You're Not Imagining It: The Ocean Has Changed Color Over 20 Years, Study Determines

Climate change is altering the color of the oceans, making them greener over time, according to a new study that analyzed 20 years of specialized satellite observations. 

The discovery offers the first clear evidence that human-driven climate change is shifting the color palette of the seas by disrupting tiny marine organisms called phytoplankton. These long-term changes in color can open a new window into the health of the ocean ecosystems that could inform marine conservation and governance in an age of rapid global warming.  

Anthropogenic climate change, which is caused by humanity’s consumption of fossil fuels, is already affecting Earth’s oceans in countless ways. Sea levels are rising across the globe, hurricanes are becoming more frequent and intense, polar sea ice is vanishing at an astonishing pace, and many marine organisms are seeking refuge outside their normal geographic ranges. 

Scientists have long predicted that climate change will also transform the color of the ocean by disturbing phytoplankton that live near the surface and form the foundation of the marine food web. As photosynthetic organisms, phytoplankton contain the pigment chlorophyll, which reflects green light, causing the ocean to be generally greener in places that support large populations of these marine microbes. 

Ocean-observing satellites can see the shifts in marine chlorophyll, but snagging a clear signal of anthropogenic climate change from this colorful noise is difficult due to the large variations in phytoplankton activity from year to year. Previous research has suggested that several decades of chlorophyll observations from space would be required to pick out signs of climate change in ocean color.

Now, a team led by B.B. Cael, principal scientist at the National Oceanography Centre, Southampton, has discovered this long-sought climate signal way ahead of schedule by analyzing unique observations of so-called “remote-sensing reflectance” from NASA’s Aqua satellite, which was launched in 2002. 

Aqua carries an instrument called the Moderate Resolution Imaging Spectroradiometer (MODIS) that can monitor seven different colors, rather than just the chlorophyll hues, allowing it to detect “shifts in ocean color—and, by extension, in surface-ocean ecosystems—that are driven by climate change,” according to a study published on Wednesday in Nature.  

After reviewing research on the topic, Cael told Motherboard he “realized that MODIS-Aqua was just about to have its 20th birthday, knew that NASA was thinking about shutting Aqua down, and thought ‘well there has to be more ‘signal’ in the full color spectrum than the one number (chlorophyll) that’s often estimated from it.”. 

“There is less ‘noise’ in the individual colors than in that number, & we’re probably never going to have another chance at a 20-year single-satellite record, so if we are ever going to see a climate change trend in ocean satellite data, it would have to be now—let’s see!” he said in an email.

To that end, Cael and his colleagues examined MODIS-Aqua multi-spectral observations of the global oceans from July 2002 to June 2022. The results revealed significant changes in color across 56 percent of the ocean over that time, mostly in tropical regions within 40° of the equator, that could not be explained by natural annual variations in phytoplankton production. 

The team concluded that the “results suggest that the effects of climate change are already being felt in surface marine microbial ecosystems, but have not yet been detected,” according to the study. 

“This was a rare example (in my experience at least!) of hypothesizing that you would see something and then seeing it,” Cael said. “Where we see trends is where the year-to-year change in color is low. We detect a trend wherever that’s the case. That’s what I was expecting (& hoping) to see.”

In other words, the team has exposed a new polychromatic tracer of human-driven climate change in the oceans that could yield valuable insights about the effects of warming global temperatures on marine ecosystems. But while modern human lifestyles are evidently making the oceans greener, the exact mechanisms behind this shift remain a mystery.

For instance, Cael and his colleagues investigated whether the color shifts were directly connected to rising surface temperatures, and found no link. The researchers could also not easily explain the observed trends with shifts in light patterns at the ocean surface or differences in the populations of predators that consume phytoplankton. 

One possible driver of the color changes, according to Cael, is a climate-fueled stratification of nutrients in upper ocean waters, which could cut off sources of nourishment to these tiny lifeforms. 

“More stratification means more difference in density of surface waters (where the light is) versus deep waters (where the nutrients are), which means less mixing of surface and deep waters, which means less nutrients getting to the plankton,” Cael explained. “This would potentially lead to a shift to smaller plankton, which live better off of low-nutrient conditions. This could be related to the color changes we see, though it’s hard to say for sure.” 

“Different plankton scatter and absorb light differently, changing the optical properties of the water and therefore the reflectance (‘ocean color’),” he continued. “So the reflectance changing means that the ecosystem is changing—even if it’s hard to say exactly how with the current state of our knowledge about plankton ecosystems. These color changes may mean a shift to smaller or bigger plankton, more or less predators or prey, different types of plankton that affect carbon storage or fisheries differently, among other things.”

Phytoplankton are not only the basic fuel that supports aquatic food webs around the world, they also play an essential role to all life on Earth as a global carbon sink and a producer of atmospheric oxygen. Given that these small creatures have such an outsized effect on the biosphere, monitoring their health and activities is critical to anticipating our future in a warming world. 

Fortunately, NASA plans to launch the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission, the first global ocean “hyperspectral” satellite, in January 2024. PACE will be able to see hundreds of subtle colors on the sea surface and “has the potential to revolutionize how much we can interpret how changes in color reflect changes in ecosystem state,” according to Cael. 

“The big question now is how we can use the data we have better, or use these new data from PACE, to match changes in ocean color to changes in the ocean ecosystem,” he noted. “The other big question is to investigate whether we see similar trends in longer records of ocean color that are stitched together from multiple satellites. We used the longest single-satellite record there is, because there are lots of mathematical issues with detecting trends in records that are composed by stitching data from multiple satellites together, but how to best stitch multiple satellites’ data together to deal with these problems is an active area of research.” 

“There are good multi-satellite records out there that might have trends in them; maybe the whole ocean’s colors are changing,” Cael concluded.