The oceans have changed color significantly in the last 20 years and will continue this trend in the medium and long term, most likely as a consequence of human-induced climate change, according to a study led by experts from the Massachusetts Institute of Technology (MIT, United States). ) and the UK National Oceanography Centre, with support from NASA.
The results of this research, published this week in an article in the journal Nature, show the detection of changes in the color of the ocean in the last two decades that cannot be explained by natural variability from one year to another alone. These color changes are difficult to see with the naked eye but are evident with the help of data such as those offered by satellites, and have occurred in 56% of the ocean waters analyzed, an extension that is larger than the total terrestrial surface. from the earth.
The researchers found most notable changes in regions of the tropical oceans near the equator, where they have become increasingly greener. The change in the color of the ocean indicates that the ecosystems within the ocean surface must also be changing, since the color of the ocean is a literal reflection of the organisms and materials in its waters, the authors recall.
At this point, the researchers can't tell how exactly marine ecosystems are changing to reflect the color change. But they are pretty sure of one thing: human-induced climate change is probably the driver.
"I've been running simulations that have told me for years that these ocean color changes are going to happen," says study co-author Stephanie Dutkiewicz, a senior research scientist in MIT's Department of Earth, Atmospheric, and Planetary Sciences and the Center for the Science of Global Change. “Seeing the color change happen is really not surprising, just terrifying. And these changes are consistent with human-induced changes in our climate."
"The data now presented provides further evidence of how human activities are affecting life on Earth over a large expanse of space," adds lead author BB Cael, from the National Oceanography Center in Southampton, UK. "It's another way that humans are affecting the biosphere."
The color of the ocean is a visual product of everything within its upper layers. In general, deep blue waters reflect very little life, while greener waters indicate the presence of ecosystems, and primarily phytoplankton, plant-like microbes that abound in the upper ocean and contain the green pigment chlorophyll. . The pigment helps plankton collect sunlight, which they use to capture carbon dioxide from the atmosphere and convert it into sugars.
Phytoplankton is the base of the marine food chain that supports increasingly complex organisms, including krill, fish, seabirds and marine mammals. Phytoplankton is also a powerful muscle in the ocean's ability to capture and store carbon dioxide. Therefore, scientists are interested in monitoring phytoplankton on the surface of the oceans and seeing how these essential communities might respond to climate change. To do so, scientists have tracked changes in chlorophyll, based on the ratio of the amount of blue vs. green light reflecting off the ocean surface, which can be monitored from space.
But about a decade ago, Henson, who is a co-author on the current study, published a collaborative paper with other experts showing that if scientists were tracking only chlorophyll, it would take at least 30 years of continuous monitoring to detect any trends. The reason, the team argued, was that the large natural variations in chlorophyll from year to year would outweigh any anthropogenic influence on chlorophyll concentrations. Thus, it would take several decades to pick out a significant signal driven by climate change amid normal noise.
In 2019, Dutkiewicz and colleagues published a separate paper, showing through a new model that the natural variation in other ocean colors is much smaller compared to that of chlorophyll. Therefore, any signs of changes caused by climate change should be easier to detect than the normal smaller variations of other colors in the ocean. They predicted that such changes should be evident within 20, rather than 30, years of follow-up.
In the current study, Cael and the team analyzed measurements of ocean color taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean color for 21 years. MODIS takes measurements at seven visible wavelengths, including the two colors researchers traditionally use to estimate chlorophyll.
The color differences that the satellite picks up are too subtle for human eyes to distinguish. Much of the ocean appears blue to our eyes, while the true color can contain a mix of more subtle wavelengths, from blue to green and even red.
Cael carried out a statistical analysis using the seven colors of the ocean measured by the satellite from 2002 to 2022 together. He first observed how much the seven colors changed from one region to another during a given year, which gave him an idea of their natural variations. He then walked away to see how these yearly variations in ocean color changed over a period longer than two decades. This analysis showed a clear trend, above the normal interannual variability.
To see if this trend is related to climate change, he then looked at Dutkiewicz's model from 2019. This model simulated Earth's oceans under two scenarios: one with the addition of greenhouse gases and the other without. The greenhouse gas model predicted that a significant trend should appear within 20 years and that this trend should cause changes in ocean color over about 50 percent of the surface of the world's oceans, almost exactly what Cael found in his analysis of real-world satellite data. .
"This suggests that the trends we are seeing are not random variation in the Earth system," says Cael. "This is consistent with anthropogenic climate change."
