Climate Change is Affecting the Color of the Ocean

Caitlin Dempsey

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Climate change is affecting a lot of different ecosystems and processes in the world. Ocean warming, ocean acidification, coral bleaching events, wrong-way benthic migration, and changes in global ocean currents are just some of the changes that researchers have measured as the world’s largest waterbodies respond to a warming Earth.

Climate change and the changing colors of the Earth’s water systems

Changing color is another pattern that is emerging as a result of climate change. A study published in 2020 found that climate change is the driving force behind an increase in snow algal blooms in Antarctica that leave snow seasonally turning green, orange, and red. In a separate study, a team of researchers analyzed Landsat images over a 38-year period and found that a significant number of large U.S. rivers are changing colors.

Climate change and phytoplankton

Climate change also has an impact on phytoplankton. These microscopic marine algae are sensitive to changes in ocean temperature and salinity.   The appearance of color in the ocean is affected by what’s in or not in the water.

Ocean water molecules absorbs all portions of light except for blue which is reflected back.  Areas of barren deep ocean appear dark blue from space.  Phytoplankton contain chlorophyll, which is a pigment that absorbs predominately in the blue portions of sunlight in order to produce carbon for photosynthesis.

In the sunlit layer of the ocean with a high density of phytoplankton, more green light is reflected back, giving the water a dark-green hue.  Green-blue reflectance has been measured by satellites since the late 1990s.

This underwater photo shows how arctic water (left) can be turned green by a bloom of phytoplankton.  Photo: Karen Frey (Clark University) via NASA, 2012.
This underwater photo shows how arctic water (left) can be turned green by a bloom of phytoplankton. Photo: Karen Frey (Clark University) via NASA, 2012.

Analyzing 20 years of satellite data to map where the ocean is changing color

The latest study analyzed the color of the oceans and found that climate changes is affecting the ocean ecosystems. A study headed by B. B. Cael, a lead scientist at the U.K.’s National Oceanography Centre, analyzed 20 years worth of NASA ocean color data from between 2000 and 2022 from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard the Aqua satellite. Researchers looked primarily at ocean surface water color in tropical and subtropical areas. Waters in higher latitudes tend to be darker and ocean water near coastlines contained too much data noise to be analyzed.

The analysis was able to map out ocean areas where chlorophyll, the substance that makes plants green, had changed. For remote sensing scientists, chlorophyll levels have typically served as the primary indicator for assessing the quantity and productivity of phytoplankton.

A map of the worlds oceans with darker green areas indicates areas where the changes in phytoplankton were the highest.
Map created by NASA highlighting where the where ocean surface color changed between 2002 and 2022. The darker green areas indicates areas where the changes were the most significant. Map: Wanmei Liang, NASA with data from Cael et al., 2023.

What the study found that 56% of the ocean’s surface had changed color during that time period, particularly in areas near the Equator. Factors like changes in the depth of the mixed layer of the ocean or how water is layered in the upper ocean are known to affect the structure and amount of plankton in the ocean, which in turn affects the ocean’s color. These trends are more noticeable in tropical and subtropical regions with less yearly variation.

The researchers also wanted to know if the changes they observed in the ocean’s color were due to climate change. To find out, they used a complex computer model that simulates the ocean’s ecosystem and how it might change due to high levels of greenhouse gases. They compared this model to actual satellite data. The model suggests that changes in the ocean’s color could become noticeable within 20 years or less in about 46% of the ocean, which is close to the 56% of change in ocean color that they found analyzing satellite data.

2019 study using  satellite measurements of reflected light to measure the affects of climate change on phytoplankton

An earlier study published in 2019 by researchers from MIT and the University of Southampton also analyzed how satellite measurements of reflected light could be used to predict the affect of climate change on phytoplankton. Using an ocean physics, biogeochemistry and ecosystem model tweaked to incorporate reflected light measurements, the researchers modeled a  global temperature increase of 3 degrees Celsius by 2100 to predict the impact on phytoplankton.

The model predicted that 50 percent of the world’s oceans will shift in color with subtropical regions becoming more blue (and devoid of phytoplankton) while polar regions will become more greener as warmer waters become more hospitable to phytoplankton.

Two maps showing the world with overlays showing changes in the ocean colors.
The researchers used a measure called the “hue angle” to quantify the true color of the ocean, with values ranging from 0° to 360°. In areas of the open ocean with high productivity, the hue angle is around 100°, which corresponds to green. In less productive areas, like the subtropical gyres, the hue angle is around 230°, which is blue. The study found that in some places the hue angle has changed by up to 10°, while in others it has decreased by up to 5°. These findings compare data from 1998–2015 with model predictions for 2085–2100. Maps: Dutkiewicz et al.(2019)

More study is needed on the impact of climate change on the color of the oceans

The authors of the 2023 study note that a change in ocean color has the potential to have a ripple effect on marine life, especially phytoplankton, which are sensitive to light conditions. Any changes in light conditions could potentially alter the entire marine ecosystem. More study is needed to understand the ramifications of color change in the world’s oceans.

This article was originally published on February 4, 2019 and has seen been updated with more recent research.

References

Cael, B. B., Bisson, K., Boss, E., Dutkiewicz, S., & Henson, S. (2023). Global climate-change trends detected in indicators of ocean ecology. Nature619(7970), 551-554. https://doi.org/10.1038/s41586-023-06321-z

Doermann, L. (2023, October 2). Climate change lends new color to the ocean. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/151894/climate-change-lends-new-color-to-the-ocean

Dutkiewicz, S., Hickman, A. E., Jahn, O., Henson, S., Beaulieu, C., & Monier, E. (2019). Ocean colour signature of climate change. Nature Communications, 10(1), 578. https://doi.org/10.1038/s41467-019-08457-xMuch of the surface ocean will shift in color by end of 21st century: study.  MIT News    

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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