Ocean productivity has been declining globally for the past three decades, with repercussions for fisheries, marine biodiversity, and the planet’s climate regulation systems.
Ocean productivity refers to how much food and energy the ocean produces to support marine life.
According to a new study led by the Council for Scientific and Industrial Research (CSIR), analysing 26 years of satellite data, climate models underestimate the global decline in the productivity of the oceans resulting from ocean warming.
Their finding follows an analysis of decades of remote sensing data and comparing these contemporary trends to future projections from a wide range of climate models.
CSIR principal researcher Dr Tommy Ryan-Keogh led the study alongside CSIR chief researcher Dr Sandy Thomalla, in collaboration with Professor Alessandro Tagliabue from the University of Liverpool.
Ryan-Keogh said ocean productivity is one of the most critical processes on the planet because it helps to regulate global climate and supports essential ecosystem services such as fisheries and it is driven by phytoplankton - the tiny plants, invisible to the human eye that live in the sunlit surface of ocean waters.
Phytoplankton are also responsible for almost half of all the oxygen in the world. They use carbon dioxide from the water to make oxygen.
Despite the importance of phytoplankton, scientists do not have a clear understanding of how climate change can impact them. Some climate models suggest that ocean productivity will increase in the future, while others suggest that it will decline.
Thomalla said it is important to note that ocean productivity is 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”.
Ryan-Keogh said to begin to address this disagreement and subsequent uncertainty between model projections of ocean productivity, the researchers published a study entitled “Global Declines in Net Primary Production Underestimated by Climate Models”.
In this study, the researchers used a novel approach combining 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.
“This is one of the first studies to use data from the past 26 years to assess how ocean productivity may be changing and to link these current changes to the future state of our oceans,” said Ryan-Keogh.
To predict the anticipated changes, the researchers developed a model ranking scheme that assessed how well each model can represent the relationships between productivity and different environmental drivers.
The models that best represent these relationships and are ranked highest all predict future declines in ocean productivity.
“We believe that even the best-ranking models are underestimating the sensitivity of declines in productivity to ocean warming,” said Ryan-Keogh.
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.”
Thomalla said: “This study is a call to attention. Our 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.”
Chief among the causes of global and ocean warming is CO2 (carbon dioxide) emissions stemming from the use/reliance on fossil fuels such as coal, oil, and natural gas for human activities in various industries.
Dr David Glassom, a Marine Biology lecturer at the University of KwaZulu-Natal, said this issue is urgent.
While the report emphasises that ocean productivity has been declining for at least three decades, productivity has almost certainly been declining for longer than that, he said.
“So, even if CO2 (Carbon dioxide) emissions stop now, the effects will persist well beyond a human lifespan. In this sense, you could say that irreversible effects have already occurred.
“The combination of loss of ocean productivity, especially in combination with other climate effects on ocean life, including fish and other anthropogenic effects such as pollution and overfishing will definitely affect economies that rely on fishing,” Glassom said.
He added that while the industries are unlikely to disappear, they will definitely have to adapt to new circumstances and work harder to achieve their catches.
“As the report points out, the loss of productivity will not be uniform over time or in different places. The effects on small-scale fishers will similarly change from place to place. Small-scale fishing is often a marginal business, even under normal conditions, so the threat to communities that rely on it is greater than to the public at large.
“At the least, small-scale fishers may find themselves having to spend more time and travel further to attain adequate catches, incurring increased costs, while long-term reduced income from fishing is a real likelihood,” he said.
On solutions to mitigate the effects, he said, the obvious step that needs to be taken is an immediate and steep reduction in the emission of greenhouse gasses. However, the effects of gases already emitted will last for decades to centuries.
“It has been shown that reducing local stresses on ocean habitats can provide some resilience in the short term to climate effects.
“Steps to reduce stresses can include pollution reduction, to assure ocean health at local scales, the declaration of more Marine Protected Areas (MPAs) to allow fish stocks the best chance of recovery, and urgent efforts to provide other livelihoods in fishing communities to minimise economic impacts,” he said.
MPAs are areas of the ocean set aside for long-term conservation aims.
He added that greatly increased effort in monitoring is important to understand the effects of declining productivity.