Climate change will dramatically alter colour of Earth's oceans

Cristina Cross
February 5, 2019

She says that current methods for monitoring phytoplankton provide information about local or regional changes but this new method, which uses satellite data, may offer a clearer, better picture of ocean change.

Climate change will bring a color change to half of the world's oceans by the end of the21st century, the study says. The subtropics-which include California, Texas and Florida-will become more blue, while areas near the poles, where warmer temperatures will lead to more diverse phytoplankton, will become greener.

The lead author says the ocean will still reflect the basic "blue" color palette we now see, but the blue areas like the subtropics will look more blue, and greener regions near the poles will be more green. "That basic pattern will still be there".

Dutkiewicz's co-authors include Oliver Jahn of MIT, Anna Hickman of the University of Southhampton, Stephanie Henson of the National Oceanography Centre Southampton, Claudie Beaulieu of the University of California at Santa Cruz, and Erwan Monier, former principal research scientist at the MIT Center for Global Change Science, and now assistant professor at the University of California at Davis, in the Department of Land, Air and Water Resources.

Phytoplankton are small, microscopic plants that float through the water column, due to their ability to absorb and reflect light, communities of phytoplankton affect the color of the ocean.

Water molecules can not soak up blue portions of the spectrum, reflecting it back out giving more desolate waters a deeper blue appearance.

Since the late 1990s, satellites have been taking continuous measurements of the ocean's colour to determine the amount of chlorophyll-and, in turn, phytoplankton-in an oceanic region.

Ocean colour varies from green to blue, depending on the type and concentration of phytoplankton, or algae, in any given area. The more phytoplankton present, the more green the water appears. "So it's a complicated process, how light is reflected back out of the ocean to give it its color".

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"Phytoplankton provide the organic matter that feeds nearly everything else in the ocean", Dutkiewicz said.

In the past, scientists have used satellite measurements of chlorophyll, a light-harvesting pigment found in phytoplankton, to try and understand the impact of climate change. But Dutkiewicz says chlorophyll estimates don't necessarily reflect climate change - significant swings in chlorophyll could come down to global warming, but they could also be due to "natural variability" due to natural phenomena such as weather.

"An El Niño or La Niña event will throw up a very large change in chlorophyll because it's changing the amount of nutrients that are coming into the system", Dutkiewicz said. The phytoplankton that live in the sunlit part of the ocean are hugely important, as they serve as the base of the marine food web.

"We're going to be able to see - not by eye but by instrument - that the colour of the ocean is changed".

While these changes may seem small, scientists say that they are deep and long-lasting. These organisms are responsible for much of the colour we see.

So the team allowed the model to work after setting mean temperatures to rise by up to 3 degrees Celsius up to 2100. The projected change by the team shows a deepening of blue in the tropical regions and green in the poles, as the regions around the equator become too hot, making the poles warmer.

But it's been hard to detect and measure these changes, says Dutkiewicz, partly because there's so much variability in the ocean from year to year.

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