The North Atlantic sub-polar gyre and phytoplankton bloom under potential future climate scenarios

The North Atlantic phytoplankton bloom (NAB) forms the base of the food web upon which marine ecosystem services throughout the northern North Atlantic rely. The NAB's nutrient supply is predominantly driven by deep winter mixing, which is most pronounced in the sub-polar gyre (SPG) where it has been predicted to collapse within the next few decades. Here we use idealized experiments with an Earth system model to investigate how mixed layer depth, surface nutrients and chlorophyll concentrations over the SPG may respond to positive carbon emissions followed by both zero and negative emissions mitigation efforts initiated at various global warming levels (GWLs). Results show a decline in all three variables during positive emissions, with the warmer the GWL the less likely the prospect of recovery back to pre-industrial conditions. In the scenario where carbon emissions are held at zero once reaching a GWL of 1.5°C, the NAB starts to recover and is extrapolated to return to preindustrial levels only after several centuries to millennia. However, zero emissions after 2.5°C and warmer shows a continued decline in the NAB. These results suggest a tipping threshold may lie between 1.5-2.5°C GWL, beyond which only negative emissions can achieve recovery of the NAB. Even in the lowest warming scenario examined, 1.5°C, recovery of the NAB under negative emissions does not occur within one century.