AMOC Stability Depends on Warming Speed, Not Heat Level
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- René van Westen at the University of Utrecht led a climate model simulation finding that AMOC stability is controlled by the speed of warming rather than a fixed temperature threshold.
- Under very slow CO2 increases of 0.5 ppm per year, the AMOC remained stable beyond 5°C of warming; under current rates (roughly 10x faster), it collapses at about 2°C above pre-industrial levels—potentially around 2060.
- The mechanism: rapid warming makes surface waters lose density faster than deep waters, stratifying the ocean and blocking the vertical mixing that drives the current.
- David Armstrong McKay at the University of Sussex cautioned that higher-complexity models vary widely and have biases, so the finding is not yet a 'clear-cut, real-world projection.'
- Camille Hankel at the University of Washington argued that slowing the rate of CO2 increase immediately may be more crucial for averting AMOC weakening than limiting total cumulative emissions.
- A collapse would drop European winter temperatures by 10 to 30°C and dramatically shift global rainfall patterns, per the research team.
Why it matters: Climate policy has long focused on capping total cumulative emissions, but Camille Hankel's framing suggests the pace of CO2 reduction may matter more than the absolute cap for AMOC survival—potentially reframing how policymakers weigh speed-versus-magnitude tradeoffs. For vulnerable regions like Europe, a 2060 collapse timeline makes the next two decades of emissions decisions materially consequential.
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