The color of the ocean has changed dramatically over the past 20 years, and this global trend is likely the result of human-induced climate change, scientists from the Massachusetts Institute of Technology, the UK’s National Oceanographic Center, and other countries report.

In a study published July 12 in the journal Nature, The team writes that over the past two decades they have found changes in ocean color that cannot be explained by natural variation from year to year alone. While these color changes are imperceptible to the human eye, they occurred in 56 percent of the world’s oceans—an area larger than Earth’s total land area.

The researchers discovered that regions of the tropical ocean, particularly near the equator, are becoming greener over time. The changing color of the ocean indicates that ecosystems on the ocean surface must also change, as the color of the ocean is a true reflection of the organisms and materials in its waters. Currently, researchers cannot say exactly how marine ecosystems change to reflect color change. But they are almost certain of one thing: The driving force is climate change, driven by human activities.

To track changes in ocean color, the scientists analyzed ocean color measurements taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been tracking ocean color for 21 years. Image source: NASA and Joshua Stephens using USGS Landsat data and MODIS data from LANCE/EOSDIS Rapid Response.

“I’ve done simulations for years that tell me these ocean color changes are going to happen,” said co-author Stephanie Dutkiewicz, a senior scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences and the Center for the Science of Global Change. “It’s not surprising to see how that actually happens, it’s scary. And these changes are consistent with human-induced changes in our climate.”

“This provides further evidence of how human activity has impacted life on Earth in large areas,” adds lead author B. Cale, PhD ’19 from the National Oceanographic Center in Southampton, England. — This is another way humans affect the biosphere.».

Co-authors of the study include Stephanie Hanson of the National Center for Oceanography, Kelsey Bisson of Oregon State University, and Emmanuel Boss of the University of Maine.

above the noise

The color of the ocean is a visual product of what is in its upper layers. In general, dark blue waters reflect little life, while greener waters indicate the presence of ecosystems and primarily phytoplankton (plant-like microbes common in the upper reaches of the ocean and containing the green pigment chlorophyll). The pigment helps plankton collect sunlight, which they use to capture carbon dioxide in the atmosphere and turn it into sugar.

Phytoplankton is the backbone of the marine food web, supporting increasingly complex organisms, from krill to fish, seabirds and marine mammals. Phytoplankton is also a powerful muscle in the ocean’s ability to capture and store carbon dioxide. That’s why scientists want to monitor phytoplankton on the surface of the oceans and see how these important communities might respond to climate change. To do this, the scientists monitored changes in chlorophyll based on the ratio of the amount of blue and green light reflected from the ocean surface observable from space.

But nearly a decade ago, Hanson, one of the authors of the current study, published a paper with others showing that it would take at least 30 years of continuous monitoring for scientists to detect any trends if they only monitored chlorophyll. which was precisely due to climate change. This was because large natural fluctuations in chlorophyll from year to year would exceed any anthropogenic effect on chlorophyll concentrations, the team said. Therefore, it would take several decades to select a meaningful signal caused by climate change among the usual noise.

In 2019, Dutkiewicz and colleagues published a separate paper showing with a new model that the natural variation in other ocean colors is much smaller than that of chlorophyll. Therefore, any signal of change caused by climate change should be easier to detect than smaller, normal variations in other ocean colors. They estimated that such changes should occur within 20 years rather than 30 years of monitoring.

“So it doesn’t make sense to look for a trend in all these other colors instead of just chlorophyll?” says. “It’s worth looking at the whole spectrum, not just trying to guess a number from parts of the spectrum.”

power of seven

In the current study, Keil and his team analyzed ocean color measurements taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite, which has been tracking ocean color for 21 years. MODIS makes measurements at seven visible wavelengths, including the two colors researchers have traditionally used to estimate chlorophyll.

The color differences picked up by the satellite are too faint for the human eye to perceive. While most of the ocean appears blue to our eyes, true color can contain a mix of finer wavelengths, from blue to green or even red.

Cael performed a statistical analysis using a combination of seven ocean colors measured by the satellite from 2002 to 2022. First, he looked at how much the seven colors changed from region to region in a given year, and this gave him an idea of ​​their natural variation. He then zoomed out to see how these annual changes in ocean color changed over the longer period of two decades. This analysis revealed a clear trend above normal annual variability.

To find out if this trend is related to climate change, she turned to the Dutkiewicz model from 2019. This model simulated the Earth’s oceans under two scenarios: one with added greenhouse gas and the other without. The greenhouse gas model predicted that within 20 years, a significant trend should emerge, causing changes in ocean color on about 50 percent of the world’s ocean surface – almost the same thing Keel found in his analysis of actual satellite data.

“This suggests that the trends we’re seeing are not random changes in the Earth system,” says Kell. “This is consistent with anthropogenic climate change.”

The team’s results show that monitoring ocean colors other than chlorophyll can provide scientists with a clearer and faster way to detect changes in marine ecosystems caused by climate change.

“The color of the oceans has changed,” says Dutkevych. “And we can’t say how. But we can say that the color changes reflect changes in plankton communities that will affect everything that feeds on plankton. Since different types of plankton have different capacities for this, this will also change the amount of carbon in the ocean. So we hope people will take it seriously. These changes are just “It’s not predicted by models. Now we see that happening, and the ocean is changing.” Source