Phytoplankton in peril, global ocean productivity plummeting, CSIR-led study warns

Decreasing ocean productivity trends across Africa (Photo-CSIR)

12 Feb 2025

New research led by the CSIR has uncovered a troubling global trend – a significant decline in ocean productivity over the past 26 years, with consequences for marine biodiversity, fisheries, and the planet’s climate regulation systems.

An alarming new study led by the Council for Scientific and Industrial Research (CSIR), analysing 26 years of satellite data, reveals climate models are underestimating the decline in ocean productivity, a critical process driven by phytoplankton that supports and regulates the climate.

The researchers found that even the best climate models fail to capture the full extent of the decline, signalling potentially severe consequences for marine ecosystems, fisheries and carbon absorption.

What is ocean productivity and why should we be concerned that it’s declining? It’s all about phytoplankton.

CSIR researchers explained that ocean productivity essentially refers to how much food and energy the ocean produces to support marine life; it is mainly driven by phytoplankton – tiny organisms that use sunlight to grow and provide nutrients for marine life.

They also help absorb carbon dioxide from the atmosphere, playing a vital role in climate regulation and sustaining the global ecosystem.

If ocean productivity is declining, that means there are fewer nutrients for marine life, which can affect fisheries, the trophic system, marine resources that uphold human livelihoods and the planet’s climate regulation systems.

Unfortunately, this is exactly what the new study by the CSIR has found. Climate models have been underestimating to what extent it has been declining.

Satellite-derived trends in ocean productivity from 1998 to 2023. (Image: CSIR)

Phytoplankton in peril

In an interview with Daily Maverick, Dr Thomas Ryan-Keogh, a principal researcher at the CSIR, explained that ocean productivity is one of the most important processes on the planet, as it fuels marine food webs and helps to regulate the global carbon cycle.

In the same way that a vegetable garden or a farm needs the right amount of nutrients, sunlight and water to produce a good harvest, phytoplankton need the same thing.

They need the right amount of sunlight. They need the right conditions and the right nutrients to thrive, grow and be productive, but climate change is altering those conditions, it’s changing the environment that they live in.

However, the research and climate models to date have painted an unclear picture of how it will respond to a changing climate, making it hard for policymakers and the general public to understand the urgency needed to actively address the ongoing changes in ocean productivity accurately.

This is what Ryan-Keogh, CSIR chief researcher Dr Sandy Thomalla and Professor Alessandro Tagliabue from the University of Liverpool sought to clear up with their new study, Global decline in net primary production underestimated by climate models, recently published in Nature’s scientific journal Communications Earth & Environment.

To do this, they combined ocean productivity estimates from 26 years of satellite data, from 1998 to 2023, and projected end-of-century trends from available earth system models to assess the climate impacts on marine ecosystems and the state of oceans.

During a briefing on Wednesday where the CSIR released this study, Ryan-Keogh said there had been a lack of consensus in the direction of change of future ocean productivity, and the magnitude of this change, resulting in mean change across models being negligible with a large uncertainty.

Some climate models suggest that ocean productivity will increase in the future, while others suggest that it will decline, but overall, the study found that even the best-ranked models are underestimating the magnitude of decline.

To predict the anticipated changes in ocean productivity, the researchers developed a model ranking scheme that assessed how well each model could represent the relationships between ocean productivity and different environmental drivers.

According to the study, the models which best represented these relationships and were ranked highest, all predicted future declines in ocean productivity.

However, it found that the future projections of even the best-ranked models were underestimated and much smaller than the changes the researchers were observing now.

ocean (Graph: CSIR)

The researchers argue that the next generation of climate models will need to correct this sensitivity to represent the ongoing changes in ocean productivity accurately.

This means that these models will project even greater declines, even under high-mitigation scenarios that currently predict relatively stable ocean productivity at the end of the century.

Declining ocean productivity

Analysing all the data from these satellites required a lot of computing power. Fortunately, Ryan-Keogh and his team are based at the Centre for High Performance Computing in Rosebank, home to Africa’s only supercomputer.

“The amount of the oceans warming is not uniform across the globe. In particular, places like the poles are warming a lot faster, and that’s because warm water is not going to absorb that much heat. But cold water can heat up quite a lot, so places like the Arctic and the southern ocean around Antarctica, those are particularly vulnerable,” Ryan-Keogh said.

Thomalla said it was important to note that ocean productivity was not declining everywhere.

“The ocean is patchy, and there are some areas where productivity is increasing, but when we average it across the global ocean, it is decreasing. If we zoom in on the African coastline, for example, we can see large declining trends in ocean productivity inshore, relative to the smaller increasing trends seen in open ocean waters further offshore,” Thomalla said.

africa Decreasing ocean productivity trends across Africa. (Image: CSIR)

Mitigation scenarios

Ryan-Keogh presented two mitigation pathways the world could take by the end of the century.

In the low mitigation scenario, where the world does not reduce its CO2 emissions and move towards green economies, the researchers found this would result in very large increases in temperature (anywhere between an increase of 3.6 to 4.4°C). This will result in very large decreases in ocean productivity, which means we will lose a lot of ocean ecosystem services.

Conversely, the second scenario was a high mitigation scenario where the world does reduce its CO2 emissions and reliance on fossil fuels, which results in much lower increases in temperatures (between 1.4 to 2.7°C). This would result in much smaller decreases in ocean productivity in the future, and we won’t necessarily lose all of our ocean ecosystem services.

Ryan-Keogh said, “The more that we mitigate now, the less ocean primary production will decline in the future.”

What they argue in the study is that even under those high mitigation scenarios where the world significantly reduces its CO2 emissions, because some warming is already locked in, there will be declines in ocean production.

This is what the CSIR’s model is demonstrating. But the degree, by how much it declines, depends on which pathway the world decides to take.

“If we don’t do anything and we see a 4.6°C increase by the end of the century, you could be looking at massive dead zones across the ocean with no food web to speak of, climate collapse, tipping points in ecosystems like the rainforest, and the loss of significant glaciers. All of these tipping points are important and we need to try and prevent them from happening.

“The only way we can do that is with these high mitigation scenarios. We have to find ways to reduce our reliance on fossil fuels, which is not always easy,” Ryan-Keogh said.

Thomalla said that this was a call to attention. Climate models are currently underestimating the future change in ocean productivity.

“These global declines will have ramifications for the ocean carbon cycle and marine ecosystems, affecting the trophic system that underpins biodiversity, fisheries and the marine resources on which humans rely,” Thomalla said.

Ryan-Keogh reiterated that reducing reliance on fossil fuels was key to arresting this decline, but that there needed to be more active ways to reduce other ocean stressors, like reducing pollution released into the oceans.

One way the public and policymakers could positively affect marine environments during this crisis was by expanding and increasing enforcement of Marine Protected Areas (MPA).

“Every time a government establishes an MPA to protect that ecosystem, you see sea life flourish. The moment we stop applying pressure on an ecosystem, it finds a way to bounce back. They are very resilient ecosystems, and the less we do to them, the more that they can rejuvenate, replenish and flourish again. Ultimately, that benefits us because they will keep the global ecosystem going,” Ryan-Keogh said. DM

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Phytoplankton in peril

Declining ocean productivity

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