Marine ecosystems worldwide are increasingly threatened by marine heatwaves, which are prolonged periods of abnormally high sea surface temperatures. These thermal events place immense stress on sensitive organisms like corals, leading to widespread damage and mortality. To effectively monitor and predict the ecological impact of these heatwaves, scientists require a standardized metric. The Degree Heating Week (DHW) serves as the primary scientific tool used globally for this purpose, quantifying both the intensity and the duration of thermal stress.
Defining the Degree Heating Week
The Degree Heating Week is a measure of cumulative thermal stress experienced by marine life, typically calculated over a 12-week or 84-day period. This metric combines two critical factors: how much warmer the water is than normal and for how long that elevated temperature persists. The DHW unit is expressed as “degree Celsius-weeks” (°C-weeks), which inherently encapsulates both the intensity and the duration of the heat anomaly. This cumulative nature makes it a valuable predictive index for marine ecosystem health. The long-term average temperature for an area’s warmest month, known as the Maximum Monthly Mean (MMM), forms the baseline for this calculation.
Deriving the Metric: HotSpots and Accumulation
The calculation of the Degree Heating Week begins with determining the daily sea surface temperature anomaly, a metric known as the Coral Bleaching HotSpot. This HotSpot value is the amount by which the current daily sea surface temperature exceeds a predetermined bleaching threshold, which is set at \(1^\circ\text{C}\) above the Maximum Monthly Mean (MMM) for that location. Only when the water temperature equals or exceeds this bleaching threshold does a positive HotSpot value contribute to the DHW accumulation.
The DHW value is then calculated by taking a running sum of these daily HotSpot values over the preceding 12 weeks. This continuous accumulation process means that the DHW value represents the total thermal energy absorbed by the marine environment over the last three months. As each new day’s HotSpot value is added, the HotSpot value from the day 85 days prior “falls off” the accumulation window, keeping the calculation consistently focused on the most recent 12 weeks of thermal history.
Thresholds for Marine Damage
The primary application of the DHW is to translate a numerical value into a prediction of ecological damage, with established thresholds linking the metric to the likelihood of coral bleaching and mortality. A DHW value greater than zero indicates that the area has experienced accumulated heat stress, signaling potential risk. When the DHW reaches \(4\text{ }^\circ\text{C-weeks}\), it signifies a significant thermal load where widespread coral bleaching is likely to occur.
The severity of the impact increases substantially as the value climbs higher. If the accumulated heat stress reaches or exceeds \(8\text{ }^\circ\text{C-weeks}\), the thermal stress is severe enough to cause widespread coral mortality. DHW values act as an early warning system, allowing conservation managers to anticipate and prepare for severe heat stress events. This predictive capability is used globally to inform decisions regarding conservation actions or further assessment of impacted reef areas.