The typical color of the sea surface can vary depending on the water quality, subsoil and incidence of light, and the density of algae also plays a role. Now a study proves that the earth's oceans are getting greener and greener. The subtle color changes detected by satellite imagery and statistical analysis suggest that sea-surface ecosystems are changing, likely as a result of human-induced global warming. So far it is unclear how this change will affect the marine food webs and the ability of the oceans to bind CO2.
The color of the ocean provides information about what is happening in its upper layers. Deep blue waters indicate a few microorganisms, while a greenish color mostly comes from phytoplankton. These tiny, single-celled algae carry out photosynthesis with the help of the green pigment chlorophyll, not only absorbing large amounts of CO2 from the atmosphere, but also providing the basis for the marine food web. However, shifts in the type and number of plankton communities can unbalance ecosystems. Color changes can indicate these shifts. However, since the chlorophyll content and thus the green color of the ocean is subject to strong seasonal fluctuations, long-term trends have so far been difficult to record.
Ocean color satellite data
A team led by BB Cael from the National Oceanography Center in Southampton has now used satellite data to show that the color of the oceans has actually changed over the past 20 years. The scientists used images of the world's oceans that NASA's Aqua research satellite took between July 2002 and June 2022. The satellite captured seven wavelengths of light reflected from the water's surface, making it possible to detect subtle changes that are invisible to the human eye.
Previous studies had usually focused on the ratio of green to blue light, as this provides the best information about the chlorophyll content and thus the phytoplankton concentration in seawater. But while short-term fluctuations can be mapped well, long-term trends remain hidden because the chlorophyll content is subject to strong seasonal fluctuations. "So I wondered if it wouldn't make more sense to look for a trend in all the other colors than just chlorophyll," says Cael. "It pays to look at the whole spectrum."
Subtle shift to green
In this way, the researchers found significant trends for 56 percent of the total ocean area - an area larger than the entire land area of the earth. According to the data, the color of these sea areas has subtly changed over the past 20 years. In particular, the tropical ocean regions near the equator have become greener and greener over time. "On the other hand, there are significant trends in chlorophyll for only twelve percent of the ocean area," reports the team.
The results agree with model calculations that had predicted corresponding changes in the color spectrum. "I ran simulations for years that told me these changes in ocean color are going to happen," says co-author Stephanie Dutkiewicz of the Massachusetts Institute of Technology in Cambridge. "To see it actually happening isn't surprising, but it's terrifying. And these changes are consistent with human-caused changes in our climate.”
effects so far unclear
However, the color changes were not directly associated with the surface temperature of the water. "So they are probably due to other influencing factors, including the thickness and mixing of the different water layers," the researchers write. "These factors are known to affect the structure and biomass of the plankton community, and are expected to change with climate." However, since these parameters are measured less accurately and continuously than surface temperature, it is more difficult for them to identify trends over the relatively short period of 20 years.
What exactly the color changes mean for the ecosystems in the oceans and how this affects the ability of the oceans to store CO2 must now be clarified by future studies. "So far we can't say exactly what caused the color changes," says Dutkiewicz. "But we can say that the color changes reflect corresponding changes in plankton communities that will affect anything that feeds on plankton." How much carbon the ocean will absorb will also change, because different plankton species have different abilities to do so. So we hope people take this seriously.”
Source: BB Caen (National Oceanography Centre, Southampton, UK) et al., Nature, doi: 10.1038/s41586-023-06321-z