Climate change is turning our oceans green

By Stephen Beech via SWNS

Oceans are no longer blue as they have turned a greener color over the last 20 years - due to man-made climate change, reveals new research.

Scientists have detected "significant" changes in the color of seas around the world over the past two decades that they say cannot be explained by natural, year-to-year variations alone.

The color shifts, though subtle to the human eye, have occurred over 56 percent of the world’s oceans — an expanse larger than the total land area on Earth, according to the findings published in the journal Nature

In particular, the research team found that tropical ocean regions near the equator have become steadily greener over time.

They say the shift in ocean color indicates that ecosystems within the surface of the ocean must also be changing, as the color of the ocean is a literal reflection of the organisms and materials in its waters.

The researchers cannot say how exactly marine ecosystems are changing to reflect the shifting color.

But the team, including scientists from the UK's National Oceanography Centre, are fairly certain of one thing: human-induced climate change is likely the driver.

Study co-author Dr. Stephanie Dutkiewicz, a senior research scientist at Massachusetts Institute of Technology (MIT) in the US, said: “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.”

Lead author Dr. B. B. Cael, of the National Oceanography Centre in Southampton, said: “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.”

The team explained that the ocean’s color is a visual product of whatever lies within its upper layers.

They said that, generally, waters that are deep blue reflect very little life, whereas greener waters indicate the presence of ecosystems, and mainly phytoplankton - plant-like microbes that are abundant in upper ocean and that contain the green pigment chlorophyll.

The pigment helps plankton harvest sunlight, which they use to capture carbon dioxide from the atmosphere and convert it into sugars.

Dr. Dutkiewicz said phytoplankton are the foundation of the marine food web that sustains progressively more complex organisms, on up to krill, fish, and seabirds and marine mammals.

She said phytoplankton are also a powerful "muscle" in the ocean’s ability to capture and store carbon dioxide.

Scientists are keen to monitor phytoplankton across the surface oceans and to see how the essential communities might respond to climate change.

To do so, researchers have tracked changes in chlorophyll, based on the ratio of how much blue versus green light is reflected from the ocean surface, which can be monitored from space

But around a decade ago, Professor Stephanie Henson, who is a co-author of the current study, published a paper with others, which showed that, if scientists were tracking chlorophyll alone, it would take at least 30 years of continuous monitoring to detect any trend that was driven specifically by climate change.

They argued that the large, natural variations in chlorophyll from year to year would "overwhelm" any anthropogenic influence on chlorophyll concentrations.

It would therefore take several decades to pick out a meaningful, climate-change-driven signal amid the normal noise.

In 2019, Dr. Dutkiewicz and her colleagues published a separate paper, showing through a new model that the natural variation in other ocean colors is much smaller compared to that of chlorophyll.

Therefore, any signal of climate-change-driven changes should be easier to detect over the smaller, normal variations of other ocean colors.

They predicted that such changes should be apparent within 20, rather than 30 years of monitoring.

Dr Caelsaid: “I thought, doesn’t it make sense to look for a trend in all these other colors, rather than in chlorophyll alone?

“It’s worth looking at the whole spectrum, rather than just trying to estimate one number from bits of the spectrum.”

In the current study, Dr. Cael and the team analyzed measurements of ocean color taken by the MRI Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean color for 21 years.

MODIS takes measurements in seven visible wavelengths, including the two colors researchers traditionally use to estimate chlorophyll.

The differences in color that the satellite picks up are too subtle for human eyes to differentiate. Much of the ocean appears blue to our eye, whereas the true color may contain a mix of subtler wavelengths, from blue to green and even red.

Dr. Cael conducted a statistical analysis using all seven ocean colors measured by the satellite from 2002 to 2022 together.

He first looked at how much the seven colors changed from region to region during a given year, which gave him an idea of their natural variations.

He then zoomed out to see how the annual variations in ocean color changed over a longer stretch of two decades.

The analysis turned up a clear trend, above the normal year-to-year variability.

To see whether the trend is related to climate change, he then looked at Dr Dutkiewicz’s model from 2019.

That model simulated the Earth’s oceans under two scenarios: one with the addition of greenhouse gases, and the other without it.

The greenhouse-gas model predicted that a significant trend should show up within 20 years and that the trend should cause changes to ocean color in about 50 percent of the world’s surface oceans — almost exactly what Dr. Cael found in his analysis of real-world satellite data.

Dr. Cael said: “This suggests that the trends we observe are not a random variation in the Earth system.

“This is consistent with anthropogenic climate change.”

Dr. Dutkiewicz added: “The color of the oceans has changed.

"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.”