To calculate Rising Ocean Temperature Risk, the following variable is used:
Rising Ocean Temperature Risk assesses hazard related to marine surface warming. This indicator is not based on an absolute temperature threshold, but measures frequency and duration of periods when surface water temperature is anomalous relative to its historical variability.
Annual risk level is determined by number of months when SST exceeds 90th historical percentile for that specific month. This metric is defined 'Marine Heatwave Months' (MHM) and provides indication of how often and for how long ocean is in extreme heat state.
Our methodology for assessing Rising Ocean Temperature Risk is directly based on definition and analysis of Marine Heatwaves, a research field of great current importance. The solid approach assesses frequency and duration of thermal anomalies, which are key factors determining impact on marine ecosystems.
The logic of classifying risk based on number of months in heatwave state (MHM) is fully supported by most important scientific report on oceans and climate.
The logic of increasing risk scale with thermal stress duration is validated by decades of research on ecological impacts, particularly coral bleaching.
In conclusion, our methodology uses a percentile anomaly-based approach, in line with standard scientific definition of marine heatwaves (Hobday et al.). The choice to classify annual risk based on number of months in stress state (MHM) is fully supported by IPCC conclusions and vast literature on ecological impacts, which identify duration of thermal stress as critical factor for marine ecosystem health.
Our methodology for assessing Rising Ocean Temperature Risk is directly based on definition and analysis of Marine Heatwaves, a research field of great current importance. The solid approach assesses frequency and duration of thermal anomalies, which are key factors determining impact on marine ecosystems. • A hierarchical approach to defining marine heatwaves This is the article that provided the scientific community with a rigorous and standardized definition of Marine Heatwave (MHW). The official definition is based on daily sea surface temperature (SST) data exceeding a seasonally variable threshold (typically 90th percentile historical) for at least 5 consecutive days. We use monthly average SST > 90th historical percentile, a robust and scientifically valid climatological proxy of MHW concept. Although it simplifies daily metric, it effectively captures same phenomenon: periods of anomalous and persistent heat. Counting 'Marine Heatwave Months' (MHM) allows quantifying cumulative duration of thermal stress on annual scale.
The logic of classifying risk based on number of months in heatwave state (MHM) is fully supported by most important scientific report on oceans and climate. • Special Report on the Ocean and Cryosphere in a Changing Climate This IPCC report dedicates an entire section to marine heatwaves (particularly Chapter 6). One of its main conclusions is that climate change is drastically increasing frequency, duration and intensity of MHWs. The report states with very high confidence that MHW frequency has doubled since 1982 and will become increasingly common. Our MHM metric is a direct measure of both frequency (how many events occur) and cumulative duration (for how many months ecosystem is under stress). Our risk scale increasing with MHM perfectly reflects IPCC conclusions on increasingly severe impacts associated with longer and more frequent heatwaves.
The logic of increasing risk scale with thermal stress duration is validated by decades of research on ecological impacts, particularly coral bleaching. • Global warming and recurrent mass bleaching of corals This is one of reference studies demonstrating how coral reef mass bleaching is directly caused by marine heatwaves. Bleaching severity depends not only on peak temperature, but especially on thermal stress duration. Concepts like 'Degree Heating Weeks' (used by NOAA) were developed precisely to quantify this cumulative stress. Our MHM indicator is conceptually analogous: more months ocean spends above critical threshold, greater cumulative stress and more severe expected ecosystem damage, such as bleaching, mass mortality and species migration.
In conclusion, our methodology uses a percentile anomaly-based approach, in line with standard scientific definition of marine heatwaves (Hobday et al.). The choice to classify annual risk based on number of months in stress state (MHM) is fully supported by IPCC conclusions and vast literature on ecological impacts, which identify duration of thermal stress as critical factor for marine ecosystem health.