Climate change is altering the colour of the oceans

Source: By Alind Chauhan: The Indian Express

The colour of the Earth’s oceans has significantly altered over the past two decades, most likely due to human-induced climate change, according to a new study. Over 56 per cent of the oceans, more than the total land area on the planet, has experienced the shift in colour, it added.

The study, ‘Global climate-change trends detected in indicators of ocean ecology’, was published earlier in July in the journal Nature. It was carried out by BB Cael and Stephanie Henson of the United Kingdom-based National Oceanography Centre, Kelsey Bisson of the Oregon State University (USA), Emmanuel Boss of the University of Maine (USA), and Stephanie Dutkiewicz of the Massachusetts Institute of Technology (USA).

Although the change in colour of the oceans doesn’t impact marine life directly, it indicates that marine ecosystems are in a state of flux and they could completely go out of balance in the future, severely affecting ocean life and humans dependent on them. BB Cael, lead author of the study, said that changes in these ecosystems could impact how productive they are, “which could, in turn, affect how much carbon the ocean stores and how much food supply there is for fisheries.”

But what makes the oceans colourful in the first place?

In most regions across the world, the oceans appear blue or navy blue for a reason. This happens due to “the absorption and scattering of light,” according to a report by NASA. When the sunlight falls on deep and clear water, colours with longer wavelengths, such as red, yellow and green, are absorbed by the water molecules but blue and violet, which have a much shorter wavelength, are reflected back.

“When sunlight hits the ocean, some of the light is reflected back directly but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light we see is composed of the shorter wavelength blues and violets,” the report explained.

But when the water isn’t deep or clean, an ocean can appear to be of a different colour. For instance, along Argentina’s coastline, where major rivers merge into the Atlantic Ocean, the ocean exudes a brown tint because of dead leaves and sediments spewing from the rivers. In other parts of the world, the oceans appear green, which happens due to the existence of phytoplankton on the upper surface of the water.

Phytoplanktons are microscopic marine algae that contain the green-coloured pigment chlorophyll. The pigment helps them absorb sunlight, which they use to capture carbon dioxide from the atmosphere and convert it into sugars. Moreover, chlorophyll absorbs the red and blue portions of the light spectrum — or photosynthesis — and reflects green light.

“So, the ocean over regions with high concentrations of phytoplankton will appear as certain shades, from blue-green to green, depending upon the type and density of the phytoplankton population there,” the report added.

What methods were used to carry out the study?

To conduct the study, Cael and his team first analysed data from Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA’s Aqua satellite, which has been monitoring ocean colour since 2002 — the measurements are taken in terms of the amount of light coming off the surface of the oceans, at all seven of the different wavelengths of light, from violet to red. The examination of 20 years worth of data indicated that in more than 50 per cent of the world’s oceans, the colour has changed.

Then, to check if the phenomenon has occurred due to climate change, researchers used a climate model – a computer representation of the Earth. According to a report by MIT, “this model simulated the planet’s oceans under two scenarios: one with the addition of greenhouse gases, and the other without it. The greenhouse-gas model predicted that a significant trend should show up within 20 years and that this trend should cause changes to ocean colour in about 50 percent of the world’s surface oceans — almost exactly what Cael found in his analysis of real-world satellite data.”

Why is the colour of the oceans changing?

The study says one of the most affected areas is the Tropical ocean regions, near the equator, where the water is turning from blue to green. But this doesn’t mean that the rest of the affected areas are also turning greener.

“The colour changes are complex and different in different locations. In general, we see an increase in the amount of green light coming off the ocean, but in some places, we see more change in some green wavelengths than others, in some places we see increases or decreases in different red or blue wavelengths,” Cael said. The researcher added that although the study couldn’t determine “what” exact changes are happening in the oceans, it could point out “why” the changes are probably happening.

The findings suggest that a shift in colour is happening in those regions where the oceans are getting more stratified. Ocean stratification is the natural separation of an ocean’s water into horizontal layers by density, with warmer, lighter, less salty, and nutrient-poor water layering on top of heavier, colder, saltier, nutrient-rich water. Usually, ocean ecosystems, currents, wind, and tides mix these layers, creating smoothed temperature and salinity transitions between them.

But because of climate change, stratification has increased, making it harder for water layers to mix with each other, which has severe consequences — oceans are able to absorb less carbon dioxide from the atmosphere and the oxygen absorbed isn’t able to mix properly with cooler ocean waters below, threatening the survival of marine life. Moreover, nutrients aren’t able to travel up to the surface of the oceans from below. This directly impacts phytoplankton, which thrives, as mentioned before, on the upper surface of the oceans.

“This decrease in the nutrient supply should favour smaller plankton, which tends to fare better in lower-nutrient conditions, which would result in changes in the composition of the plankton (a diverse collection of organisms found in water) community, which would have signatures in the colour,” Cael explained.

Changes in the composition of the plankton population have larger effects on the marine ecosystem. Plankton has two major types: phytoplankton, which are plants, and zooplankton, which are animals. Phytoplankton are eaten by zooplankton, which are then eaten by other marine animals such as crabs, fish and sea stars, and therefore, plankton are critical in supporting marine and freshwater food webs. Any alteration in their population could throw off the whole marine ecosystem.

“The colour of the oceans has changed,” Stephanie Dutkiewicz, co-author of the study, said in a statement. “And we can’t say how. But we can say that changes in colour 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.”