The sky and the ocean both appear blue in colour, but not because either one is a reflection of the other – it’s down to how they absorb or scatter sunlight. While Earth’s atmosphere scatters the blue end of sunlight’s spectrum the most, its ocean absorbs the light at the red end and leaves the blue behind.
Both of these effects result in a blue colour visible to the human eye, and not only on the ground in the ocean’s case – given that more than 70% of Earth’s surface is covered by ocean, it also appears as a “blue planet” from space.
However, according to a new study published in Nature last month, the world’s oceans overall appear to be changing colour: They’re getting greener.
Using satellite data, the group behind the study observed greening trends across the surface of more than half of the planet’s oceans over the past couple of decades.
“On the whole, low-latitude oceans have become greener in the past 20 years,” states the study.
Why do they think this is happening? Well, possibly because of climate change.
Scientists thought they would need many more years of data before they could spot signs of climate change in the colour of the oceans https://t.co/mBGfBV2Fgy
— nature (@Nature) July 14, 2023
How Was the Colour Change Detected?
The group analysed 20 years of ocean colour data collected by one of the scientific instruments onboard “Aqua” – one of NASA’s Earth Science satellites.
This instrument is known as the Moderate Resolution Imaging Spectroradiometer (MODIS), which determines ocean colour by measuring light reflected off the surface of the water.
Upon analyzing MODIS’ time series dataset from 2002-2022, the group observed significant colour change trends over 56% of the world’s oceans during this time – particularly in the subtropical and tropical regions.
What’s Causing the Greening?
Although the total environmental impact of human-driven climate change is still unclear, we do know it’s having a profound impact on Earth’s ecosystems – marine ecosystems included.
And as the Nature study highlights, climate change has been shown to affect surface-ocean ecosystems at specific locations, but it can be hard to monitor such changes on a global scale over long periods of time.
That’s where satellite remote sensing of ocean colour comes in: Because changes in the colour of the ocean can indicate changes to the life existing on its surface.
According to the group, the observed colour change trends in the world’s oceans over the past couple of decades reflect relative changes in marine ecosystems – changes which they suggest could be caused by climate change.
Although they’re not sure of the exact ecological shift that’s causing the colour change – and they also mention the greening could be caused by “an increase in detrital particles” – the study’s scientists suggest that the greening could also be down to the effect of climate change on phytoplankton communities.
Regardless of the cause, the group note that the observed changes will have “ecological implications.”
What Are Phytoplankton?
With a name derived from the Greek for “plant” (phyton) and “wanderer” (plankton), phytoplankton are microscopic plant-like organisms that live near the ocean’s surface and are of great ecological significance.
Not only are phytoplankton integral to the aquatic food web and marine biodiversity, but they also form a vital component of the ocean carbon cycle. They sequester enormous amounts of carbon dioxide from the atmosphere, and produce a significant portion of the oxygen that we breathe through photosynthesis, acting as a crucial part of the “ocean biological carbon pump.”
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That’s where the green colour comes in: Although phytoplankton are a diverse group comprised of organisms like bacteria, protists and single-cell plants, they all photosynthesise and so all contain chlorophyll – the green pigment which absorbs sunlight.
Therefore, areas of the ocean that appear greener in colour are generally speaking likely to be home to more phytoplankton.
Why Do They Think Climate Change is Involved?
As the study states: “A key question is whether the identified trends are driven by climate change.”
To see how climate change might play into the colour change, the group used a computer simulation to model changes in the global ocean ecosystem during the 21st century (2000-2105) under two different scenarios: One with high greenhouse gas emissions and one with emissions consistent with the levels of 1860 as a control.
“Thus, the differences between the simulations indicate anthropogenically driven climate change,” states the study.
When comparing the trends that emerged in the high emissions simulation with the real observed trends in the world’s oceans, similarities became apparent, suggesting climate change as the driving factor behind the colour change.
How do they think climate change is affecting phytoplankton? The group notes that although changes in sea surface temperature (SST) do not appear related to the observed trends, changes in the structure and stratification of the ocean’s layers – factors known to change with climate and also affect phytoplankton – might be the culprits.
“Altogether, these results suggest that the effects of climate change are already being felt in surface marine microbial ecosystems,” states the study.
What’s the Significance of These Findings?
Due to the ecological importance of phytoplankton for ocean ecosystems, biodiversity and carbon storage, the observed ocean colour change – possibly indicating changes in the structure and biomass of phytoplankton communities – could have implications for the marine environment.
The group behind the study note how their findings may be relevant for ocean conservation and governance, specifically pointing out how monitoring changes in surface-ocean microbial ecosystems could help inform where to establish marine protected areas (MPAs) as part of the recently adopted and historic United Nations High Seas Treaty.
They also highlight the importance of satellite data and the value of long-term missions such as NASA’s MODIS-Aqua, stating that “for future work, merged multi-satellite products, as well as work that is currently underway to improve them, are essential.”
In fact, NASA are actually advancing towards the scheduled 2024 launch of a new satellite science mission to study ocean colour, biology and health called PACE (Plankton, Aerosol, Cloud, ocean Ecosystem), announcing in April this year that the spacecraft has been successfully assembled.
Furthermore, the group also call for ongoing work to delve deeper into studying such trends further to understand what kind of surface-ocean ecosystem shifts could be behind the colour change, stating:
“Given the key role of plankton ecosystems in marine food webs, global biogeochemical cycles and carbon cycle–climate feedbacks, detecting change in these ecosystems is of great utility.”
Editor’s Note: The opinions expressed here by the authors are their own, not those of Impakter.com. — In the Featured Photo: “Spring Color in the North Sea.” Featured Photo Credit: NASA Visible Earth. This image (taken May 5, 2018) originally appeared in the NASA Earth Observatory story Spring Color in the North Sea. NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey and MODIS data from LANCE/EOSDIS Rapid Response. Story by Mike Carlowicz.