Rethinking Marine Heatwaves: Towards a Probabilistic Marine Heat Stress

Marine heatwaves (MHWs) are commonly defined as statistically extreme temperature events relative to a climatological baseline. While this approach provides a consistent description of ocean temperature extremes, it does not directly account for the thermal exposure experienced by marine organisms or its biological consequences. Here, we propose the Probabilistic Marine Heat Stress Wave (PMHSW) framework, which shifts the focus from temperature anomalies toward biologically relevant thermal exposure. The framework integrates temperature exceedance above a biological threshold across time and depth to estimate cumulative thermal exposure and relates this exposure to the probability of a defined biological response. The probabilistic formulation allows differences among organisms, habitats and thermal tolerances to be represented while retaining a continuous description of biological heat stress. Using synthetic examples, we demonstrate how the response function can be estimated from biological observations and how a binary event definition can be obtained when needed. The PMHSW framework is intended as a complementary approach to existing MHW definitions, particularly when the objective is to assess the potential biological consequences of marine thermal extremes under present and future climate conditions.