Marine heatwaves under net-zero and overshoot scenarios

Marine heatwaves are hazardous events particularly threatful to the ocean ecosystem. Observations show that their frequency and intensity are increasing in response to global warming. Evaluation of future marine heatwaves’ characteristics were primarily made using transient states of the Earth system. In this context, metrics were assessed at transient global warming levels (TWL), following the Paris Agreement goal to limit global warming well below 2.0°C or even 1.5°C above pre-industrial levels. However, assessment at TWL cannot be proxies for global warming stabilization storylines which require net-zero emission. In addition, current trends in global warming suggest that the Paris Agreement limits will be exceeded. Here, we analyse marine heatwaves’ characteristics at 2.0 and 4.0°C stabilized global warming levels (SWL) under net-zero and overshoot scenarios. For that, we run long term simulations following the TipMIP protocol and using the CNRM-ESM2-2 model. A positive 0.2°C.decade-1 ramp-up allows to reach the target temperatures where 300-years net-zero runs are branched. Overshoots are carried out, after 50-years of stabilization, using a symmetrical negative ramp-down. These results enable (i) to understand the global and regional evolution under net-zero and (ii) to evaluate possible hysteresis effects undergone with overshoots and net-zero pathways. In broader perspective, this work focuses on the implications for marine heatwaves’ key metrics as their consequent impacts could differ according to the pathway followed.