[Reading Science] Global Warming Changes Even the Color of the Ocean

Research Team at UK National Oceanography Centre
20 Years of Data Analysis
Between 40°N and 40°S Latitude Seas
56% Changed Color, Gradually Turning Greener

Shocking research results have revealed that the color of seawater worldwide has been changing from dark blue to green over the past 20 years due to climate change.

On March 18, 2017, NASA's Aqua satellite captured an image of the sea surface around Tunisia, Africa, where green algae blooms caused the sea color to turn green. Photo by NASA website

A research team from the UK's National Oceanography Centre published this paper on the 12th (local time) in the international academic journal Nature. The color of the sea can change for various reasons. For example, in cases of 'eutrophication,' where nutrients flowing from land due to floods, typhoons, or rising from deep underwater become excessive. Green plankton such as chlorophyll, which feed on these nutrients, proliferate massively, turning the seawater green. The problem is that it is difficult to distinguish whether this is a temporary phenomenon or a long-term trend. Scientists have believed that observing changes for at least 40 years is necessary to predict long-term trends such as the impact of global warming. There is also difficulty in collecting data. Many satellites have measured the color of the sea, but since each used different methods, it was hard to consolidate the data.

However, the research team succeeded in distinguishing this with only the latest 20 years of observational data. This indicates that recent climate change has a significant impact on marine ecosystems. The team analyzed data collected over the past 20 years by NASA's Aqua satellite, launched in 2002, using the MODIS sensor. Instead of analyzing a single wavelength to measure chlorophyll levels, they used a method analyzing all seven visible light wavelengths reflected from seawater.

As a result, the team concluded that long-term color changes have occurred in about 56% of the tropical and subtropical sea surfaces between 40 degrees north and 40 degrees south latitude, which make up most of the world's seawater. Tropical and subtropical seas have almost no changes in surface temperature, so their color does not fluctuate significantly by season or year. Therefore, short-term changes can be more clearly identified. The concentration changes in color varied depending on the measured light wavelength, but generally, seawater was found to be gradually turning greener over time. The team also identified a correlation with climate change. Comparing the seawater color changes with simulation models of how marine ecosystems change in response to increased greenhouse gases in the atmosphere showed consistency.

The research team is investigating the mechanisms behind these long-term seawater color changes. First, it appears that the rise in surface water temperature is not a direct cause. Satellite image analysis showed that areas where surface water temperature rose did not coincide with areas of color change. Instead, they are focusing on the relationship with nutrient distribution in seawater. When surface water warms, the upper layers of the sea become stratified, making it harder for nutrients from deeper layers to reach the surface. When nutrients become scarce, relatively smaller plankton species survive more easily than larger plankton. This change in nutrient levels affects the marine ecosystem, which in turn changes the overall color of the sea. Of course, the mechanisms behind seawater color changes are believed to be complex. Even the research team admits that they do not yet fully understand why this is happening. B. Kael, a researcher at the UK's National Oceanography Centre, said, "The reason we need to pay attention to the seawater color changes identified in this study is that it signals to humanity that something is changing in the marine ecosystem."

Meanwhile, this research has increased interest in NASA's PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) satellite, scheduled for launch in January next year. This satellite will measure the color of seawater worldwide using a hyperspectral imaging sensor, capturing many more wavelengths than any previous satellite. Ivona Setinic, a researcher in charge of the PACE satellite project, explained, "This study confirms the necessity of global marine hyperspectral imaging missions like the PACE satellite project," adding, "PACE will help us understand the significance of the observed changes in marine ecosystem structure."