- New research sheds light on the unexpected color shift and raises questions about its ecological implications
More than half of the world’s oceans have experienced a surprising transformation over the past two decades, displaying a greener hue that scientists now attribute to the effects of global warming. The findings, published today in the journal Nature, challenge previous assumptions that it would take many more years of data to detect climate change-related shifts in ocean color.
Lead author B. B. Cael, an ocean and climate scientist at the National Oceanography Centre in Southampton, UK, expressed his concern, stating, “We are affecting the ecosystem in a way that we haven’t seen before.”
The color of the ocean can change due to various factors, including the upwelling of nutrients from the depths, which leads to the flourishing of phytoplankton blooms rich in the green pigment chlorophyll. By analyzing the wavelengths of sunlight reflected off the ocean’s surface, researchers can estimate the presence of chlorophyll and the abundance of living organisms like phytoplankton and algae. It was anticipated that biological productivity would change as ocean temperatures rose due to climate change.
However, fluctuations in the amount of chlorophyll in surface waters from year to year made it challenging to differentiate between natural variations and climate change-induced trends. Scientists previously believed that detecting such trends would require up to 40 years of observations.
Additionally, the varying methodologies used by different satellites to measure ocean color further complicated the analysis. To address this, Cael’s team turned to the MODIS sensor aboard NASA’s Aqua satellite, which has been orbiting Earth since 2002, exceeding its initial projected lifespan. Instead of relying on a single wavelength to track chlorophyll, they examined data from seven different wavelengths of light, providing a comprehensive perspective on ocean color changes.
With two decades’ worth of MODIS data, the researchers identified significant shifts in 56% of the world’s ocean surface, predominantly within tropical and subtropical waters between latitudes 40º S and 40º N. These regions, characterized by consistent colors throughout the year due to mild seasons, allowed small long-term changes to become more discernible.
The intensity of the color change varied depending on the measured wavelength, but overall, the oceans were becoming greener over time.
To investigate the potential link to climate change, the observations were compared to a model simulating the response of marine ecosystems to rising greenhouse gas levels. The observed changes aligned with the model’s predictions, supporting the notion that climate change is driving the shift.
As for the cause of the color change, Cael ruled out a direct effect of increasing sea surface temperatures since the observed areas did not correspond to regions with significant temperature rises. One theory suggests that the redistribution of nutrients in the ocean could be responsible. As surface waters warm, they become more stratified, hindering the upward movement of nutrients. This change in nutrient availability could favor the survival of smaller phytoplankton over larger ones, leading to ecological shifts reflected in the overall color of the water.
However, researchers emphasize that this is just one hypothesis among many, and further investigation is needed to determine the precise causes behind these changes. Cael notes, “The reason we care about the color is because the color tells us something about what’s happening in the ecosystem.”
This discovery amplifies the anticipation surrounding NASA’s upcoming mission to monitor ocean color, the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite, set to launch in January 2024. PACE will utilize hyperspectral capabilities, measuring ocean color in more wavelengths than any previous satellite, which will provide valuable insights into the ecological implications of observed trends in ocean ecosystem structure.
Ivona Cetinić, an oceanographer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is involved in the PACE project, emphasized the significance of these findings, stating, “All of this definitely confirms the need for global hyperspectral missions such as PACE. The spacecraft should allow us to understand the ecological implications of the observed trends in ocean ecosystem structure in years to come.”
As researchers delve deeper into understanding the changing color of the oceans, their findings have far-reaching implications for our understanding of climate change and its impact on marine ecosystems. Click here to read the full report.