The Ocean is Changing Color – And Climate Change is Likely To Blame
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Using satellite imagery that dates back two decades, researchers have detected subtle color changes in the Earth’s oceans that cannot be explained by normal year-to-year ecological variability alone. In a study published in Nature, the scientists say that these color changes are likely a consequence of human-induced climate change.
Blue, green and everything in-between
An ocean’s color reflects what is going on with the organisms and materials present in its upper layers. For example, deep blues indicate regions with very little life, whereas parts with a green hue indicate the presence of phytoplankton – plant-like microbes that contain significant amounts of the green pigment chlorophyll.
The presence of phytoplankton is of particular interest to climate scientists, as it forms the foundation of the complex marine food chain and plays a key role in global carbon dioxide capture and storage. Scientists are therefore keen to monitor phytoplankton across the surface oceans in order to assess how these essential microbes and their related ecosystems are responding to climate change. To do this, scientists study the ratio of how much blue versus green light is being reflected from different regions of the ocean’s surface in pictures taken by satellites.
However, several years ago, a paper published by Professor Stephanie Henson – a research fellow at the National Oceanography Centre in the U.K. and a co-author on this latest study – suggested that tracking chlorophyll alone might not be enough to spot climate change-related trends. In fact, Henson estimated that it would take at least 30 years of continuous monitoring to detect relevant trends in chlorophyll, as other natural year-to-year fluctuations would overwhelm any specific climate-driven shifts.
But the ocean is not just blue or green; it also reflects other wavelengths of light on the visible spectrum.
“So I thought, doesn’t it make sense to look for a trend in all these other colors, rather than in chlorophyll alone?” said B. B. Cael, an ocean and climate scientist at the National Oceanography Centre and the lead author of this latest Nature study. “It’s worth looking at the whole spectrum, rather than just trying to estimate one number from bits of the spectrum.”
Multi-color analysis fits well with climate models
In this new study, the research team looked at ocean color measurements taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean color for 21 years. The MODIS records color in seven visible wavelength bands, including the wavelength used traditionally to monitor chlorophyll, as well as other wavelengths that we might not normally associate with the ocean, such as red.
The researchers examined ocean images taken between 2002 to 2022 across all seven color bands. First, they looked at how the colors changed from region to region during a given year, to get a sense of the natural variation in each wavelength band. Then they examined how these annual variations change over the full two-decade study period.
These analyses turned up a clear trend, one that was above the normal expected year-to-year variability. They observed noticeable color shifts in 56% of the world’s ocean surfaces – mostly in tropical and subtropical waters. Due to their location which protects them from extreme seasons, these waters tend not to vary much in color throughout the year, the researchers explained. As a result, small long-term changes are more apparent in these regions.
To assess whether this trend might be linked to climate change, the researchers looked to a model previously published by study co-author Stephanie Dutkiewicz in 2019. This model simulated the oceans under two different scenarios: one which ignored greenhouse gases and the other with these gases included. The greenhouse gas model had predicted that a significant trend should show up in the world’s oceans within 20 years and that this trend should cause noticeable color changes in around 50% of the world’s oceans – an almost exact parallel to what was seen in the real-world satellite data.
“This suggests that the trends we observe are not a random variation in the Earth system,” Cael said. “This is consistent with anthropogenic climate change.”
“I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen,” added Dutkiewicz, a senior research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences and the Center for Global Change Science. “To actually see it happening for real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate.”
Color change likely reflects human impact on the environment
Researchers are not completely certain about how marine ecosystems are changing to produce these shifts in color – the changes don’t match up with areas where ocean temperatures are known to have risen. But they are certain that the changes observed in recent decades are a result of human-induced climate change.
“The color of the oceans has changed,” Dutkiewicz said. “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.”
Given their latest results, which demonstrate the utility of monitoring ocean colors beyond what is relevant to chlorophyll, the researchers say that this kind of monitoring could give scientists a clearer, faster way to detect climate change-driven changes to marine ecosystems in the future.
Reference: Cael BB, Bisson K, Boss E, Dutkiewicz S, Henson S. Global climate-change trends detected in indicators of ocean ecology. Nature. 2023. doi: 10.1038/s41586-023-06321-z
This article is a rework of a press release issued by the Massachusetts Institute of Technology. Material has been edited for length and content.