundance varies significantly from year to year, and detecting long-term changes in color from such noisy satellite data is difficult. Climate models indicate that it could take 30 to 40 years of data to pull out a trend, Dutkiewicz said.
To more quickly reveal any patterns in ocean color, the researchers tried an unusual approach: With 20 years (2002–2022) of near-daily ocean imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS), a camera aboard NASA’s Aqua satellite, the team looked at multiple colors within the spectrum of visible light, rather than just blue and green.
Cleaning the Noise
“It is difficult to differentiate noise from signal, as every observation is full of noise,” said coauthor Emmanuel Boss, who leads the Maine In-situ Sound & Color Lab at the University of Maine. To see beyond seasonal variabilities and El Niño, the team used a statistical technique in which they could analyze several variables—all the colors observed by MODIS—at once.
The team found that in more than half the oceans, mainly close to the tropics, there was a clear shift in hue toward green, even with only 20 years of available data.
Because the oceans near the poles are not exposed year-round and darkness lasts for months, the researchers focused on the equatorial oceans. “But it doesn’t mean the poles are not changing,” Boss added. They also didn’t look near coasts; these areas are naturally quite noisy because of recurrent phenomena such as upwelling—when cold, deep, and nutrient-rich water rises to the surface and takes the place of warmer surface water.
Shifts in Marine Organisms
“We don’t know what is causing the trend. It could be dissolved organic material, changes in the type and quantity of phytoplankton…all these aspects can affect ocean color,” Boss said. “We’re hoping more colleagues will try and find what has been causing these changes.” The authors noted that the color changes didn’t follow changes in sea surface temperature.
Climate change might be to blame, according to the authors. That hypothesis resulted not only from what they found in the data set but also from the fact that the data matched best one of the models the team was working with—one with high carbon emissions and a visible climate change scenario.
The study is relevant because it sheds light on changes in phytoplankton populations, which sit at the base of many marine food webs, said Luciana Prado, a physical oceanographer at Rio de Janeiro State University in Brazil who was not involved in the study. Primary consumers ranging from zooplankton to jellies consume phytoplankton, whereas secondary consumers, including many fish, feed on them. (Finally, top consumers like sharks and marine mammals frequently feed on fish.)
“The change in water color could mean there might be a new distribution of elements and nutrients that impact the quantity and quality of phytoplankton in the oceans,” Prado added.
“This is a sophisticated, unconventional new approach for looking at ocean color,” said Aurea Ciotti, a bio-optics oceanographer at São Paulo University who was not involved in the study. This kind of study is very challenging, she said, because it works with vastly different scales. “These studies work with huge swaths of ocean area and look at microscopic organisms that live for hours or a few days. It is a great challenge to calculate that without oversimplifying how oceans work. The good news is nowadays we have technology that allows for more accurate calculations, so that we don’t need to oversimplify data that much,” she observed.
The study resulted in a somewhat counterintuitive finding that the scientific community will need some time to digest, Ciotti said. “With climate change, it would make total sense if the oceans became bluer, or poorer in life and nutrients. The use of other techniques opens new doors for us to understand that reality might not always fit our expectations.”
—Meghie Rodrigues (@meghier), Science Writer