Predictions about the future of the Great Barrier Reef’s coral may be too grim, according to Australian researchers. 

A new study looks at a forecasting measure known as Degree Heating Months (DHM), which researchers say could overstate heat stress compared to the widely used Degree Heating Weeks (DHW).

The study found that DHM-based projections predicted 33 to 1,584 per cent more coral bleaching incidents than DHW, depending on the model used. 

This variance is rooted in how the two metrics calculate heat stress: while DHW measures it over weeks, DHM extends it to months, often inflating stress levels. 

Notably, projections of coral cover from 2030 to 2050 showed that DHW estimates were consistently double those produced by DHM.

The implications are significant for reef management. 

DHM, the researchers argue, is “not suitable for reef management decision-making” as it consistently predicts more severe bleaching than DHW. This exaggeration also skews ecosystem models, which are used to shape conservation policies.  

For example, under moderate emissions scenarios, coral cover is projected to stabilise at 3 to 5 per cent using DHW but drops to zero with DHM. 

The study also found DHM could push back predicted timelines for critically low coral cover by 10–20 years under various scenarios, giving false hope about the reef's resilience. 

“Using DHM to estimate future coral cover may be misleading and needs a rethink,” the researchers warn.  

Although DHM has been widely used, its flaws are becoming harder to ignore as climate models advance. 

The team suggests using DHW instead, arguing it is more aligned with real-world bleaching thresholds and outcomes. 

With the Great Barrier Reef already under intense climate pressure, improving forecasting tools is crucial for targeted intervention and survival strategies.  

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