Of 15 global Earth system models of how climate change will impact oceans, those that project a future decline in primary production in oceans correlate most closely with remote sensing satellite data from 1998 to 2023, Council for Scientific and Industrial Research (CSIR) principal researcher Dr Tommy Ryan-Keogh says.

The CSIR research team found that five models, which ranked highest in terms of the projections of their algorithms compared with the 26 years of satellite data, suggest that the primary production of oceans will decline into the future.

The best-performing algorithms suggest that the world may face a 7% to 10% decline in primary productivity of oceans, although this is likely to vary across ocean regions, with some areas set to decline in productivity, while others may see an increase, he says.

“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,” says CSIR chief researcher Dr Sandy Thomalla.

“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,” she explains.

The changes in ocean productivity are important to consider for food security and to prevent the bottom of the ocean food-web from declining, says Ryan-Keogh.

Analysis of the 26 years of satellite data showed a widespread decline in ocean productivity affecting ocean ecosystem services. The study also indicated that climate models underestimate the global decline in the productivity of the oceans resulting from ocean warming.

About three-billion people worldwide depend on seafood as a main protein source and, in some countries, as much as 50% of the population relies on seafood as a source of protein, he says.

Primary production refers to the blooms of phytoplankton, driven by photosynthesis, that drive season variations in oceans' productivity.

Ocean productivity is a critical process on the planet because it helps to regulate global climate and supports essential ecosystem services, such as fisheries.

For each of the 15 Earth system models, the team, which included Ryan-Keogh, Thomalla and University of Liverpool's Professor Alessandro Tagliabue, modelled the systems' projections for the 26 years for which the team had data available and then modelled their projections to the end of the century.

However, even the highest-accuracy models do not accurately model the changes to oceans observed in the data over the 26 years and, therefore, do not create much faith in their accuracy to project future changes in ocean productivity, Ryan-Keogh notes.

Further, the declines in primary production modelled by the most accurate models still underestimated the magnitude of the decline, and the correlation between the models' projections and observed data is still not accurate enough, he says.

Any changes in the productivity of oceans are likely to impact developing and Global South countries to a greater degree than developed countries, he adds.

The team modelled two pathways; one in which climate mitigation is low and one in which climate mitigation is high and carbon dioxide is reduced.

Under the low-mitigation scenario, large temperature increases between 3.6 °C and 4.4 °C result in large decreases in ocean productivity, which would result in the loss of a lot of the ocean ecosystem services, says Ryan-Keogh.

The higher mitigation pathway results in lower temperature increases of between 1.4 °C and 2.7 °C, leading to smaller decreases in primary ocean production in the future and the loss of less of the ocean ecosystem services.

The more swiftly the world mitigates climate change and reduces greenhouse-gas emissions, the less primary production will decrease in the future, he says.

“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,” says Thomalla.

The lack of consensus on the direction and magnitude of the projected change in ocean productivity from the models undermines efforts to assess climate impacts on marine ecosystems with confidence.

It may suggest that a new research area of exploration be pursued collectively, she says.