The Speed of Warming, Not Just Its Size, Could Decide an Ocean Current's Fate

For years, climate scientists have debated a single number: how many degrees of warming it would take to shut down the ocean current system that keeps Western Europe mild. A new study from Utrecht University, published August 13 in Nature Climate Change, argues that question has been framed wrong. In their climate-model simulations, the same current collapsed at just 2°C of warming when carbon dioxide rose quickly, yet stayed strong past 5°C when it rose slowly. The temperature was not what mattered. The speed was.

KEY NUMBER
The model puts the critical warming rate at around 0.3°C per decade, a pace the researchers say the world is already approaching.

What Happened

Lead author René van Westen and colleagues at Utrecht's Institute for Marine and Atmospheric Research ran the Community Earth System Model twice, holding the eventual CO2 increase constant but changing how fast it arrived. Under the slow-warming run, the AMOC's strength held steady even as global temperature climbed past 5°C. Under the fast-warming run, matched to something close to today's actual emissions pace, the circulation collapsed at roughly 2°C. Since the widely cited estimate of a 4°C collapse threshold comes from studies that mostly used fast-forcing scenarios, the team argues that number was never a fixed property of the ocean, but an artifact of how quickly those simulations pushed the climate to change.

Why It Matters

The AMOC moves warm surface water north and colder, saltier water back south at depth, redistributing heat across the planet and moderating winters in Northwestern Europe. Much current climate policy, including the logic behind "overshoot" pathways that allow temporary spikes past a target with the hope of pulling temperatures back down later, is built around hitting a peak-temperature number. This study complicates that framing: if the ocean's stability depends on how fast it gets pushed rather than how far, a slow, gradual approach to the same eventual warming level could avoid a collapse that a rapid approach would trigger, even if both end up in the same place. That logic could extend to other slow-adjusting climate systems. It also lines up with separate projections that rising AI-driven fossil-fuel output could add billions of tonnes of CO₂ a year, a trend that would push emissions rates up rather than down.

Why the Rate Matters More Than the Peak

Co-author Henk Dijkstra explains the mechanism as a question of adjustment time. Under slow warming, the entire ocean column, from the surface to the deepest layers, has decades to gradually reorganize its temperature and salinity in step with the changing climate. Under fast warming, the deep ocean can't keep pace with surface changes. That mismatch destabilizes the circulation before it can adapt. Co-author Reyk Börner notes that the team deliberately chose a slow-warming scenario far below today's actual pace specifically to isolate the effect of speed from the effect of eventual temperature.

What Happens Next

The Utrecht group says its next step is testing whether the rate-dependent stabilizing mechanism holds up in other climate models and under more realistic, variable emissions paths rather than the two constant-rate scenarios used here. Van Westen has framed the practical message as one about deceleration: reaching a given temperature target more slowly, even without lowering that target, may lower near-term collapse risk. Whether policymakers built around overshoot pathways, which explicitly plan for a temporary peak followed by cooling, will treat the pace of emissions as a variable worth managing on its own remains an open question.

Takeaway

The widely repeated "4°C" collapse threshold was never a fixed property of the Atlantic's circulation. It was a number produced mostly by fast-forcing model runs. The new study shows that changing only the speed of warming, while holding the eventual temperature roughly constant, moves that threshold by several degrees in either direction. That's a distinction between what a model assumes about pace and what it concludes about temperature. It's the specific gap this study closes.