Ocean color has changed significantly over the past 20 years

The colors of our oceans have undergone a significant transformation in the last two decades, indicating a likely global outcome of human-induced climate change.

Scientists from the Massachusetts Institute of Technology (MIT), the National Oceanography Center in the U.K., and other institutions reported this alarming development. The researchers presented their findings today on ocean color changes in the scientific journal Nature.

The team emphasized that this is not just the product of natural fluctuations from year to year. They found that these color shifts, although subtle to human eyes, have affected more than half of the world’s oceans. This covers a vast expanse larger than Earth’s total land area.

Marine ecosystems are changing along with ocean colors

The researchers noted a particularly interesting color shift. Tropical ocean regions near the equator have been becoming steadily greener.

They explained that the ocean’s color is a reflection of its inhabitants and the materials present in its waters. As a consequence, this shift in color means the ecosystems within the ocean surface are undergoing changes as well.

At this point, the scientists can’t identify exactly how the marine ecosystems are changing in response to the shifting colors. However, they strongly suspect that human-induced climate change is the primary driver.

Study co-author Stephanie Dutkiewicz is a senior research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences and the Center for Global Change Science. She says, “I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen. To actually see it happening for real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate.”

Supporting Dutkiewicz, B. B. Cael, PhD ’19 of the National Oceanography Center in Southampton, U.K., and lead author of the study, adds, “This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent. It’s another way that humans are affecting the biosphere.”

What impacts ocean color?

Ocean color is affected by what lies in its upper layers. Deep blue waters generally indicate a scarcity of life. Greener waters suggest the presence of vibrant ecosystems, chiefly those of phytoplankton.

These plant-like microbes are abundant in the upper ocean and contain green pigment chlorophyll, aiding them in harvesting sunlight. They absorb carbon dioxide from the atmosphere and convert it into sugars.

Phytoplankton are essential in the marine food web. They provide sustenance for progressively complex organisms such as krill, fish, seabirds, and marine mammals. Moreover, they play a critical role in the ocean’s ability to capture and store carbon dioxide.

Thus, monitoring phytoplankton across the surface oceans and understanding their response to climate change is a priority for scientists. They have tracked changes in chlorophyll, gauged by the ratio of blue to green light reflected from the ocean surface.

However, according to Stephanie Henson of the National Oceanography Center, co-author of the current study, and her colleagues, tracking chlorophyll alone would take at least 30 years of continuous monitoring to detect any climate change-driven trend.

They argued that this was due to large natural variations in chlorophyll concentration. This would obscure any human-induced changes.

How the study was conducted

To address this challenge, Dutkiewicz and her colleagues published a paper in 2019, showcasing a new model. This model demonstrated that the natural variation in other ocean colors is much smaller compared to that of chlorophyll.

Therefore, any climate change-driven alterations would be easier to detect amid the smaller, normal variations of other ocean colors. They predicted that such changes would be noticeable within 20 years instead of 30.

Motivated by this, Cael decided to look at the whole spectrum of ocean colors instead of just chlorophyll. Using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, Cael performed a statistical analysis.

Clear trend identified across seven ocean colors

He examined all seven colors measured by the satellite over the last two decades. His analysis revealed a clear trend that surpassed the normal year-to-year fluctuations.

Comparing his results with Dutkiewicz’s 2019 model, Cael found a remarkable match. The model predicted significant trends within 20 years due to the addition of greenhouse gases.

This will affect about 50% of the world’s surface oceans. These findings aligned almost exactly with Cael’s real-world satellite data analysis.

“This suggests that the trends we observe are not a random variation in the Earth system,” Cael explains. “This is consistent with anthropogenic climate change.”

The study’s conclusions indicate that monitoring a variety of ocean colors beyond chlorophyll could offer scientists a faster, clearer method to detect changes in marine ecosystems due to climate change.

“The color of the oceans has changed,” Dutkiewicz confirms. “And we can’t say how. But we can say that changes in color reflect changes in plankton communities, that will impact everything that feeds on plankton. It will also change how much the ocean will take up carbon, because different types of plankton have different abilities to do that. So, we hope people take this seriously. It’s not only models that are predicting these changes will happen. We can now see it happening, and the ocean is changing.”

More about ocean color

The color of the ocean is determined by the interaction of sunlight with the substances or particles present in the water. Sunlight contains all colors of light, but water absorbs colors at different rates.

In clean, pure water, longer wavelengths of light, such as red, orange, and yellow, are absorbed more strongly, and short-wavelength light like blue is absorbed less. This gives large bodies of water a blue appearance.

There are several factors that can affect the color of ocean water:

Phytoplankton

Phytoplankton are tiny marine plants that live near the surface of the ocean. They contain chlorophyll to perform photosynthesis, which gives them a green color. When there are a lot of phytoplankton in the water, the ocean can look greenish.

Dissolved and Suspended Particles

The ocean contains many suspended particles such as sand, silt, and organic matter. When sunlight hits these particles, it can scatter in many directions and change the color of the water.

The ocean can appear murky or brown near the coast because of the large amounts of suspended particles. Dissolved substances like salts and chemicals can also affect the color of the water.

Depth

The color of the ocean can also change with depth. Near the surface, where sunlight can penetrate the water, the ocean appears lighter. As depth increases and sunlight diminishes, the water appears darker.

Algal Blooms

Sometimes, algal blooms can cause the water to turn red or brown. This is often referred to as “red tide”.

Temperature and Salinity

Changes in water temperature and salinity can also affect its density, which can influence how light is absorbed and scattered.

Angle of the Sun

The position of the sun in the sky can affect how we perceive the color of the ocean. When the sun is high, the ocean may appear to be a deep blue. Near sunrise or sunset, when the sun is low in the sky, the water might look reddish or golden.

Scientists can learn a lot about the ocean by studying its color. For example, they can use satellite imagery to measure the color of the ocean surface and make inferences about the amount of phytoplankton, suspended particles, and dissolved organic matter.

This can give them valuable information about the health of marine ecosystems and the global carbon cycle. Changes in ocean color can also signal the effects of climate change. Warmer waters may contain less phytoplankton, which also alters the color of the ocean.