Climate change could cause more than 50 percent of Earth's oceans to change color by the end of the 21st century, according to a newly published study. The blues and greens of our planet's oceans may intensify, and could serve as an indicator for the health of marine life.

Human-triggered alterations to our planet's climate are already taking their toll on the oceans. The effects include a rise in sea temperature and ocean acidification, habitat destruction through coral bleaching and the creation of more powerful waves.

The new MIT study has revealed that climate change will also intensify the color of Earth's seas, and that the shift in hue could mark large-scale changes to the marine ecosystem.

Before talking about the significance of color to marine health, we need to understand why the ocean is blue. The answer lies in how sunlight interacts with matter. Water molecules are capable of absorbing the majority of the Sun's rays, with the exception of light located in the blue part of the electromagnetic spectrum. These unabsorbed wavelengths of light are reflected away from the molecules, making them appear blue.

However, Earth's oceans are not comprised of water alone. They are home to a stunning range of lifeforms of all shapes and sizes, all of which have unique properties and reflect different wavelengths of light. Therefore, the color of an ocean is partially a reflection of the life dwelling within, and can be used as an indicator of its health.

Among this aquatic menagerie are lifeforms called phytoplankton, a type of microscopic marine microalgae that contain significant amounts of chlorophyll. This natural pigment excels at absorbing the blue part of the electromagnetic spectrum and reflecting green light.

Enormous swarms of phytoplankton are responsible for turning vast swathes of water green. Scientists are able to use satellite imagery and data to estimate the amount of chlorophyll, and therefore the quantity of phytoplankton, present in an ocean region.

However, it is sometimes challenging to isolate the driving force behind an observed change in phytoplankton levels. Whilst it is possible that climate change could be the cause of a population swing, it could also result from natural causes, such as an El Niño event altering the level of nutrients in the marine environment.

Scientists behind the new study sought to use computer modelling to predict the effect that climate change will have on the populations of different species of phytoplankton, as ocean temperatures continue to rise. Their model takes into account how phytoplankton reflect light, and predicts how a change in microalgae populations would alter the color of the light from the water's surface.

Rather than create a computer model from scratch, the team modified an existing tool that had been used to predict phytoplankton population changes as a result of rising temperatures and ocean acidification. The model took into account phytoplankton characteristics such as feeding and growth behaviors, and was capable of simulating ocean currents and mixing dynamics.

The existing model was altered to allow it to estimate the specific wavelengths of light that would be absorbed, and those that would be reflected back out by the ocean depending on the quantity and diversity of organisms that are present in the body of water.

Results of simulations created by the team matched closely enough with past observations made by real-world satellites, for the researchers to be confident that their updated model was capable of predicting ocean colors resulting from specific environmental changes.

The scientists then simulated what parts of Earth's oceans would look like by the end of the century under a "business as usual" approach, wherein little is done to curtail greenhouse gasses, and the global temperature has risen by up to 3 °C.

In this scenario, computer modelling predicts that by the year 2100, there will be even less microalgae in the blue regions of Earth's ocean than there is today, causing it to appear a more vivid hue of blue. Meanwhile, current algae-rich areas will experience a phytoplankton bloom triggered by rising temperatures, causing them to appear greener.

"The model suggests the changes won't appear huge to the naked eye, and the ocean will still look like it has blue regions in the subtropics and greener regions near the equator and poles," says lead author Dr. Stephanie Dutkiewicz, a principal research scientist at MIT's Department of Earth, Atmospheric, and Planetary Sciences and the Joint Program on the Science and Policy of Global Change. "That basic pattern will still be there. But it'll be enough different that it will affect the rest of the food web that phytoplankton supports."

The changes should be visible to satellites in low-Earth orbit.

A paper detailing the work has been published in the journal Nature Communications.

Source: MIT