Over the past two decades, 56% of Earth’s oceans have become greener, according to a new study by a team of scientists from the United States and the United Kingdom, published in the journal Nature on 12 July. The affected area is larger than the planet’s total land mass and scientists are still trying to understand how this is happening.
With the help from satellites, researchers managed to study the gradual and subtle color differences over the affected areas, which display greatest changes near the tropics. There, seas have become steadily greener, indicating that the ecosystem is also changing. The color of the ocean reflects a growth of living organisms in the surface waters of earthly seas.
The reason we care about this is not because we care about the color, but because the color is a reflection of the changes in the state of the ecosystem.
B. B. Cael, Ocean and Climate scientist at the National Oceanography Centre in Southampton, UK
According to thestudy, many factors can contribute to the changes in the ocean’s color. One example is when deep-sea nutrients rise up to feed on phytoplankton blooms containing green-tinted chlorophyll. Scientists can approximate the levels of chlorophyll, as well as how many organisms like algae and phytoplankton there are, by observing sunlight wavelengths reflected off the surface of the ocean.
However, in surface waters, the amount of chlorophyll can vary greatly each year, so it can be difficult to pick up differences between natural changes and those brought on by climate change. In theory, warmer ocean waters due to climate change should lead to differences in biological productivity, but scientists believe it could take as long as four decades to be able to pinpoint any clear shifts.
“These are not ultra, massive ecosystem-destroying changes, they may be subtle,” lead author of the study B. B. Cael, an ocean and climate scientist at the National Oceanography Centre in Southampton, UK, told The Guardian. “But this gives us an additional piece of evidence that human activity is likely affecting large parts of the global biosphere in a way that we haven’t been able to understand.”
In a published paper about a decade ago, co-author Stephanie Henson of the National Oceanography Center said that tracking just chlorophyll would require at least 30 years of monitoring to see any effect from climate change.
In order to find out if the changes could be caused by climate change, the scientists used the results of a simulation model that played out the possible responses of marine ecosystems to increasing levels of atmospheric greenhouse gases. They found that the changes in the model matched those of their observations.
Cael said the cause of the increased greenish hue of the ocean is likely not the warming ocean because the parts of the ocean that had changed color didn’t match those where temperatures have increased: “The distribution of nutrients could have affected the shift, as stratification of upper ocean layers occurs as surface waters warm, making it more difficult for nutrients to rise. Fewer nutrients mean smaller phytoplankton can survive better, which alters the ecosystem and could affect the overall color of the water.”
NASA’s next big mission to observe the color of the ocean will be the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite — set to launch in early 2024.
“All of this definitely confirms the need for global hyperspectral missions such as PACE,” said Ivona Cetinić, an oceanographer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who works on PACE. The satellite “should allow us to understand the ecological implications of the observed trends in ocean ecosystem structure in years to come.”
