Published: Tue, February 05, 2019
Science | By Michele Flores

Climate change will dramatically alter colour of Earth's oceans

Climate change will dramatically alter colour of Earth's oceans

The colour of the ocean will intensify by the end of the century, becoming more blue or green in some regions because of rising temperatures, a study has said. Scientists have used these measurements to derive the amount of chlorophyll, and by extension, phytoplankton, in a given ocean region.

Scientists already know that climate change is affecting plankton, with warmer waters leading to different algae species blooming in new waters, for instance. "We are interested in phytoplankton because they are tiny marine plants, they contribute about half of global photosynthesis, they are the base of the marine food web". According to Nasa, warming changes key properties of the ocean and can affect phytoplankton growth, since they need not only sunlight and carbon dioxide to thrive, but also nutrients.

By contrast, ocean water nearer the poles will react differently to the warming trend.

Hickman said: "Crudely speaking, where the water is now quite blue because the phytoplankton [have a] relatively low biomass, you are going to see the water getting more blue, and where the ocean is relatively more green because the biomass is higher, you are going to see [it] getting [greener]". 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. Likewise, the more abundant they are, the less blue the water will be.

Because of the way light reflects off the organisms, blooms of these phytoplankton create colourful patterns at the ocean's surface.

What the ocean looks like depends on the types of organisms and molecules floating near the surface.

"Sunlight will come into the ocean, and anything that's in the ocean will absorb it, like chlorophyll", Dutkiewicz says. "So it's a complicated process, how light is reflected back out of the ocean to give it its color". Scientists have predicted that if this continues, ocean colours would change by the end of the century.

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As the researchers cranked up global temperatures in the model, by up to 3 degrees Celsius by 2100 - what most scientists predict will occur under a business-as-usual scenario of relatively no action to reduce greenhouse gases - they found that wavelengths of light in the blue/green waveband responded the fastest.

"Chlorophyll is changing, but you can't really see it because of its incredible natural variability", Dutkiewicz says.

When the temperature value is tweaked, the model's output shows the change in the colour of Earth's oceans: The blue and green parts of the spectrum being the most responsive, showing drastic change with increased temperatures. "But you can see a significant, climate-related shift in some of these wavebands, in the signal being sent out to the satellites". The sensors, which measure the colour of the water, work to calibrate satellite data in real time, providing information about the health of the Salish Sea.

"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.

"It could be potentially quite serious", Dutkiewicz added.

This study aimed to show how researchers can see a clear signal of climate change's effect on phytoplankton by looking at measurements of reflected light alone-something the research community has been advocating for.

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