Changing sea colour evidences extent of climate change

The deep-blue sea is turning a touch greener, according to UK research based on NASA data.

NASA said this change is important because the colour of the ocean surface is indicative of the ecosystem that lies beneath. Communities of phytoplankton, microscopic photosynthesizing organisms, abound in near-surface waters and are foundational to the aquatic food web and carbon cycle. 

The organisation said that this shift in the water’s hue confirms a trend expected under climate change and signals changes to ecosystems within the global ocean, which covers 70% of the Earth’s surface.

Researchers led by B B Cael,  a principal scientist at the UK’s National Oceanography Centre, revealed that 56% of the global sea surface has undergone a significant change in colour in the past 20 years. After analysing ocean colour data from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on NASA’s Aqua satellite, they found that much of the change stems from the ocean turning more green.

The study focused on tropical and subtropical regions, excluding higher latitudes, which are dark for part of the year, and coastal waters, where the data are naturally very noisy.

The team discerned an ocean colour trend that had been predicted by climate modelling, but one that was expected to take 30-40 years of data to detect using satellite-based chlorophyll estimates. That’s because the natural variability in chlorophyll is high relative to the climate change trend. The new method, incorporating all visible light, was robust enough to confirm the trend in 20 years.

At this stage, the researchers said, it is difficult to say what exact ecological changes are responsible for the new hues. However, the authors posit, they could result from different assemblages of plankton, more detrital particles, or other organisms such as zooplankton. 

It is unlikely the colour changes come from materials such as plastics or other pollutants, said Cael, since they are not widespread enough to register at large scales.

“What we do know is that in the last 20 years, the ocean has become more stratified,” he said. Surface waters have absorbed excess heat from the warming climate, and as a result, they are less prone to mixing with deeper, more nutrient-rich layers. This scenario would favour plankton adapted to a nutrient-poor environment. The areas of ocean colour change align well with where the sea has become more stratified, said Cael, but there is no such overlap with sea surface temperature changes.