Thermohaline circulation: ‘ocean conveyor belt’ can continue climate change beyond Net Zero

Could ocean currents ensure that global warming continues even after we have reached carbon-neutrality? 

One of the concepts that is crucial to academic climate science but not widely known to the public is the equilibrium climate sensitivity (ECS) – more than this year’s temperature, it’s this number that we need to be worrying about. 

A theoretical tool, the ECS is an estimation of the long-run (100+ years) surface temperature after a doubling of atmospheric CO₂, accounting for long-term ocean warming and feedbacks. Current modelling puts it in the range of 2.5°C to 4.5°C.¹  

What’s most important is that if concentrations hold steady (which would require net-zero emissions, by the way), global warming will continue for decades to come, risking a catastrophic and irreversible breakdown in our food systems.  

Full cessation of carbon emissions should begin a slow fall in GHG concentrations since anthropogenic CO2 takes a little longer to be absorbed than biogenic, but this would likely only result in flatlining – not declining – temperatures for centuries.  

But why? 

Boiling a moving pot 

Unfortunately, the ocean is not static, it is a dynamic, ever-moving mass – much closer to a river than a lake, as it were. 

Across the entire globe, the ocean is animated by surface currents moving in the opposite direction to deep currents, with several different points at which it descends or ascends. This is the thermohaline circulation.  

Thermohaline circulation is driven by differences in temperature and salinity, which affect water density. Cold, salty water is denser than warm, fresh water, causing the former to sink below the latter.  

As seawater moves toward the poles, it cools and becomes saltier due to sea ice formation, increasing its density.  

This dense water sinks to the deep ocean, forming deep water currents that drive global ocean circulation. Warm water moves toward the poles to replace the sinking water, completing the cycle and helping regulate Earth's climate. 

When the oceans warm in line with changes to the planet’s radiative forcing, as long as temperature and salinity differentials are maintained, thermohaline circulation works to further extend the time it takes for the ocean to warm up. 

If our current radiative forcing was maintained by achieving a type of Net Zero where annual carbon emissions are exactly square with maximum annual sequestration rates, temperatures could continue to rise for many years as we slowly heat this dynamic ocean. 

Will the oceans warm faster if the AMOC collapses? 

However, there’s growing evidence that a keystone of the thermohaline circulation, the Atlantic Meridional Overturning Circulation (AMOC) could imminently slow down or collapse altogether.^{2 3} 

The AMOC is a major component of the global conveyor belt, responsible for transporting warm tropical water northward and pulling cold water deep into the ocean.  

But melting ice from Greenland and the Arctic is flooding the North Atlantic with fresh water, disrupting the delicate balance of salinity and density that drives this circulation. 

If the AMOC weakens significantly or collapses, the consequences could be severe. More heat would become trapped in surface waters rather than being distributed to the deep ocean, accelerating warming at the surface and intensifying global temperature rise.⁴ 

Disruptions to ocean circulation would also reshape global weather patterns, potentially leading to stronger storms, harsher droughts, and shifts in monsoon systems.  

While overall global temperatures would continue rising, paradoxically, parts of Europe could experience significant cooling as the weakened Gulf Stream fails to bring warm waters north.^{5 6} 

1 The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. Forster et al. Sixth Assessment Report of the Intergovernmental Panel on Climate Change. 2023. 

2 Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Peter D. Ditlevsen and Susanne Ditlevsen. Atmospheric and Oceanic Physics. 2023.    

3  Weakening of the Atlantic Meridional Overturning Circulation driven by subarctic freshening since the mid-twentieth century. Garbiel M. Pontes and Laurie Menviel. Nature Geoscience. 2024.   

4 Open Letter by Climate Scientists to the Nordic Council of Ministers. 2024. 

5 Shifts in national land use and food production in Great Britain after a climate tipping point. Ritchie et al. Nature Food. 2020.   

6 Running AMOC in the farming economy. Tim G. Benton. Nature Food. 2020.