Coral Bleaching Risk Calculator
Compute coral bleaching risk using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
DHW = HotSpot x Duration; HotSpot = SST - MMM
DHW accumulates thermal stress: SST above maximum monthly mean times weeks. DHW 4+ means significant bleaching; 8+ means severe with mortality.
Worked Examples
Example 1: Moderate Thermal Stress
Problem:SST 29.5 C, MMM 27.8 C, 6 weeks, 70% light, pH 8.05.
Solution:HotSpot=1.7 C\nDHW=1.7x6=10.2\nAcid stress=0.5\nDHW>=8: Alert Level 2
Result:DHW: 10.2 | Alert Level 2
Example 2: Minor Warming
Problem:SST 28.2 C, MMM 27.5 C, 3 weeks, 50% light, pH 8.10.
Solution:HotSpot=0.7 C\nDHW=0.7x3=2.1\nAcid=0\nHotSpot<1: Stress
Result:DHW: 2.1 | Stress level
Frequently Asked Questions
What causes coral bleaching?
Coral bleaching occurs when corals expel symbiotic algae (zooxanthellae) due to environmental stress primarily elevated water temperatures. These algae provide up to 90 percent of coral energy through photosynthesis. When sea surface temperatures exceed the local maximum monthly mean by just 1 degree for several weeks the symbiosis breaks down. Bleached corals are severely weakened and die if stress persists beyond 4 to 8 weeks.
How does ocean acidification compound bleaching?
Ocean acidification makes corals more vulnerable to thermal bleaching by forcing them to expend more energy maintaining calcium carbonate skeletons. This reduces capacity to cope with temperature stress lowering the bleaching threshold. Current ocean pH has dropped from pre-industrial 8.18 to about 8.05. Experiments show corals in acidic conditions bleach at temperatures 0.5 to 1 degree lower than those in normal pH water.
What is the HotSpot value in coral monitoring?
HotSpot represents the current positive temperature anomaly above the local bleaching threshold defined as the maximum monthly mean SST. Calculated as HotSpot = SST minus MMM with only positive values retained. A HotSpot of 1+ degree indicates conditions warm enough for coral stress. It provides an instantaneous snapshot while DHW provides cumulative stress. NOAA publishes daily HotSpot maps for reef areas worldwide.
How long does coral recovery take after bleaching?
Recovery depends on severity. Mildly bleached corals (under 4 DHW) can regain zooxanthellae within 3 to 12 months. Moderate bleaching (4-8 DHW) requires 2 to 5 years for full tissue recovery. Severely bleached reefs with mass mortality need 10 to 30 years for structural recovery assuming larval supply from healthy reefs. Repeated events before recovery permanently shift communities toward algae-dominated states.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy