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A possible tipping threshold for a key North Atlantic ecosystem has been identified in a new study led by researchers at the National Oceanography Centre (NOC) and the TerraFIRMA project. Published in the Journal of Geophysical Research, the research explored how the North Atlantic subpolar gyre and its phytoplankton bloom might respond to different levels of global warming and emissions pathways.

The North Atlantic Ocean has an annual spring bloom of microscopic algae that supports its marine food web and ecosystems. This bloom occurs because winter mixing brings up nutrients from the deep ocean to the surface, particularly in the subpolar gyre. With global warming, research indicates that mixing in the gyre is likely to decline rapidly over the coming decades.

Using computer simulations of the UK Earth System Model under different carbon dioxide (CO₂) emission scenarios, the researchers found that under continued positive emissions cause the bloom to become progressively smaller as mixing in the gyre declines. Reaching net zero before global warming exceeds 1.5-2°C allows the bloom to recover, although returning to pre-industrial conditions would take more than five centuries. Once this threshold is exceeded, net zero alone is insufficient to reverse the decline. While future negative emissions technologies, such as carbon dioxide removal, could potentially restore the bloom even from as much as 5°C of global warming, recovery timescales would be far beyond human lifetimes and policy timescales.

Lead author Dr Sophy Oliver at NOC said, “The encouraging message from this study is that there is a pathway towards recovery. Limiting global warming gives the North Atlantic ecosystem a much better chance of recovering in the long term. However, our results also show that the ocean operates on timescales much longer than our own lifetimes – even under the most favourable scenarios, recovery takes centuries.”

Dr Andrew Yool, senior scientist and a model expert at NOC, said: “These experiments help us understand how the ocean might respond to different climate pathways and, importantly, how much opportunity we have to influence those pathways. They underline the risks of relying too heavily on future technologies that are not yet available at scale. Reducing emissions still remains the most direct and reliable way to limit the risks to ocean ecosystems and the wider climate.”

NOC principal scientist Dr Katya Popova, an expert in the impacts of climate change on marine ecosystems, commented: “This work highlights the need to assess broader ecosystem consequences, including effects on the marine food web and carbon storage. Changes in the gyre do not stay in the gyre and they may serve as an early warning indicator to wider ecosystem impacts, allowing for more timely adaptation to changing ocean conditions.”

The researchers emphasise that the study uses idealised model experiments from only one Earth system model, and that further experiments across a wider range of models are needed to build a more robust picture of how the gyre and bloom may respond to different emissions scenarios. Fortunately, international modelling groups are beginning to reproduce these experiments across a diverse range of models to better understand how sensitive the Earth is to the timing of achieving net zero.

The research was led by the National Oceanography Centre, with support from Plymouth Marine Laboratory, the UK Met Office and the National Centre for Atmospheric Science, University of Reading. It was funded by the NERC projects TerraFIRMA, UKESM and AtlantiS, the EU-funded OptimESM and TipESM projects, and PROMOTE, funded by ARIA.

Full research article: https://doi.org/10.1029/2025JC023561

NOC press release: https://www.noc.ac.uk/news/new-study-identifies-possible-tipping-threshold-key-north-atlantic-ecosystem