Oceans to turn bluer due to global warming

James Marshall
February 4, 2019

The study was published in the scientific journal Nature Communications.

The team modelled what would happen to the oceans by the end of this century if the world warmed by 3C, which is close to where temperatures are likely to be, if every country sticks to the promises they have made in the Paris climate agreement.

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 as warm, subtropical waters get warmer, populations of phytoplankton are projected to decrease, changing the waters to more blue colors.

The sea will turn a brighter shade of blue due to global warming, a new study predicts. "That basic pattern will still be there".

Phytoplankton are microscopic algae that float at the bottom of the ocean food web, forming a key part of most ocean ecosystems.

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 ocean's color depends on how sunlight interacts with whatever is in the water. Water molecules alone absorb nearly all sunlight except for the blue part of the spectrum, which is reflected back out. Hence, relatively barren open-ocean regions appear as deep blue from space.

Phytoplankton absorb and reflect different wavelengths of sunlight, which appear as different colors.

Since the late 1990s, satellites have taken continuous measurements of the ocean's color. This can then provide a more accurate reflection of the climate trend than calculating chlorophyll levels from this data, which only gives a sense of the amount of phytoplankton.

"What we've shown is that the colour in the blue green range is going to show that signal of change sooner, in some places in maybe the next decade", said Dr Dutkiewicz.

But it isn't just global warming that causes a change in algal blooms and chlorophyll; naturally occurring weather events like El Nino could also temporarily cause a spike in phytoplankton bloom, causing an increase in chlorophyll.

In order to account for these natural events, the researchers tweaked a previous global model used previously to predict phytoplankton changes in response to rising temperatures and ocean acidification to instead predict how climate change is affecting phytoplankton. This model takes information about phytoplankton, such as what they consume and how they grow, and incorporates this information into a physical model that simulates the ocean's currents and mixing.

It was also programmed to estimate the specific wavelengths of light that are absorbed and reflected by the ocean, taking into consideration the amount and type of organisms in a given region. 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.

Importantly, she said, the shift in reflectance of blue/green light appeared to give an earlier indication of changes to phytoplankton than estimates of the amount of chlorophyll present, a measure now used to monitor phytoplankton levels. "But you can see a significant, climate-related shift in some of these wavebands, in the signal being sent out to the satellites". "If they were to magically change - or if we were to kill them off completely - there would be a lot of carbon coming out of the ocean and back into the atmosphere, and creating more problems that we have now".

As for Boston, she doesn't expect that we'll see much color change in the harbor itself - it's too full of sediment and runoff for the phytoplankton to make much of a color difference.

Other reports by Click Lancashire

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