Climate change could mean blue oceans as heat kills phytoplankton

James Marshall
February 4, 2019

A new study from researchers at the Massachusetts Institute of Technology finds that more than half of the world's oceans will shift color by the year 2100, due to changes in the types and location of phytoplankton. The phytoplankton's chlorophyll, which absorbs blue and reflects green. 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.

Writing in the journal Nature Communications, Hickman and colleagues from the United Kingdom and USA report how they came to their conclusions by using a computer model that predicts how factors such as temperature, ocean currents and ocean acidity affects the growth and types of phytoplankton in the water, as well as levels of other coloured organic matter and detritus.

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.

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.

The change in oceans' colour takes place because of the difference in quantities of phytoplankton, the microscopic algae that gives water bodies their distinct green colour.

This will drive a colour change in more than 50 percent of the world's seas by 2100. The researchers also simulated the way phytoplankton absorb and reflect light, and how the ocean's color changes as global warming affects the makeup of phytoplankton communities. Dutkiewicz fed satellite measurements of reflected light into a computer model, and correlated it to the number and type of ocean organisms.

This will happen because some species of phytoplankton will respond well to a warmer environment and will create larger blooms of more diverse marine organisms.

"We're not going to suddenly go from having a blue ocean to a red ocean or something like that, but there will be very, very subtle changes", says Dutkiewicz, a principal research scientist at MIT's Center for Global Change Science. But further offshore, the North Atlantic may appear a deeper blue.

Since the late 1990s, satellites have taken continuous measurements of the ocean's color. Significant swings in chlorophyll may be due to global warming, but it could also be due to "natural variability" such as cyclical increases in chlorophyll from natural weather-related events such as El Niño or La Niña.

"More of the ocean is going to show a change in colour over the next few decades than we would see in chlorophyll, the changing colour is going to be more of a warning signal". 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. 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.

As well as changes in the blue of the oceans, we are also likely to see changes in the green.

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. Different types of phytoplankton absorb light differently, and if climate change shifts one community of phytoplankton to another, that will also change the types of food webs they can support.

"There will be a noticeable difference in the colour of 50 per cent of the ocean by the end of the 21st century", Dutkiewicz says. "It could be potentially quite serious".

Other reports by Click Lancashire

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