Climate models point to several local forces shaping productivity in the world’s major coastal upwelling regions.
Eastern Boundary Upwelling Systems are coastal regions where deep, nutrient-rich water rises toward the surface. They support high levels of marine productivity and important fisheries. Researchers examined how productivity could change during the 21st century using Earth system models from the sixth phase of the Coupled Model Intercomparison Project.
Weaker winds explain some projected declines, especially in parts of the Canary, Benguela and California systems. But across much of the remaining area, the models point to other influences, including changes in ocean circulation, stronger layering of the ocean and shifts in the supply of nutrients below the surface.
What shapes future productivity
The study examined projected changes in net primary production, the growth of marine plants that forms the base of ocean food webs, across four major coastal upwelling systems. Model differences were larger than differences between future scenarios, and the models showed limited agreement on the overall direction of change.
In parts of the Canary and Benguela systems near the equator, and in the historically most productive part of the California system, weaker winds along the coast reduce upwelling, the delivery of nutrients to sunlit surface waters and, consequently, productivity. These areas together make up 25% of the total area of the four systems. In the remaining 75%, projected changes require additional explanations, including changes driven by large-scale ocean transport and shifts in wind patterns, stronger ocean stratification, and changes in subsurface nutrient stores. Consistent responses across models appeared mainly at the scale of individual subsystems rather than across each entire system.
Models and uncertainty
The researchers analyzed 21st-century projections from Earth system models included in CMIP6, comparing productivity with several possible driving mechanisms. This is model-based evidence rather than a direct measurement of future ocean conditions.
The models showed greater disagreement with one another than across scenarios, and they did not consistently agree on the direction of productivity changes across whole upwelling systems. Agreement was stronger in some subsystems, but the study reports limited confidence in broad future trends.
