Along the west coast of North America, seasonal upwelling has long been the engine of one of the most productive marine ecosystems on the planet. By bringing cold, nutrient-rich waters to the surface, it sustains plankton, fisheries, and coastal communities. But this same process is now revealing how climate change is reshaping coastal oceans – often in ways that are invisible at the surface.
In a recent CMCC Lecture, Bassi Fellow Samantha Siedlecki (University of Connecticut) explores how the Northern California Current System is experiencing compound ocean change, where multiple stressors – warming, ocean acidification, and deoxygenation – act together and reinforce one another. Long-term observations show a clear signal: as atmospheric CO₂ rises, surface ocean CO₂ follows, pH declines, and conditions become increasingly corrosive for marine organisms that rely on calcium carbonate to build shells. These changes are among the most confidently projected global climate signals – but near the coast, their intensity and impacts can be profoundly altered.
“As a coastal oceanographer, I see the coast as the place where people make connections to the ocean,” comments Siedlecki after the webinar. “It’s where global change becomes locally relevant. Fishermen notice differences in the species they catch, and people see changes at the beach. Coasts provide an opportunity to bring large-scale changes into a local context.”
Studying these coastal changes is far from straightforward. Global climate models reliably simulate large-scale trends in pH and oxygen, but as these signals reach the coast, local processes such as mixed-layer dynamics, river inputs, and sediment interactions can significantly modify their impact. This means that high-resolution, localized projections are essential to understand the true extent of stressors on marine life. By applying dynamical downscaling and advanced forecasting systems, scientists can capture these fine-scale processes and provide actionable information for managers and communities.
“Connecting global change to coasts requires a lot of information. My group runs high-resolution simulations with complex datasets, which is quite computationally heavy. Observational records are often short, especially for phenomena like ocean acidification. Therefore, to understand what has happened at the coastal level, we use models to extend modern observations back in time,” says Siedlecki. “We also use these tools to forecast and project into the future. With the right information, we can generate forecasts from weather timescales to climate timescales hundreds of years ahead. The challenge is helping people understand how to use this information and trust these new tools, whilst identifying the decisions these forecasts can support. There’s a lot of promise and we already have some successful case studies.”
High-resolution coastal simulations reveal that surface and subsurface waters are changing at different rates. While surface waters are warming faster, bottom waters are experiencing more severe declines in oxygen and carbonate saturation. This matters deeply for species such as the Dungeness crab, one of the most economically important for fisheries on the US West Coast.
These crabs are sensitive to changes in oxygen, pH, and temperature. Siedlecki’s research shows that over the past three decades, compounded exposure to these stressors has increased, particularly in subsurface waters, which are changing faster than surface layers. Maps produced in collaboration with crab managers illustrate the expansion of corrosive and hypoxic zones, highlighting how habitat compression could threaten this vital fishery if trends continue.
This is an example of the effects of compound change, where multiple stressors acting simultaneously cannot be understood by looking at a single factor in isolation. Surface and bottom waters respond differently, and coastal retention of upwelled waters modifies the rates of acidification and deoxygenation. These insights are not only critical for ecosystem science but also for guiding fisheries management, conservation, and climate adaptation strategies. Understanding these coastal processes, Siedlecki emphasizes, is essential to protect both marine life and the communities that depend on it.
A key insight from this research is that it’s not just the strength of upwelling that is changing, but its retention and vertical structure, linked to shifts in mixed-layer depth. These dynamics amplify deoxygenation and acidification on the continental shelf – processes that global climate models struggle to capture. As a result, subsurface monitoring and localized climate projections become essential tools for understanding risks and supporting fisheries management and coastal resilience.
Coastal ecosystems sit at the frontline of climate change, where global signals meet local processes. Understanding compound change and translating it into actionable, place-based information is critical for protecting marine resources and the communities that depend on them.
“I really hope that people will one day be able to look at ocean forecasts the way they do weather, right on their mobile phone,” says Siedlecki. “That’s the vision: to have information so readily accessible that people can develop trust in these new tools.”
More information:
Watch the lecture and learn more about CMCC research on ocean change and coastal resilience:
