Scientists from MIT, the National Oceanography Center in the U.K., and other institutions have reported a significant change in the color of the ocean over the past 20 years, suggesting that human-induced climate change is likely responsible. Their study, published in Nature, reveals that ocean color shifts have occurred in more than half of the world's oceans, exceeding the total land area on Earth.
The researchers found that tropical ocean regions near the equator have become progressively greener over time. This change in color indicates that marine ecosystems within the surface ocean are also undergoing transformations, as the color of the ocean reflects the organisms and materials present in its waters.
While the study doesn't pinpoint the exact nature of these ecosystem changes, the scientists are confident that human-induced climate change is the driving force behind them. Stephanie Dutkiewicz, a co-author of the study and senior research scientist at MIT, explains that computer simulations have long predicted such changes in ocean color, making their actual observation both expected and alarming. The alterations in ocean color serve as further evidence of the broad impact of human activities on Earth's biosphere.
The research team consists of Stephanie Dutkiewicz, B. B. Cael Ph.D. '19 (lead author), Stephanie Henson, Kelsey Bisson, and Emmanuel Boss, affiliated with various institutions such as MIT, the National Oceanography Center, Oregon State University, and the University of Maine, respectively.
Above the noise
The color of the ocean reflects its composition within the upper layers. Deep blue waters signify a lack of life, while greener waters indicate the presence of ecosystems, primarily phytoplankton—microscopic plant-like organisms abundant in the upper ocean that contain chlorophyll, a green pigment. Phytoplankton utilize sunlight to capture carbon dioxide from the atmosphere and convert it into sugars, forming the foundation of the marine food web and playing a crucial role in carbon sequestration.
Understanding how phytoplankton and marine ecosystems respond to climate change is of great interest to scientists. Monitoring changes in chlorophyll, measured by the ratio of blue to green light reflected from the ocean surface, has been a common approach using satellite observations.
However, a previous study co-authored by Henson highlighted the challenge of detecting climate change signals based solely on chlorophyll measurements. Due to the significant natural year-to-year variations in chlorophyll concentrations, it would require continuous monitoring for at least 30 years to discern a climate-change-driven trend amidst the normal fluctuations.
In 2019, Dutkiewicz and her team published a different study demonstrating that other ocean colors exhibit much smaller natural variations compared to chlorophyll. Consequently, they proposed that monitoring changes in these other colors, rather than solely focusing on chlorophyll, could provide clearer indications of climate-change-related shifts. They predicted that such changes should be discernible within 20 years of monitoring.
Considering this, Cael suggests that examining the entire spectrum of ocean colors rather than focusing on a single parameter like chlorophyll would yield a more comprehensive understanding of trends and provide insights into the effects of climate change on marine ecosystems.
The power of seven
In the recent study, the research team utilized data from the Aqua satellite's Moderate Resolution Imaging Spectroradiometer (MODIS) to analyze ocean color measurements spanning 21 years. The satellite captures measurements in seven visible wavelengths, including the traditional colors used to estimate chlorophyll levels.
These subtle color differences captured by MODIS are indistinguishable to the human eye. While we perceive much of the ocean as blue, it actually contains a mixture of wavelengths ranging from blue to green and even red.
To uncover patterns, Cael conducted a statistical analysis considering all seven ocean colors measured by the satellite from 2002 to 2022. He assessed the natural variations in these colors from year to year within different regions, providing insights into their normal fluctuations. Zooming out, he observed how these annual color variations changed over the two-decade period, revealing a distinct trend that surpassed the expected year-to-year variability.
To determine whether this trend could be attributed to climate change, Cael referred to Dutkiewicz's 2019 model. The model simulated the Earth's oceans under two scenarios: one with increased greenhouse gas concentrations and one without. According to the greenhouse-gas model, a significant trend should emerge within 20 years, leading to changes in ocean color across approximately 50 percent of the world's surface oceans. Remarkably, Cael's analysis of real-world satellite data aligned closely with these predictions.
This consistency strongly suggests that the observed trends are not random fluctuations but rather a result of anthropogenic climate change.
The findings emphasize the importance of monitoring ocean colors beyond just chlorophyll for a more comprehensive and rapid detection of climate-change-driven transformations in marine ecosystems.
Dutkiewicz underscores the significance of the study's results, emphasizing that the color of the oceans has indeed changed. While the specific nature of these changes remains unclear, they are indicative of shifts in plankton communities, which will have cascading effects on the entire food chain reliant on plankton. Additionally, these changes will influence the ocean's capacity to absorb carbon dioxide since different types of plankton vary in their ability to perform this vital function. The study highlights the urgency of recognizing these shifts as tangible evidence of ongoing changes in the ocean ecosystem, warranting serious attention and action.